Nervous System. Nervous System. Nervous System. Neurons NEURONS. Neurons (nerve cells) are the cells of the nervous system that function in

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1 Nervous System The nervous system is divided into two major parts, (1) the central nervous system (CNS) and (2) the peripheral nervous system (PNS). Anatomy and Physiology Text and Laboratory Workbook, Stephen G. Davenport, Copyright 2006, All Rights Reserved, no part of this publication can be used for any commercial purpose. Permission requests should be addressed to Stephen G. Davenport, Link Publishing, P.O. Box 15562, San Antonio, TX, Nervous System Central Nervous System The central nervous system consists of the brain and the spinal cord. Functions of the central nervous system include integration, control, consciousness, and mental activity. Peripheral Nervous System The peripheral nervous system consists of all the nervous system components, such as the nerves and neurons, that extend from or are located outside of the central nervous system. The peripheral nervous system is divided into the (1) sensory (afferent) division and the (2) motor (efferent) division Afferent (sensory) division The sensory division involves information collected from the somatic division, visceral division, and special senses and delivered to the central nervous system (CNS). Efferent (motor) division The efferent division involves information flow from the central nervous system (CNS) to the somatic division and the visceral division. Nervous System Somatic Division The somatic division of the peripheral nervous system is the division involved with the voluntary control of body movements. The somatic division is divided into the sensory (afferent) and the motor (efferent) components. The sensory component functions in receiving stimuli and conducting information to the CNS concerning voluntary body movements. The motor (efferent) component functions to deliver information from the CNS to the skeletal muscles, thus, directing their contraction. Visceral Division The visceral division of the peripheral nervous system is the division involved with involuntary control of body movements such as those of the cardiovascular, digestive, urinary, respiratory, etc., systems. The sensory component functions in receiving stimuli and conducting information concerning involuntary control to the CNS. The motor component functions to deliver information from the CNS to control involuntary movements. The motor component for involuntary control is routed through the autonomic nervous system (ANS). The autonomic nervous system, divided into the parasympathetic and sympathetic divisions, directs motor control to smooth muscle, cardiac muscle, and glands. Special Senses The special senses include taste, hearing, equilibrium, vision, and smell. Information from the five senses is routed to the CNS by way of the afferent (sensory) division. Information processed by the central nervous system may produce an efferent (motor) response that is directed to either/both the efferent somatic or visceral divisions. Figure 17.1 An overview of the organization of the nervous system. Neurons NEURONS Neurons (nerve cells) are the cells of the nervous system that function in (1) the generation and (2) the conduction of the nerve impulse, and the (3) secretion of a neurotransmitter at their terminals. Neurons have a cell body with one or more processes (nerve fibers) extending from them. 1

2 Structure of a Multipolar Neuron Cell Body Neuron Cell Body The cell bodies of neurons are located in the gray matter of the central nervous system and in structures called ganglia in the peripheral nervous system. The cell body of a neuron contains abundant cytoplasm with numerous organelles. The organelles that are easily observed with the light microscope include (1) a large nucleus that contains one to several dark-stained nucleoli and (2) dark-stained granules called Nissl substance, or bodies (rough endoplasmic reticulum). Figure 17.2 A typical multipolar neuron showing numerous processes associated with the cell body. Only one axon originates at the cell body, all the other processes are dendrites. Structure of a Multipolar Neuron - Processes Dendrites Depending upon the type of neuron, one to many processes called dendrites may be present at the cell body. Dendrites have traditionally been described as the neuron s processes that function in the conduction of impulses toward its body. Another description is that the dendrites are the structures that function as the receptive portions of the neuron. The latter function is the one used is this study. Structure of a Multipolar Neuron - Processes Axon Usually, a neuron has only one process called an axon associated with its cell body. The site on the cell body where the axon originates is called the axon hillock. Once the axon leaves the cell body, the axon can split into branches called collaterals. The axon ends in fine branches called telodendria. The end of each tenodendrion is called an axon terminal that functions in a synapse, the site of where the nerve impulse passes to another neuron, a muscle, or a gland. Traditionally, an axon has been described as the portion of the neuron that functions in the conduction of impulses away from the cell s body. Another description is that the axon is the process that generates and conducts the impulse, and releases a neurotransmitter at its axon terminals. CLASSIFICATION OF NEURONS Classification According to Function Classification According to Function Three classifications of neurons according to function are (1) sensory neurons, (2) motor neurons, and (3) association neurons (interneurons). Sensory neurons Neurons that transmit impulses generated at their receptors toward the central nervous system are sensory, or afferent, neurons. They constitute the sensory (afferent) division of the peripheral nervous system. Motor neurons Neurons that transmit impulses from the central nervous system to effectors (glands and muscles) are motor, or efferent, neurons. They constitute the motor (efferent) division of the peripheral nervous system. Association neurons (Interneurons) Neurons of the CNS that transmit impulses from one neuron to another are generally called association neurons, or interneurons. 2

