Major Structures of the Nervous System

Transcription

1 Major Structures of the Nervous System Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptors Tortora & Grabowski 9/e ã2000 JWS 12-1

2 Nervous System Divisions Central Nervous System (CNS) consists of the brain and spinal cord Peripheral Nervous System (PNS) consists of cranial and spinal nerves that contain both sensory and motor fibers connects CNS to muscles, glands & all sensory receptors Tortora & Grabowski 9/e ã2000 JWS 12-2

3 Nerve a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. Cranial Nerve nerves that emerge from the brain & follows a defined path Spinal Nerves nerves that emerge from the spinal cord & follows a defined path Ganglia small masses of nervous tissue, consisting primarily of neuron cell bodies and are located outside of the brain and spinal cord. They are associated with the brain and spinal nerves. 12-3

4 Enteric plexuses networks of neurons located in the walls of organs of the gastrointestinal tract that help regulate the digestive system Sensory receptor a structure of the nervous system that monitors changes in the external or internal environment.

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6 Subdivisions of the PNS Somatic (voluntary) nervous system (SNS) neurons from cutaneous and special sensory receptors to the CNS motor neurons to skeletal muscle tissue Autonomic (involuntary) nervous systems sensory neurons from visceral organs to CNS motor neurons to smooth & cardiac muscle and glands Sympathetic division (flight or fight) /Explain effects on Organs Parasympathetic division (return body to normal) Tortora & Grabowski 9/e ã2000 JWS 12-6

7 CENTRAL NERVOUS SYSTEM brain spinal cord sensory nerves axons of motor nerves somatic subdivision (motor functions) These nerves carry signals to and from skeletal muscles, tendons, and skin. Sensory autonomic subdivision (visceral functions) These nerves carry signals to and from internal organs (gut, heart, glands, etc.). parasympathetic nerves sympathetic nerves Motor Fig. 35.6, p. 591 PERIPHERAL NERVOUS SYSTEM Tortora & Grabowski 9/e ã2000 JWS 12-7

8 Controls and integrates all body activities within limits that maintain life Three basic functions 1. Sensing changes with sensory receptors - internally & externally 2. Integration (interpreting, remembering) of those changes in the int. &ext. environment 3. Motor- reacting to those changes with effectors ; by muscular contractions(smooth, cardiac & skeletal) & glandular secretions Tortora & Grabowski 9/e ã2000 JWS 12-8

9 Neuroglial Cells Do not produce action potentials(ap) Half of the volume of the CNS Smaller cells than neurons 25 times more numerous than neurons Cells can divide rapid mitosis in tumor formation (gliomas) 4 Neuroglial Cell types in CNS astrocytes, oligodendrocytes, microglia & ependymal 2 Neuroglial Cell types in PNS Schwann and satellite cells Tortora & Grabowski 9/e ã2000 JWS 12-9

10 Neurons Functional unit of nervous system Most do not divide /limited number Have capacity to produce action potentials Cell body single nucleus with prominent nucleolus Nissl bodies (will stain dark) rough ER & free ribosomes for protein synthesis Nerve fiber general term for any neuronal process that emerges from the cell body of a neuron. Dendrites- the receiving on input portions of a neuron.

11 Parts of a Neuron Neuroglial cells Nucleus with Nucleolus Axons or Dendrites Cell body Tortora & Grabowski 9/e ã2000 JWS 12-11

12 Axon Conduct impulses away from cell body Long, thin cylindrical process of cell Arises at axon hillock Impulses arise from trigger zone Side branches (collaterals) end in fine processes called axon terminals Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters

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14 Neurotransmitters a molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber or gland cell.

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16 Classifying Neurons Structural classification is based on the number of processes (axons or dendrites) extending from the cell body.

17 Multipolar neurons have several dendrites and only one axon and are located throughout the brain and spinal cord. The vast majority of the neurons in the human body are multipolar. Classifying Neurons

18 Classifying Neurons Bipolar neurons have one main dendrite and one axon. They are used to convey the special senses of sight, smell, hearing and balance. Found in the retina of the eye, theinner ear, and the olfactory (olfact = to smell) area of the brain.

19 Classifying Neurons Unipolar neurons contain one process which extends from the body and divides into a central branch that functions as an axon and as a dendritic root. Unipolar structure is often employed for sensory neurons that convey touch, pressure and pain.

