Organization of Nervous System

Transcription

1 Control Communication Sensory Integration Motor Nervous System Functions Organization of Nervous System CENTRAL NERVOUS SYSTEM CNS Brain and Spinal Cord PERIPHERAL NERVOUS SYSTEM PNS Cranial Nerves and Spinal Nerves (Nerve tissue outside of CNS) 10/26/2004 S. Davenport 1 10/26/2004 S. Davenport 2 PNS and Sensory Division CENTRAL NERVOUS SYSTEM CNS Integration and Control PERIPHERAL NERVOUS SYSTEM PNS SENSORY (AFFERENT) DIVISION Somatic and Visceral Sensory Division (afferent PNS) Special senses (such as for sight, smell, and hearing) Visceral receptors (such as baroreceptors, chemoreceptors, mechanoreceptors) Somatic receptors (such as skeletal muscle stretch receptors) Brain Spinal cord Cranial nerves CNS PNS Spinal nerves SOMATIC RECEPTORS Special senses, Proprioceptors, etc. VISCERAL RECEPTORS Chemoreceptors, Baroreceptors, Mechanoreceptors Somatic and visceral (have some receptors in common such as pressure receptors) 10/26/2004 S. Davenport 3 10/26/2004 S. Davenport 4 1

2 SOMATIC NERVOUS SYSTEM To skeletal muscles Voluntary PNS and Motor Division CENTRAL NERVOUS SYSTEM CNS Integration and Control PERIPHERAL NERVOUS SYSTEM PNS MOTOR (efferent) DIVISION Somatic and Autonomic AUTONOMIC NERVOUS SYSTEM Cardiac muscle, smooth muscle, and glands (some) Involuntary Motor Division (efferent PNS) Parasympathetic (autonomic, visceral) Sympathetic (autonomic, visceral) Somatic (skeletal muscle, voluntary) Brain Spinal cord Cranial nerves CNS PNS Spinal nerves Sympathetic Fight or flight response Parasympathetic Rest and repose response Sympathetic (sweat glands, involuntary) 10/26/2004 S. Davenport 5 10/26/2004 S. Davenport 6 Nerve Tissue Nervous tissue forms the nervous system consists mostly of the (1) brain, (2) spinal cord, and (3) nerves. Two fundamental types of cells form the basis of nervous tissue: (1) neurons (nerve cells) (2) neuroglia (supporting cells) Neurons Structure Functions 10/26/2004 S. Davenport 7 10/26/2004 S. Davenport 8 2

3 Neurons There are many types of neurons Most neurons have cell body (soma) cell processes (axon and dendrites) Functions include Generation and transmission of electrical events resulting in the inhibition or excitation of postsynaptic neuron (or effector) Neuroglial cells Mostly function in supporting and insulating the nervous tissue. There are several different types of neuroglial cells. Typical Neurons Must live (amitotic) and function for the life of the individual. Function is the generation and conduction of an electrical event, the nerve impulse. Result is the excitation or inhibition of the associated (synapsed) neuron, muscle, or gland. 10/26/2004 S. Davenport 9 10/26/2004 S. Davenport 10 Histology of Nerve Tissue Label: Axon Collaterals Dendrites Neuroglia Nucleus Soma (body) Structure of the Neuron Soma (body) The portion of the cell with houses abundant cytoplasmic organelles and is associated with the cell processes Cell bodies are either located within the gray matter of the CNS (most) or in structures called ganglia of the PNS Nucleus Control center of the cell 10/26/2004 S. Davenport 11 10/26/2004 S. Davenport 12 3

4 Structure of the Neuron Dendrites The cell processes which function as the receptive portion of the neuron. May be modified and described as receptors such as corpuscles, spindles, etc. Conduct toward the cell body Generate electrical information as graded potentials (not nerve impulses, or action potentials) Structure of the Neuron Axon Originate from cell body at region called hillock Typically axons are called nerve fibers (are thin and may be several feet in length) May branch to form collaterals Usually ends in terminal branches each with knoblike ending called synaptic knobs, axonal terminals, or boutons. 10/26/2004 S. Davenport 13 10/26/2004 S. Davenport 14 Structure of the Neuron Axon Can be functionally divided into three regions (1) generator, (2) conductor, (3) and secretor. Generation occurs at its most proximal region called the axon hillock Conduction proceeds along the length of the fiber Secretion (the release of neurotransmitter) occurs upon the arrival of the nerve impulse at the axon terminals Axons are organized into regions called tracts (or columns) of the CNS or into the nerves of the PNS. 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. 10/26/2004 S. Davenport 15 10/26/2004 S. Davenport 16 4