3 Figure 17.3 An overview of the functional classification of neurons. Classification According to Structure Three classifications of neurons according to structure are (1) unipolar neurons, (2) bipolar neurons, and (3) multipolar neurons. Unipolar neurons A unipolar neuron has a single continuous fibrous process that is associated with its body. Its single process is produced by the merging of its receptive dendrites with the conductive axon that terminates with synaptic contacts either in the brain or spinal cord. Unipolar neurons are sensory neurons associated with the peripheral nervous system. Bipolar neurons A bipolar neuron has two fibrous process, each process is associated with the cell s body. Bipolar neurons are sensory neurons associated with the neural pathways of the senses involving sight, hearing, and smell. Multipolar neurons A multipolar neuron has more than two processes associated with its cell body. Only one of the processes is the axon, all of the other processes are dendrites. Multipolar neurons are common in the CNS functioning as association (interneurons), and exiting the CNS functioning as motor neurons. NEUROGLIA Neuroglia are the cells and their associated branching fibers that support neural tissue Figure 17.4 An overview of the structural classification of neurons. Neuroglia Neuroglia are the cells and their associated branching fibers that support neural tissue. Central nervous system Four varieties of neuroglia found in the are (1) ependymal cells, (2) astrocytes, (3) oligodendrocytes, and (4) microglia. Peripheral Nervous System Two varieties of neuroglia found in the peripheral nervous system are (1) satellite cells and (2) Schwann cells. Neuroglia of the Central Nervous System Ependymal cells Ependymal cells are found lining the cerebrospinal fluid containing cavities of the CNS. In the brain ependymal cells line the ventricles, and in the spinal cord they line the central canal. The ependymal cells function in the production and regulation of cerebrospinal fluid (CSF). Astrocytes Astrocytes, the most numerous glial cells, are glial cells named for their star-shape. Among their functions are support and nutrient exchange/regulation between neurons and adjacent capillaries. 3

4 Neuroglia of the Central Nervous System Oligodendrocytes Oligodendrocytes are the glial cells that produce the myelin sheaths of CNS axons. The oligodendroglia produces sheet-like extensions that form the myelin sheets. Microglia Microglia are phagocytic cells of the CNS. They remove debris, waste, pathogens, and other materials. Neuroglia of the Peripheral Nervous System Satellite cells Satellite cells are the glial cells that surround the cell bodies of neurons in ganglia, the only sites in the PNS that contain cell bodies of neurons. Schwann cells Schwann cells are the glial cells that are associated with all axons in the PNS. Schwann cells either tightly wrap axons to produce myelin sheaths (myelinated axons) or remain in close association to produce unmyelinated axons. Lab Activity 1 Motor Nerve Cells Observe a microscopic preparation of Motor nerve cells, smear (Nerve cells, spinal cord smear). Identify the multipolar neurons and neuroglia cells (nuclei). Locate an isolated neuron and identify its cell body, nucleus, Nissl substance (bodies), and processes. Figure 17.5 Scanning power photograph of a multipolar neuron from a slide preparation labeled Motor nerve cells, spinal cord smear. Lab Activity 1 Motor Nerve Cells Cell body The cell body contains most of the cell s organelles and cytoplasm. The cytoplasm contains the usual cell organelles except centrioles (the lack of centrioles makes the cells amitotic). The nucleus is easily observed with one to several nucleoli. Dark-stained areas of rough endoplasmic reticulum called Nissl substance (bodies) can usually be observed with high magnification. Cell processes Usually, on smear preparations the cells are severely traumatized, which makes the microscopic identification of the dendrites and the single axon difficult. The axon is long, may show distal branches, and does not contain Nissl substance (bodies). In comparison, the dendrites are short, have numerous branches, and contain Nissl substance. Lab Activity 1 Motor Nerve Cells Neuroglia The supporting cells of the nerve tissue, the neuroglia, are seen as the dark-stained nuclei distributed throughout the preparation. Mostly consisting of astrocytes, the neuroglia are highly traumatized during tissue preparation leaving their nuclei scattered throughout the preparation. 4

5 Figure 17.6 Low power photograph of motor nerve cells from a slide preparation labeled Motor nerve cells, spinal cord smear. The general structure of multipolar neurons is observed. Figure 17.7 High power photograph of multipolar nerve cells from a slide preparation labeled Motor nerve cells, spinal cord smear. MYELINATED and UNMYELINATED AXONS Peripheral Nervous System Myelinated and Unmyelinated Axons of the PNS In the peripheral nervous system, neuroglia called Schwann cells are arranged sequentially along all axons. The plasma membranes of Schwann cells contain a phospholipid called myelin. If the Schwann cells surround and tightly wrap an axon, they produce a myelinated axon with each Schwann cell producing an area of concentrically wrapped plasma membrane called the myelin sheath. The Schwann cell s cytoplasm covered by the plasma membrane is displaced outward to the myelin sheath and forms the membranous covering of the fiber, the neurolemma. Small gaps, called nodes of Ranvier, are formed between adjacent Schwann cells. Unmyelinated axons are formed when Schwann cells do not tightly wrap axons. In unmyelinated axons, a single Schwann cell usually associates with several axons and only partially encloses the axons. Figure 17.8 Illustration showing structural differences between myelinated and unmyelinated axons in the peripheral nervous system. Figure 17.9 Illustration showing structural differences between myelinated and unmyelinated axons in the peripheral nervous system. 5