20 Functional Classification of Neurons Sensory / Afferent Neurons transport sensory information from skin, muscles, joints, sense organs & viscera to CNS Motor / Efferent Neurons send motor nerve impulses to muscles & glands & other neurons Interneurons / Association Neurons connect sensory to motor neurons 90% of neurons in the body Tortora & Grabowski 9/e ã2000 JWS 12-20

21 Neurons and Neuroglia As the thinking cells of the brain, each neuron does, in miniature, what the entire nervous system does as an organ: Receive, process and transmit information by manipulating the flow of charge across their membranes. Neuroglia (glial cells) play a major role in support and nutrition of the brain, but they do not manipulate information. They maintain the internal environment so that neurons can do their jobs. Tortora & Grabowski 9/e ã2000 JWS 12-21

22 Astrocytes In CNS Star-shaped cells Help form blood-brain barrier by covering blood capillaries Metabolize neurotransmitters Regulate K+ balance Provide structural support

23 Oligodendrocytes In CNS Most common Glial cell type Each cell forms myelin sheath around more than one axon in CNS 12-23

24 Microglia Cells In CNS Small cells found near blood vessels Phagocytic role -- clear away dead cells

25 Ependymal cells In CNS Form epithelial membrane lining cerebral cavities & central canal Produce & circulate cerebrospinal fluid (CSF)

26 Schwann Cells In PNS The Schwann cells encircle PNS axons Each cell produces part of the myelin sheath surrounding an axon in the PNS Tortora & Grabowski 9/e ã2000 JWS 12-26

27 Satellite Cells In PNS Flat cells surrounding neuronal cell bodies in peripheral ganglia Support neurons in the PNS ganglia Tortora & Grabowski 9/e ã2000 JWS 12-27

28 Neuroglia

29 Neuroglia Myelination is the process of forming a myelin sheath which insulates and increases nerve impulse speed. Myelin is formed by Oligodendrocytes in the CNS and by Schwann cells in the PNS.

30 Axon Coverings in PNS All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells Neurolemma is cytoplasm & nucleus of Schwann cell / Myelin Sheath is plasma membrane of Schwann Cell gaps called nodes of Ranvier Myelinated fibers appear white jelly-roll like wrappings made of lipoprotein = myelin acts as ion insulator speeds conduction of nerve impulses Unmyelinated fibers slow, small diameter fibers only surrounded by neurolemma but no myelin sheath wrapping Tortora & Grabowski 9/e ã2000 JWS 12-30

31 Myelination in PNS In PNS Schwann cells myelinate (wrap around) individual axons in the PNS during fetal development Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath Tube guides growing axons that are repairing themselves Tortora & Grabowski 9/e ã2000 JWS 12-31

32 Myelination in the CNS Oligodendrocytes myelinate axons in the CNS Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons No neurolemma is formed Little regrowth after injury is possible due to the lack of a distinct tube or neurolemma Tortora & Grabowski 9/e ã2000 JWS 12-32

33 The group of cells bodies in the peripheral nervous system are known as ganglia. The axons leaving these cells are called nerves. Most ganglial cells are sensory neurons which gather nerve information from sensory systems and motor neurons and transfer processed information to muscles, glands and internal organs. Tortora & Grabowski 9/e ã2000 JWS 12-33

34 Nuclei are the clusters of neuron cell bodies found in the central nervous system. The axons leaving these cells are called tracts. Nuclei make the grey matter while tracts make the white matter in the central nervous system.

35 Gray and White Matter White matter = myelinated processes (white in color) Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color) In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS Tortora & Grabowski 9/e ã2000 JWS 12-35

36 Resting Membrane Potential More negative ions along inside of cell membrane & more positive ions along outside of membrane at REST potential energy difference at rest is -70 mv cell is POLARIZED Resting Membrane Potential exists because; 1. Concentration of ions different inside & outside extracellular fluid rich in Na+ and Cl- cytosol full of K+, organic phosphate & amino acids 2. Membrane permeability differs for Na+ and K greater permeability for K+ inward flow of Na+ can t keep up with outward flow of K+ 3. Na+/K+ pump removes Na+ as fast as it leaks in 3 Na out of the cell & 2 K into the cell Tortora & Grabowski 9/e ã2000 JWS 12-36

37 Continuous versus Saltatory Conduction Continuous conduction (unmyelinated fibers) step-by-step depolarization of each portion of the length of the neuron Saltatory conduction - over myelinated axons in PNS depolarization only at Nodes of Ranvier where there is a high density of voltage-gated ion channels current carried by ions flows through extracellular fluid from node to node / much faster Tortora & Grabowski 9/e ã2000 JWS 12-37

38 Saltatory Conduction Nerve impulse conduction in which the impulse jumps from node to node Tortora & Grabowski 9/e ã2000 JWS 12-38

39 Chemical Synapses Action potential reaches end bulb and voltage-gated Ca+ 2 channels open Ca+2 flows inward triggering release of neurotransmitter Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors the more neurotransmitter released the greater the change in potential of the postsynaptic cell One-way information transfer Tortora & Grabowski 9/e ã2000 JWS 12-39

40 Removal of Neurotransmitter Diffusion move down concentration gradient into the blood stream Enzymatic degradation acetylcholinesterase brakes it down Uptake by neurons or glia cells neurotransmitter transporters Tortora & Grabowski 9/e ã2000 JWS 12-40