5 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 Structural Classification of Neurons Unipolar Bipolar Multipolar 10/26/2004 S. Davenport 17 10/26/2004 S. Davenport 18 Structural Classification Unipolar neuron Has a single process associated with the cell body. Functions as sensory (afferent) neuron Bipolar neuron Has two processes associated with the cell body Functions as sensory neuron (locations include retina and olfactory mucosa) Multipolar neuron Has more than two processes (usually many) associated with the cell body. Functions as motor (efferent) neuron Classification of Neurons Neurons Structural Classification Functional Classification (based upon number of processes (based upon direction of conduction associated with cell body) in reference to CNS) Unipolar Bipolar Multipolar Sensory (afferent) Motor (efferent) Association 10/26/2004 S. Davenport 19 10/26/2004 S. Davenport 20 5

6 Functional Classification of Neurons Sensory neurons Association neurons Motor neurons Sensory Neurons 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. Consist of somatic and visceral neurons and are mostly unipolar Golgi tendon organs (stretch receptors) Sensory neuron Spinal cord 10/26/2004 S. Davenport 21 10/26/2004 S. Davenport 22 Types of Receptors Exteroceptors External environment in form of touch, temperature, pressure, sight, smell, and hearing. Proprioceptors Monitor position and movement of skeletal muscles and joints Interoceptors Monitor systems such as urinary, digestive, respiratory, cardiovascular provide pressure, pain, and input into autonomic pathways. 10/26/2004 S. Davenport 23 Motor Neurons Motor, or efferent, neurons transmit impulses from the central nervous system to effectors (glands and muscles). They constitute the motor (efferent) division of the peripheral nervous system. Consist of somatic and visceral neurons and are mostly multipolar Neuromuscular junctions Motor neuron Spinal cord 10/26/2004 S. Davenport 24 6

7 Interneurons (association) Transmit impulses from one neuron to another. They are located in the central nervous system. Association neuron Spinal cord Neuroglia of the CNS Astrocytes Microglia Ependymal Oligodendrocytes 10/26/2004 S. Davenport 25 10/26/2004 S. Davenport 26 Astrocytes (as-tro-cytes) Function in exchange Star-shaped cells which associates neurons and their surrounding capillaries. Microglia (mi-krog-le-ah) Phagocytosis neural debris and microorganisms\ Ependymal (ep-en-di-mal) Form the lining of ventricles of the brain and central canal of spinal cord. Oligodendrocytes (ol-i-go-den-dro-site) Form insulating covering called myelin sheath around the axons of the CNS 10/26/2004 S. Davenport 27 Neuroglia of the PNS Satellite cells Schwann cells 10/26/2004 S. Davenport 28 7

8 Schwann and Satellite Cells Schwann cells (neurolemmocytes) are associated with axons of PNS. May produce myelinated or unmyelinated fibers Satellite cells surround neuron cell bodies which are located within ganglia 10/26/2004 S. Davenport 29 Myelination of Axon All axons of the PNS are associated with Schwann cells All axons of the CNS are associated with oligodendrocytes Myelin (my-e-lin) is a lipoid substance located in the membranes of Schwann cells (neurilemmocytes) and oligodendrocytes Axons are either MYELINATED OR UNMYELINATED depending upon whether the axon is wrapped or loosely associated with its associated cells 10/26/2004 S. Davenport 30 Schwann Cells Myelinated fibers in PNS Myelinated fiber The sequential wrapping of each Schwann cell membrane around the axon produces a myelinated fiber. The space between adjacent Schwann cells is called the node of Ranvier. Neurilemma is nucleus and cytoplasm outside of myelin sheath Schwann Cells Unmyelinated fibers of PNS The loose association of each Schwann cell membrane around the axon produces an unmyelinated fiber. 10/26/2004 S. Davenport 31 10/26/2004 S. Davenport 32 8