6 Myelinated and Unmyelinated Axons of the CNS In the central nervous system, the neuroglia called oligodendrocytes associate with the axons to form myelinated axons. Oligodendrocytes associate with several axons by produce sheet-like extensions that wrap around, thus, myelinating the axons. Mostly, myelinated fibers are organized into the areas of white matter of the brain and spinal cord. Unmyelinated fibers are common in the gray matter of the brain and spinal cord. Figure Illustration showing myelination of fibers (axons) in the central nervous system by oligodendrocyte. Lab Activity 2 Medullated Nerve Observe a microscopic preparation labeled Medullated nerve, teased. A nerve is a part of the peripheral nervous system that consists of parallel axons (fibers) and their associated Schwann cells enclosed in connective tissue wrappings. Teasing the nerve separates the axons (fibers) for individual observation. Some preparations are specifically prepared (treated with osmic acid) to show the internal details of the myelin sheath, the node of Ranvier, and the axon. Otherwise, the preparation will usually show only the surfaces of the Schwann cells and the nodes of Ranvier. Figure Illustration of teased nerve fibers. Figure High power photograph of teased nerve fibers (at the node of Ranvier). Lab Activity 2 Medullated Nerve Schwann cells Schwann cells are located sequentially along the axon. Each Schwann cell tightly wraps the axon to form a myelin sheath. The Schwann cell s cytoplasm surrounded by plasma membrane are located to the outside of the myelin sheath and are called the neurilemma. Myelin sheath The myelin sheath is formed by the wrapping of the myelin containing plasma membrane of the Schwann cell around the axon. The myelin sheath is easily observed in preparations treated with osmic acid. Myelin treated with osmic acid is darkly stained. If not treated with osmic acid, myelin sheaths are identified as remnants. 6

7 Lab Activity 2 Medullated Nerve Nodes of Ranvier Nodes of Ranvier are gaps formed between adjacent Schwann cells. They allow exposure of the axon to the extracellular environment. Axon An axon is the process (branch) of a neuron which (1) generates and (2) conducts nerve impulses, and (3) releases neurotransmitter at its terminal synapses. The portions of the axons in nerves are the long processes (fibers) that function in conduction of nerve impulses. NERVE A nerve is a part of the peripheral nervous system and consists of parallel axons (fibers) and their associated Schwann cells enclosed in connective tissue wrappings (sheaths). Nerves Nerves may contain (1) only myelinated fibers, (2) only unmyelinated fibers, or (3) a combination of both. According to the directions of impulse conduction, nerves are classified as (1) sensory - Sensory nerves contain fibers of sensory (afferent) neurons that convey impulses to the central nervous system (2) motor - Motor nerves contain fibers of motor (efferent) neurons and convey impulses away from the central nervous system to effectors. (3) mixed (both sensory and motor) - Mixed nerves contain a mixture of both sensory and motor fibers. Organization of a Nerve Each axon (fiber) and its associated Schwann cells are surrounded by a connective tissue sheath called the endoneurium. A connective tissue sheath called the perineurium organizes individual fibers (axons) and their associated endoneuria into groups called fascicles. An outermost connective tissue sheath called the epineurium organizes the fascicles into a nerve. Epineurium The epineurium is the outer connective tissue sheath of the nerve. The epineurium surrounds groups of fibers (axons) called fascicles. Perineurium The perineurium is the connective tissue sheath that organizes fibers into fascicles. Fascicles Fascicles are groups of fibers surrounded by the connective tissue sheath called the perineurium. Endoneurium The endoneurium is the inner connective tissue sheath that surrounds each individual axon (fiber). Organization of a Nerve Axons The axon functions in the generation and the conduction of the nerve impulse, and releases a neurotransmitter at its terminal synapses. The portions of the axons in nerves are the long processes (fibers) that function in conduction of nerve impulses. All axons of the peripheral nervous system are associated with Schwann cells. In a section of a nerve, an axon is identified as a tiny dark circular structure associated with a Schwann cell. Myelinated axons are centrally located within the myelin sheaths of the Schwann cells. Axons and their associated Schwann cells are surrounded by a connective tissue sheath called the endoneurium. Organization of a Nerve Schwann cells Schwann cells are associated with the axons of the peripheral nervous system. Myelinated axons are formed by the wrapping of the myelin containing plasma membranes of Schwann cells around the axons. Unmyelinated axons are formed when a Schwann cell associates with several axons and does not tightly wrap the axons. Neurilemma The neurilemma is the membranous covering of a nerve fiber. The neurilemma is formed by the thin region of cytoplasm and the plasma membranes of the Schwann cells. Myelin sheath Myelin sheaths are the fatty sheaths that are formed by the tight wrapping of the Schwann cells around the axon. Unless the specimen is specifically stained for myelin (medullated nerve with osmic acid), the myelin sheath is lightly stained and exists as a remnant. 7

8 Figure General structure of a nerve. A nerve is a part of the peripheral nervous system and consists of parallel axons (fibers) and their associated Schwann cells enclosed in connective tissue wrappings. Figure Illustration of a cross section of a nerve. A nerve is a part of the peripheral nervous system and consists of parallel axons (fibers) and their associated Schwann cells enclosed in connective tissue wrappings. Lab Activity 3 Cross Section of a Nerve Observe a preparation of a Nerve, c.s. (c.s.- cross section). A cross section of a nerve may be presented singly on a slide preparation or may be accompanied by a longitudinal section (l.s.). Preparations with both sections are typically labeled Nerve, c.s. & l.s. Most general preparation of nerves are not prepared with osmic acid, thus, do not show darkly stained myelin sheaths. Instead the myelin sheaths are observed as remnants and are mostly identified by location. Observe the preparation with scanning power and identify the epineurium, perineurium, fascicles, axons, and myelin sheaths. Figure Scanning power photograph of a cross section of a nerve. Since general preparations are not treated with osmic acid, the myelin sheaths are observed as remnants. Figure Illustration of myelinated fibers from a cross section (c.s.) of a nerve (high magnification). The myelin sheaths of the Schwann cells are not well preserved and are identified as remnants. Figure High power photograph of fibers (axons) from a cross section of a nerve. Preparation does not show preserved myelin sheaths. 8

9 Figure High power photograph of fibers (axons) from a cross section of a nerve prepared with Masson stain. Preparation does not show preserved myelin sheaths. Lab Activity 4 Longitudinal Section (l.s.) of a Nerve Observe a microscopic preparation labeled Nerve, l.s. A longitudinal section of a nerve is most useful in showing the relationship between the Schwann cells and the axon. As with the cross section, myelin sheaths are observed only if the specimen was processed to maintain the myelin (medullated nerve treated with osmic acid). Otherwise, the myelin sheaths exist as remnants and are lightly stained. Figure Low power photograph of a longitudinal section of a nerve. Figure High power photograph of fibers (axons) from a longitudinal section of a nerve. Lab Activity 5 Cross Section of Medullated (Myelinated) Nerve Observe a microscopic preparation labeled Medullated Nerve, c.s., osmic acid (c.s.- cross section). The observation of a nerve in cross section allows a study of (1) its connective tissue organization and (2) of axons. Myelinated axons are best observed in preparations of a medullated nerve treated with osmic acid. Identify the (1) epineurium, (2) perineurium, (3) endoneurium (4) fascicles, (5) axons, (6) Schwann cells, (7) neurilemma, and (8) myelin sheaths. Lab Activity 5 - Myelinated Nerve (with myelin sheaths stained) Observe a preparation of a Medullated Nerve, osmic acid. Treatment of medullated (myelinated) nerve preparations with osmic acid darkly stains the myelin sheaths. Observe the preparation with scanning power and identify the epineurium, perineurium, fascicles, axons, and myelin sheath. 9

10 Figure Scanning power photograph of a cross section of a medullated nerve prepared with osmic acid. Myelin sheaths are easy to identify because osmic acid stains myelin black. Figure Illustration of myelinated fibers from a cross section of a medullated nerve (high magnification). Schwann cells surrounding the axons have regions called myelin sheaths and regions of cytoplasm with a covering plasma membrane, the neurilemma. Figure High power photograph of medullated fibers (axons) from a cross section of a medullated nerve. Most of the axons are surrounded by thick myelin sheaths. SPECIALIZED NEURON ENDINGS Specialized neuron endings of the peripheral nervous system are found associated with the (1) sensory (afferent) and (2) motor (efferent) neurons. Receptors Specialized neuron endings of the peripheral nervous system are found associated with the (1) sensory (afferent) and (2) motor (efferent) neurons. The sensory division of the PNS relies on dendrites, the receptive portion of the axon, to respond to stimuli. Dendrites may be modified into specialized structures called receptors. The motor division of the PNS relies upon the synaptic transmission of the nerve impulse from the axon to the effector, a muscle or a gland. Receptors Receptors are sensory endings which respond to specific types of stimuli. A stimulus is a change that promotes a response. For sensory receptors stimuli are mediated through a change in the receptors environment. Among the stimuli that receptors respond to are changes in temperature (thermoreceptors), mechanical forces such as pressure and stretch (mechanoreceptors), and chemicals such as acids and electrolytes (chemoreceptors). 10

11 Effectors Effectors are the muscles and glands controlled by the peripheral nervous system. The axons of efferent neurons synapse with effectors and rely upon a neurotransmitter to mediate the flow of information (nerve impulse). Synapses with muscles are the neuromuscular junctions and with glands the neuroglandular junctions. Figure Simplified neural pathway between a receptor (Pacinian corpuscle) and an effector (neuromuscular junction - neuron synapse with skeletal muscle fiber) RECEPTOR Pacinian Corpuscle RECEPTOR Pacinian Corpuscle Among the many specialized receptors of the peripheral nervous system, the Pacinian corpuscle is large and distinct. Pacinian corpuscles are lamellated pressure receptors (mechanoreceptor) mostly located deep in the dermis of the skin and in the loose connective tissues distributed throughout the body. A Pacinian corpuscle consists of a centrally located dendrite (the receptive portion of the neuron) surrounded by layers of flattened Schwann cells, which are surrounded by a connective tissue capsule. RECEPTOR Pacinian Corpuscle Upon stimulation, the dendrite generates an electrical potential called a graded potential. A graded potential is a local response (here restricted to the dendrite), and produces an electrical signal that has an intensity related to the strength of stimulation. If the graded potential reaches an intensity sufficient to stimulate the axon to threshold, an action potential is produced and propagated to the axon s terminus, the axon terminals. The axon terminals in response to the action potential release a neurotransmitter, which functions as a chemical mediator for the transfer of the electrical information. Lab Activity 6 Pacinian Corpuscle A Pacinian corpuscle has a dendrite (receptive region) located in the center of concentric layers of flattened Schwann cells (lamellae), which is surrounded by a connective tissue capsule. 11