9 Myelinated Fiber Histology of PNS Axons Identify: Axons Myelin sheath Node of Ranvier Neurilemma Schwann cell 10/26/2004 S. Davenport 33 10/26/2004 S. Davenport 34 CNS Axons Oligodendrocytes (ol-i-go-den-dro-site) Form insulating covering called myelin sheath around CNS axons Single oligodendrocyte associates with many axons Lack neurilemma Neurophysiology: Ions and Electrical Signals 10/26/2004 S. Davenport 35 10/26/2004 S. Davenport 36 9

10 Membrane Potentials Resting membrane potential Potential maintained by neuron. Graded potential Temporary, localized change in resting membrane potential. Diminishes with distance Action potential Electrical impulse that travels along an axon. Propagated and does not diminish with distance. Synaptic activity Typically involves release of neurotransmitter from axon. Binding to postsynaptic membrane causes graded potential. Postsynaptic cell response Usually the result of an overall change due to the influence of many graded potentials (many synapses). 10/26/2004 S. Davenport 37 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 10/26/2004 S. Davenport 38 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 Resistance Extracellular 10/26/2004 S. Davenport 39 Intracellular 10/26/2004 S. Davenport 40 10

11 Electrical Terminology and the Cell Membrane How might the following terms apply to the illustrated cell membrane? Potential energy Voltage Current Resistance Extracellular Electrical Terminology and the Cell Membrane How might the following terms apply to the illustrated cell membrane? Potential energy Voltage Current Resistance Extracellular 10/26/2004 S. Davenport Intracellular 41 10/26/2004 S. Davenport Intracellular 42 Transmembrane 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 10/26/2004 S. Davenport 43 10/26/2004 S. Davenport 44 11

12 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) 10/26/2004 S. Davenport 45 Mechanical Channels Sodium channels (typical) open when subjected to mechanical stimulus 10/26/2004 S. Davenport 46 Channels Identify regions which are Mechanically gated Electrically gated Chemically gated Graded Potentials 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 10/26/2004 S. Davenport 47 10/26/2004 S. Davenport 48 12

13 IPSP and EPSP EPSP Excitatory postsynaptic potential results when interior becomes more positive IPSP Inhibitory post-synaptic potential results when interior becomes more negative 10/26/2004 S. Davenport 49 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 mechanicallygated channels). The mechanically gated Na+ channels become inactive Action potential Not local; travels great distance Involves electrically-gated channels Propagated along fiber (axon) All-or-None Principle applies 10/26/2004 S. Davenport 50 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 10/26/2004 S. Davenport 51 Resting Membrane Potential 10/26/2004 S. Davenport 52 13

14 Depolarization as Na+ Moves Inward Receptor s Na+ channels become inactive Local current opened adjacent electrically-gated Na+ channels (threshold) These channels produce local current Adjacent Na+ channels open 10/26/2004 S. Davenport 53 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 section 10/26/2004 S. Davenport 54 Na+ / K+ Pump The Na+ / K+ re-establishes the extracellular and intracellular ionic gradients Pump requires ATP Na+ is pumped outward K+ is pumped inward Propagation of Action Potential Propagation Refers to the relay of the electrical event, the action potential, along the axon Continuous Propagation Involves adjacent membrane proteins, typical of unmyelinated axons Saltatory Propagation Involves propagation from node to node, typical of myelinated axons 10/26/2004 S. Davenport 55 10/26/2004 S. Davenport 56 14