12 Figure A Pacinian corpuscle is a pressure receptor (mechanoreceptor). It has a dendrite (receptive region) located in the center of concentric layers of flattened Schwann cells (lamellae), which is surrounded by a connective tissue capsule. Lab Activity 6 Pacinian Corpuscle Dendrite The centrally located dendrite is the receptive portion of the Pacinian corpuscle. Capsule The capsule is the outer connective tissue layer of the Pacinian corpuscle. Lamellae The lamellae are concentric layers of flattened Schwann cells. EFFECTOR Neuromuscular Junctions Effector Neuromuscular Junctions The axon of a motor neuron may branch many times as it enters a muscle. At the point where an axon approaches the muscle fiber (cell), it branches into many small terminal branches (telodendria) that end in knob-like axonal terminals. The site on the muscle cell where the axon s terminals come in close contact (synapse) with the muscle fiber is called the neuromuscular junction. A small space, called the synaptic cleft, separates the axon s terminals and the adjacent region of the muscle fiber, the motor end plate. The motor end plate is the specialized region of the muscle fiber s plasma membrane that contains receptors for the neurotransmitter. Thus, the neuromuscular junction includes the axon s terminals, synaptic cleft, and the motor end plate. Lab Activity 7 Neuromuscular Junction Observe a preparation labeled Neuromuscular junctions. The preparations of neuromuscular junctions are of skeletal muscle fibers (cells). Follow several axons, each to its junction with a skeletal muscle fiber. Figure Scanning power photograph of neuromuscular junctions. A neuromuscular junction consists of the axon s terminals, a synaptic cleft, and the motor end plate of the skeletal muscle cell (fiber) 12

13 Figure High power photograph of a neuromuscular junction. Lab Activity 7 Neuromuscular Junction Axon The axons of motor neurons are the processes that generate and conduct nerve impulses to the neuromuscular junctions and at their axonal terminals release neurotransmitter. Neuromuscular junction The neuromuscular junction consists of the axon s terminals, a synaptic cleft, and the motor end plate of the skeletal muscle cell (fiber). Physiology of Conduction Electrical Terminology Potential energy State of electrical energy as measured by the potential to produce electrical effects Voltage (potential) Electrical measurement used to describe electrical potential between two points. Current Flow of electrical charge and is due to the electrical difference (voltage) between two points Resistance Opposition to electrical flow Insulators have high resistance Conductors have low resistance Size AAA Electrical Terminology Size D How might the following terms apply to these two batteries? Potential energy Are both the same? Voltage Are both the same? Current Do both produce the same? Resistance Does a battery contain a resister? Electrical Terminology and the Cell Membrane How might the following terms apply to the illustrated cell membrane? Potential energy Voltage Current Extracellular Resistance Intracellular 13

14 Electrical Terminology and the Cell Membrane How might the following terms apply to the illustrated cell membrane? Potential energy Voltage Current Resistance Extracellular Intracellular Electrical Terminology and the Cell Membrane How might the following terms apply to the illustrated cell membrane? Potential energy Voltage Current Resistance Extracellular Intracellular Membrane Potentials Extracellular Intracellular Ionic difference between intracellular and extracellular fluids Extracellular higher concentration of Na+ (and Cl-) Intracellular higher concentration of K+ and negative proteins. Net result is potential difference between extracellular and intracellular. Extracellular is positive (Na+) Intracellular is negative due to negative proteins. Resting Membrane Potential Membrane Potential Changes If the resting membrane potential is to change must be a change in the distribution of positive and/or negative charges; a redistribution of ions Movement of ions can result when ions move through channels which include Mechanically-gated (regulated) channels Open when subjected to a mechanical stimulus Voltage-gated (regulated) channels Open when subjected to an electrical stimulus Chemically-gated (regulated) channels Open when subjected to a specific chemical such as a neurotransmitter or hormone Passive (leakage) channels Ions may leak through channels (or the phospholipid bilayer) Mechanical Channels Sodium channels (typical) open when subjected to mechanical stimulus 14

15 Channels Identify regions which are Mechanically gated Electrically gated Chemically gated Generator Potential Local response (graded potential at stretch receptor) Sodium ions move across membrane Interior becomes less negative (more positive) Depolarization (changes toward less negative (positive) voltage May not reach threshold, thus no effect (action potential) May reach threshold and produce an action potential Threshold and Action Potentials Threshold Point of depolarization (stimulation) which initiates an effect (action potential) In this case the electrically-gated Na+ channels open, (which are adjacent to the active mechanically-gated channels). The mechanically gated Na+ channels become inactive Action potential Not local; travels great distance Involves electrically-gated channels Propagated along fiber (axon) Generation of Action Potential 1. Resting membrane potential is established 2. Depolarization phase Increase in sodium ion permeability Self propagating event 3. Repolarization phase Decrease in sodium ion permeability Increase in potassium ion permeability Undershoot or after-hyperpolarization occurs Redistribution of sodium and potassium by ATP driven sodium-potassium pump Depolarization as Na+ Moves Inward Receptor s Na+ channels become inactive Local current opened adjacent electricallygated Na+ channels (threshold) These channels produce local current Adj t N + Repolarization as K+ Moves Out Local current opens adjacent electrically-gated K+ channels K+ moves outward and repolarization occurs Local currents open adjacent Na+ channels Action potential is propagated to adjacent forward ti 15

16 Na+ / K+ Pump The Na+ / K+ reestablishes the extracellular and intracellular ionic gradients Pump requires ATP Na+ is pumped outward K+ is pumped inward Synapse Anatomical relationship between neurons, or neurons and an effector organ, and at which a nerve impulse is transmitted through the action of a neurotransmitter. Components of Synapse Consist of Presynaptic membrane of axonal terminal (synaptic knob or bouton) which functions in the release of neurotransmitter Postsynaptic membrane (of dendrite, postsynaptic neuron, effector, organ, etc.) which houses receptors for neurotransmitter Synaptic cleft of extracellular material between presynaptic and postsynaptic membranes which electrically isolates the membranes. 2. Calcium ion channels open 1. Action potential arrives 7. Neurotransmitter is deactivated by enzymatic action; some components may be reused Synapse 3. Calcium ions promote exocytosis of neurotransmitter, calcium ions are quickly removed 4. Neurotransmitter binds to postsynaptic receptors 5. Receptors allow passage of specific type of ions 6. Depending upon ion movement postsynaptic membrane is either depolarized (EPSP) or hyperpolarized (IPSP) Termination of Neurotransmitter Enzymes associated with postsynaptic membrane or present in cleft Reuptake by astrocytes into presynaptic terminal where degraded by enzymes Neurotransmitter diffuses away from synapse IPSP and EPSP EPSP Excitatory postsynaptic potential results when interior becomes more positive IPSP Inhibitory post-synaptic potential results when interior becomes more negative 16

17 Synaps e Synaps e A B A B The result is A or B? Which is produced? A) action potential, B) IPSP, C) EPSP? The result is A or B? Which is produced? A) action potential, B) IPSP, C) EPSP? Summation Summation is the adding together of synaptic potentials (SPs). Could be EPSPs, IPSPs, or both EPSPs and IPSPs. Temporal summation Pertaining to time; the quick succession of SPs at a few synapses are summated Spatial summation Pertaining to space; many SPs occur over the postsynaptic membrane and are summated Mechanisms of Neurotransmitters Direct acting Channel linked receptors result in the opening of ion channels Alter membrane potential of target Can produce depolarization (sodium ions move inward) and hyperpolarization (potassium ions move outward) Mechanisms of Neurotransmitters Indirect acting Involves G-protein complex Results in the production of a second messenger Second messenger may influence enzymes to Activate or inactivate proteins (translation) Regulate gene activity (transcription) Regulate membrane ion channels and potentials Autonomic Regulation 17

18 Motor Division (efferent PNS) Parasympathetic (autonomic, visceral) Sympathetic (autonomic, visceral) Somatic (skeletal muscle, voluntary) Sympathetic (sweat glands, involuntary) Brain Spinal cord Cranial nerves CNS PNS Spinal nerves Autonomic Systems Sympathetic fight or flight response Terminal neurotransmitter is epinephrine (E) or norepinephrine (NE) Parasympathetic resting and digesting, or rest and repose Terminal neurotransmitter is acetylcholine (ACh) Organs May have dual innervations, response is excitation by one system and inhibition by other system May have single innervations, response is promoted or not promoted. HUMAN BRAIN HUMAN BRAIN Four major regions of the human brain, (1) cerebrum, (2) cerebellum, (3) diencephalon, and (4) brain stem. Four major regions of the human brain, (1) cerebrum, (2) cerebellum, (3) diencephalon, and (4) brain stem. Figure Midsagittal view of human brain showing four major regions. Figure Lateral view of human brain. 18

19 Figure Superior view of human brain. Figure Inferior view of human brain. Figure Illustration showing a midsagittal section of the human brain. Figure Photograph of a midsagittal section of the human brain. CEREBRUM The cerebrum is the largest part of the brain. Cerebrum The cerebrum is the largest part of the brain. The cerebrum is divided into the right and left cerebral hemispheres, which are connected inferiorly by a large band of white matter, the corpus callosum. The cerebrum functions include integrating somatic (body) sensory and motor information, thought, memory, reason, and emotions. 19

20 Cerebrum Figure Functional areas (Brodmann areas) of the cerebrum. Cerebral hemispheres The right and left cerebral hemispheres form the superior portion of the brain. Externally, four lobes, the (1) frontal, (2) parietal, (3) occipital, and (4) temporal, are named for both their associated cranial bones and cerebral landmarks. Both hemispheres are referred to as the cerebrum. Gyri Gyri are rounded elevated ridges on the surface of the cerebrum. Sulci Sulci are shallow grooves on the surface of the cerebrum. Fissure Fissures are deep furrows. Two dominate fissures of the brain are the longitudinal fissure and the transverse fissure. Cerebrum Longitudinal fissure The longitudinal fissure is a deep groove that medially divides the cerebrum into its right and left cerebral hemispheres. Transverse fissure The transverse fissure separates the superiorly located cerebrum from the inferiorly located cerebellum. Central sulcus A central sulcus is the centrally located shallow groove of each cerebral hemisphere that divides each frontal lobe from each parietal lobe. Cerebrum Frontal lobes A frontal lobe is the most anterior lobe of each cerebral hemisphere. Each is separated posteriorly from a parietal lobe by shallow groove, a central sulcus. The functional regions of the frontal lobes include (1) somatic motor cortex that controls movement of skeletal muscles, (2) a premotor cortex for learned (memorized) motor skills and habits (3) a motor area (Broca s area) for motor control of muscles associated with speech, (4) cognition (process of knowing - awareness, perception, reasoning, and judgment), (5) language centers for word association and meaning. Cerebrum Precentral gyrus The precentral gyrus of each frontal lobe is a rounded elevated ridge located immediately anterior to each central sulcus. A precentral gyrus functions as the primary motor cortex and houses the neurons (pyramidal cells) directly involved in conscious control of skeletal muscles. Parietal lobes Each parietal lobe is located posterior to its associated central sulcus and anterior to each occipital lobe. The primary function of the parietal lobes is housing the areas that receive and integrate relayed somatic sensory information, the somatosensory areas. The primary sensory (somatosensory) areas of the postcentral gyri receive the somatosensory information. Posterior to each postcentral gyrus, the somatosensory association areas function to integrate the sensory information so that it is comprehensible. The parietal lobe also functions in sensory integration for spatial visualization (visual attention) and manipulation of objects. Cerebrum Postcentral gyrus The postcentral gyrus of each parietal lobe is a rounded elevated ridge located posterior to each central sulcus. A postcentral gyrus functions as the primary sensory (somatosensory) cortex as it houses the neurons that receive information relayed from receptors in the skin and from a group of receptors distributed in muscles, tendons, and joints, the proprioceptors. Occipital lobes The occipital lobes are located posterior to the parietal lobes. Each occipital lobe is separated anteriorly from its associated parietal lobe mostly by the parieto-occipital fissure. The function of the occipital lobes is to house the visual cortex. The posterior occipital lobe houses the primary visual cortex, the neurons that receive information relayed from the visual fields of the eyes. Anterior to the primary visual cortex, the visual association area allows interpretation of received visual information. 20