15 Conduction Velocity Myelinated fibers conduct faster than unmyelinated fibers Continuous conduction Saltatory conduction Large fibers conduct faster than small fibers Larger fibers offer less resistance What is the approximate range of conduction velocities? What is multiple sclerosis (MS)? Synaptic Activity Impulse passes from presynaptic membrane (typically the neuron s axon) to postsynaptic membrane. Typical postsynaptic membranes are located on Neurons (neuronal synapse) Muscle cells (neuromuscular synapse) Glands (neuroglandular synapse) 10/26/2004 S. Davenport 57 10/26/2004 S. Davenport 58 Chemical Synapses Involve a neurotransmitter Excitatory neurotransmitters always produce an EPSP (excitatory postsynaptic potential) Inhibitory neurotransmitters always produce an IPSP (inhibitory postsynaptic potential) The production of an action potential depends upon reaching threshold. Thus, it is the property of the receptor not the neurotransmitter. The same neurotransmitter may be inhibitory at one location and excitatory at another location 10/26/2004 S. Davenport 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) 10/26/2004 S. Davenport 60 15

16 Synapse Synapse A B A B The result is A or B? The result is A or B? Which is produced? Which is produced? A) action potential, B) IPSP, C) EPSP? A) action potential, B) IPSP, C) EPSP? 10/26/2004 S. Davenport 62 10/26/2004 S. Davenport 61 Cholinergic Synapses Release acetylcholine At all neuromuscular juntions At many CNS synapses At all neuron-to-neuron synapses in PNS At all effector sites (muscles and glands) of parasympathetic nervous system (ANS). How Neurotransmitters Work Compounds that have a direct effect on membrane potential Compound that have an indirect effect on membrane potential or cell activity Lipid-soluble gases that effect the inside of the cell 10/26/2004 S. Davenport 63 10/26/2004 S. Davenport 64 16

17 How Neurotransmitters Work Direct acting (effect) 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 10/26/2004 S. Davenport 65 10/26/2004 S. Davenport 66 Postsynaptic Potentials IPSPs (review) EPSPs (review) Summation The adding together of synaptic potentials (SPs). Could be EPSPs, IPSPs, or both EPSPs and IPSPs. Temporal summation Spatial summation Faciliation Summation Temporal summation Pertaining to time; the quick succession of SPs at a few synapses are summated 10/26/2004 S. Davenport 67 10/26/2004 S. Davenport 68 17

18 Summation Spatial summation Pertaining to space; many SPs occur over the postsynaptic membrane and are summated Nerves Cordlike organ which conducts impulses from a part of the central nervous system and another region of the body. Components of the peripheral nervous system (PNS) Include the spinal nerves, the cranial nerves, and all of their branches. 10/26/2004 S. Davenport 69 10/26/2004 S. Davenport 70 Nerve Structure Consisting of parallel axons (fibers) and their associated Schwann cells (neurilemmocytes) enclosed in successive connective tissue wrappings (endoneurium, perineurium, epineurium). Nerves May contain only myelinated fibers, only unmyelinated fibers, or a combination of both myelinated and unmyelinated fibers Classified as Sensory (afferent) Motor (efferent) Mixed (combination of afferent and efferent) 10/26/2004 S. Davenport 71 10/26/2004 S. Davenport 72 18

19 Nerve Define and Identify: Nerve Epineurium Perineurium Fasicle What is the difference between a nerve and a tract? Nerve Define and Identify Perineurium Endoneurium Schwann cell Axon Myelin sheath Neurilemma 10/26/2004 S. Davenport 73 10/26/2004 S. Davenport 74 Sensory Nerves (fibers) Typically involve unipolar neurons Impulse GENERATION begins at a dendrite (receptor) Flow of information is into the CNS Typically to an association neuron. Reflex Includes Sensory (afferent) neurons Association neurons Motor (efferent) neurons Response is predictable Golgi tendon organs (stretch receptors) Sensory neuron Spinal cord 10/26/2004 S. Davenport 75 10/26/2004 S. Davenport 76 19

20 Autonomic Regulation Motor Division (efferent PNS) Parasympathetic (autonomic, visceral) Sympathetic (autonomic, visceral) Somatic (skeletal muscle, voluntary) Brain Spinal cord Cranial nerves CNS PNS Spinal nerves Sympathetic (sweat glands, involuntary) 10/26/2004 S. Davenport 77 10/26/2004 S. Davenport 78 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. 10/26/2004 S. Davenport 79 20