21 Cerebrum Temporal lobes Each temporal lobe is located laterally on each hemisphere. Each is separated from the frontal and parietal lobes by the lateral sulcus. The functions of the temporal lobes include the (1) auditory areas, (2) language area (Wernicke s area), (3) memory, and the (4) ability to categorize objects. The primary auditory cortex is located in the superior portion of each temporal lobe and receives information relayed from the auditory receptors. Inferior to the primary auditory cortex the auditory association area allows meaningful interpretation of the auditory information. Wernicke s area functions in recognition of spoken words. Lateral sulcus Each lateral sulcus separates each temporal lobe from its associated frontal and parietal lobes. Cerebrum Cerebral cortex The cerebral cortex is the outer gray matter of the hemispheres. It is composed mostly of neuron cell bodies and unmyelinated fibers. Cerebral white matter The cerebral white matter is deep to the gray matter. It is composed mostly of myelinated nerve fibers. Lateral ventricles The two lateral ventricles are large chambers, one of which is located within each cerebral hemisphere. The lateral ventricles communicate with the third ventricle, each by way of an interventricular foramen. Each lateral ventricle houses the choroid plexus, which extends from the third ventricle. The choroid plexus is the site for the production and regulation of cerebrospinal fluid (CSF). Cerebrospinal fluid fills the lateral ventricles and drains into the third ventricle. Cerebrum Corpus callosum The corpus callosum consists of fibers (white matter) that connect corresponding areas of the right and left cerebral hemispheres. It is located superior to the lateral ventricles and deep in the longitudinal fissure. It functions in communication between the right and left cerebral hemispheres. Fornix The fornix is an arched band of white matter located inferior to the corpus callosum. It connects and functions in communication between regions of the brain called the hippocampus (functions in memory processes) and the hypothalamus. Septum pellucidum The septum pellucidum is a membrane that medially separates the two lateral ventricles. DIENCEPHALON The diencephalon is a region of the brain that is surrounded by the cerebral hemispheres. It consists mostly of the (1) thalamus, (2) hypothalamus, and (3) epithalamus. Diencephalon Figure The diencephalon consists of the epithalamus, thalami, and the hypothalamus. Thalami The thalami are two interconnected regions of gray matter, the right thalamus and the left thalamus, that form the superior portions of the lateral walls of the third ventricle. A bridge of fibers, the intermediate mass, passes across the third ventricle and connects the two thalamic regions. The primary function of the thalami is to relay incoming sensory information to various regions of the cerebral cortex. Epithalamus The epithalamus is located superior to the thalamus and forms the thin roof of the third ventricle. The pineal gland (body) extends outward from the posterior surface of the epithalamus. Pineal gland (body) The pineal gland is a posterior extension of the epithalamus. The pineal gland functions as an endocrine gland releasing melatonin, a hormone that functions in sleep cycles and reproduction. 21

22 Diencephalon Hypothalamus The hypothalamus is located inferior to the thalamus. It forms the inferior portions of the lateral walls and the floor of the third ventricle. Among the functions of the hypothalamus are: (1) regulation of body temperature, (2) sensations (drives) of hunger and thirst, (3) production of two hormones, antidiuretic hormone (ADH) and oxytocin (OT), that are released at the posterior pituitary gland, (4) production of regulatory hormones to control the anterior pituitary gland, (5) and regulation of the autonomic nervous system (especially involving the responses to stress and the coordination of information flow between the pons and medulla). Diencephalon Infundibulum The infundibulum is a hollow stalk of tissue which extends from the hypothalamus to the pituitary gland. The infundibulum serves as a pathway for (1) fibers (axons) and (2) blood vessels that leave the hypothalamus and enter the pituitary gland. Fibers (axons) from specialized endocrine producing neurons of the hypothalamus pass through the infundibulum, terminate at the posterior pituitary gland, and release the hormones (1) antidiuretic hormone (ADH) and (2) oxytocin (OT). A specialized unit of blood vessels in the hypothalamus functions to pickup regulatory hormones. The blood vessels pass through the infundibulum and deliver the regulatory hormones to the anterior pituitary. The regulatory hormones function in controlling the secretory activity of the anterior pituitary. Diencephalon Third ventricle The third ventricle is a narrow chamber centrally located in the diencephalon. The third ventricle houses a choroid plexus, a structure that produces cerebrospinal fluid (CSF). In addition to functioning as a site for the production of cerebrospinal fluid by its choroid plexus, the third ventricle receives cerebrospinal fluid from the lateral ventricles. From the third ventricle cerebrospinal fluid drains into the cerebral aqueduct of the midbrain. BRAIN STEM The brain stem is located between the diencephalon and the spinal cord. Superiorly to inferiorly, the brain stem consists of the (1) midbrain, (2) pons, and the (3) medulla oblongata. Brain Stem Midbrain Located immediately inferior to the diencephalon, the midbrain is the most superior portion of the brain stem. Functions of the midbrain include (1) serves as a pathway for ascending and descending tracts (peduncles), (2) visual and auditory reflexes (corpora quadrigemina), (3) control of muscle tone, (4) regulation of cerebral nuclei, and (5) maintenance of consciousness. Two cerebral peduncles (bundles of myelinated fibers) that contain ascending and descending fiber tracts are located on the midbrain's ventral-lateral surface. The posterior surface of the midbrain is formed by four rounded elevations called the corpora quadrigemina. The midbrain also houses several nuclei (red nucleus and substantia nigra). Figure The brain stem consists of the midbrain, pons, and the medulla. The corpora quadrigemina is the posterior region of the midbrain. The cerebral aqueduct is a channel that delivers CSF to the fourth ventricle. 22

23 Brain Stem Corpora quadrigemina The corpora quadrigemina are four rounded elevations that form the posterior surface of the midbrain. The superior two elevations are the superior colliculi and the inferior elevations are the inferior colliculi. The superior colliculi contain nuclei that mostly function in visual reflexes, especially involving movement of the head, neck, and eyes. The inferior colliculi contain nuclei that function in auditory reflexes, especially involving the movement of the head, neck, and extremities. Brain Stem Pons The pons is the rounded bulge of the brain stem located between the midbrain (superior) and the medulla oblongata (inferior). The pons functions to (1) connect the cerebellum superiorly with the midbrain and cerebrum and inferiorly with the medulla oblongata and spinal cord, (2) contain nuclei for respiration and (3) contain nuclei of four cranial nerves (trigeminal- V, abducens- VI, facial- VII, and vestibulocochlear nerves- VIII). Brain Stem Medulla oblongata The medulla oblongata is located between the pons (superior) and the spinal cord (inferior). Its ventral surface is composed of two ridges of motor fibers called the pyramids. The medulla oblongata functions include: (1) connects the spinal cord with the brain (medulla is connection into the brain stem), (2) houses autonomic centers (nuclei) that function in the regulation of respiration and circulation (cardiovascular center - control of heart rate and force of contraction and blood vessel diameter (tone), (3) contains nuclei of five cranial nerves (vestibulocochlear-viii, glossopharyngeal- IX, vagus- X, accessory- XI, and hypoglossal- XII nerves), (4) relay centers for somatic sensory information to the thalamus and cerebellum, and (5) functions as the site where motor tracts (pyramidal tracts) from the motor cortex cross to the opposite side, the decussation of the pyramids. Brain Stem Cerebral aqueduct The cerebral aqueduct is narrow channel within the midbrain that connects the third and the fourth ventricle. The cerebral aqueduct drains cerebrospinal fluid from the third ventricle to the fourth ventricle. Fourth ventricle The fourth ventricle is located posterior to the pons. Superiorly, the fourth ventricle is continuous with the cerebral aqueduct, and inferiorly it is continuous with the central canal of the spinal cord. The fourth ventricle houses a choroid plexus, a structure that produces cerebrospinal fluid (CSF). In addition to functioning as a site for the production of cerebrospinal fluid by its choroid plexus, the fourth ventricle receives cerebrospinal fluid from the third ventricle by way of the cerebral aqueduct. Cerebrospinal fluid moves out of the fourth ventricle into the subarachnoid space, a space formed under the arachnoid meninx, a membrane covering that surrounds the brain and the spinal cord. Cerebellum Cerebellum The cerebellum is located posterior to the pons and medulla and inferior to the occipital lobes of the cerebral hemispheres. The cerebellum is located posterior to the pons and medulla and inferior to the occipital lobes of the cerebral hemispheres. The cerebellum functions include the coordination of complex muscle movements and the maintenance of posture and balance. 23

24 Cerebellum Figure The cerebellum functions include the coordination of complex muscle movement and the maintenance of posture and balance. Cerebellar hemispheres The cerebellar hemispheres are located one on each side of the cerebellum s central vermis. Vermis The cerebellar vermis is the central region of the cerebellum that functions to connect the two cerebellar hemispheres. Folia Folia are the horizontally oriented rounded ridges of cerebellar hemispheres. The folia are separated by narrow grooves called sulci. Arbor vitae The arbor vitae are the branching areas of cerebellar white matter. VENTRICLES The ventricles of the brain are small interconnected internal cavities that contain the choroid plexuses and cerebrospinal fluid (CSF). Figure The ventricles of the brain. Ventricles Lateral ventricles A large lateral ventricle is found within each cerebral hemisphere. The two ventricles are medially separated by a thin partition called the septum pellucidum. The lateral ventricles contain a structure that functions in the production and regulation of cerebrospinal fluid (CSF), the choroid plexus. The lateral ventricles (first and second) communicate with the third ventricle each by an interventricular foramen, which allows CSF to flow into the third ventricle. Third ventricle The third ventricle is a narrow chamber located within the diencephalon. It receives CSF for its own choroid plexus and from the lateral ventricles. From the third ventricle CSF flows into the cerebral aqueduct to the fourth ventricle. Ventricles Fourth ventricle The fourth ventricle is located between the pons and the cerebellum. It connects to the cerebral aqueduct superiorly and is continuous with the central canal of the spinal cord. The fourth ventricle receives CSF from its own choroid plexus and from the cerebral aqueduct. From the fourth ventricle CSF flows through foramina into the subarachnoid space, where the CSF then circulates around the brain and spinal cord. 24