Chapter 11: Nervous System and Nervous Tissue I. Functions and divisions of the nervous system A. Sensory input: monitor changes in internal and external environment B. Integrations: make decisions about sensory input C. Motor output: pathways that activate glands and muscles D. Levels of organization 1. Central nervous system (CNS): brain and spinal cord 2. Peripheral nervous system (PNS): all nerves 3. PNS is divided into 2 main subdivisions: a. Sensory (afferent) division
1) Somatic: from skin, muscles and joints 2) Visceral: from visceral organs b. Motor (efferent) 1) Somatic NS: to skeletal muscles 2) Autonomic NS: controls cardiac, smooth, glands a) Sympathetic b) Parasympathetic II. Histology of nervous tissue A. Supporting cells 1. Neuroglia of the CNS: outnumber neurons by 10-50 times Divide (unlike neurons which do not divide) a. Astrocytes: star-shaped, half of neural tissue volume b. Microglia c. Ependymal: line the cavities of brain and spinal cord
d. Oligodendrocytes 2. Neuroglia of the PNS a. Satellite cells: b. Schwann cells: B. Neurons: commonly called nerve cells 1. Three unique characteristics a. Function for a lifetime b. Do not divide: amitotic c. Exceptionally high metabolic rate: cannot survive more than a few minutes without O2 2. Neuron cell body: perikaryon the biosynthetic center of neuron a. Rough ER: chromatophilic substance or Nissl bodies b. Neurofibrils 3. Neuron processes: extensions off the cell body a. Dendrites: receive impulses from another location to the cell body b. Axon: conduct impulse from the cell body to another location
1) Axon hillock: the part of the neuron where the axon attaches to the cell body 2) Axon collateral: a branch off the main axon 3) Terminal branches: found at end of an axon or collateral a) Axon terminals: where synaptic vesicles are stored at the end of the terminal branches 4) Functional characteristics a) Axonal transport: the main intracellular transport mechanism 4. Myelin sheath: protects, insulates and increases the speed of impulses a. PNS myelin: Schwann cell (neurolemmocyte) wraps itself tightly around an axon 1) PNS myelin defined: the tight wrappings of a Schwann cell around an axon 2) Neurilemma: the outermost remainder of the Schwann cell b. CNS Myelin: Oligodendrocyte performs a similar function
c. Node of Ranvier: short sections of naked axon between myelin wrappings - CNS and PNS d. Unmyelinated neurons: CNS and PNS, but Schwann and oligodendrocytes are associated differently with axons 5. Nerve: bundle of axons in the PNS 6. Tract: bundle of axons in the CNS 7. Gray matter: CNS neuron cell bodies 8. White matter: the axons from the cell bodies in the gray matter C. Classification of neurons: structural and functional 1. Structural: classification by the number of processes that extend from cell body a. Multipolar: many branching dendrites and a single axon b. Bipolar: a single axon and a single dendrite c. Unipolar: a single short process that extends from cell body
2. Functional classification: according to the direction of the impulse a. Sensory (afferent): the impulse direction is toward the CNS 1) Cell bodies: located in clumps called ganglia in the PNS 2) All sensory neurons are unipolar 3) They typically possess a long dendrite b. Motor (efferent): the impulse direction is away from the CNS 1) Motor cell bodies: are all in the CNS 2) All motor neurons are multipolar
c. Interneurons (association) connect sensory neurons to motor neurons 1) Cell bodies: most are in the CNS 2) Most are multipolar 3) 99% of all neurons are interneurons 4) Examples: pyramidal and purkinje cells 5) Simple to complex circuits III. Membrane potentials: neurophysiology or the function of a neuron A. Role of membrane ion channels 1. Gated channels: protein pores that act as ion gates a. Chemically gated channels: open when the appropriate neurotransmitter binds to it
a. Voltage gated channels: open in response to a voltage change b. Mechanically gated channels responds to mechanical deformation of a sensory receptor B. Resting membrane potential 1. Rules for maintaining resting potential a. The neuron membrane: is impermeable to (-) charged proteins b. K + constantly leaks out: leaving a net negative charge inside c. Na + mostly stays outside: only slightly permeable so positive charge due to Na + mostly remains outside d. K + and Na + level at rest: are maintained by the Na + /K + pumps 1) How the pumps work: 3 Na + are pumped out while 2 K + are pumped in, consuming 1 ATP
2) Diagram of a resting neuron 2. Membrane potentials that act as signals a. Terminology 1) Depolarization: the inside of the neuron becomes less negative 2) Repolarization: the cell returns to its resting state after depolarization 3) Hyperpolarization: the inside of the neuron becomes more negative than the normal resting state 4) Graded potentials: small depolarizations or hyperpolarizations 5) Action potential: a large depolarization that travels all the way down the length of the axon 6) Threshold: the intracellular "set point" at the axon hillock at which an action potential will be triggered a) Typically this is -50 mv 7) All-or-none: the event happens or it does not, there is no in between b. Graded potentials: 2 types 1) Receptor potential: if an external energy source is the stimulus a) Generator potential: if the stimulus reaches threshold 2) Postsynaptic potential: if another neuron is the stimulus
c. Action potentials 1) Generation of an action potential: involves opening ion gates in the neuron membrane a) Resting state: all gated channels closed b) Depolarization (1) Graded potentials: reach axon hillock (2) Na + gates open: Na+ rushes in: threshold is -50 mv (3) If threshold is reached (a) Action potential occurs: positive feedback self propagation (4) Na + gates close when +30 mv is reached c) Repolarization (1) As Na + gates close, K + gates open (2) K + rushes out repolarizing the cell (3) Cell achieves -70 mv again (4) Na + /K + pump reestablishes normal ion positions d) Hyperpolarization 2) Diagram of an action potential 3) Propagation of an action potential a) Self-propagating: (positive feedback)
b) Diagram of positive feedback AP propagation 4) Absolute and relative refractory periods a) Absolute refractory period: a short period of time that neuron is incapable of responding to another stimulus no matter how strong b) Relative refractory period: only a stronger than normal stimulus will fire neuron again during this time interval 5) Conduction velocity of axons a) Axon diameter b) Degree of myelination (1) Terminology (a) Saltatory conduction Impulse jumps from node to node (i) Nodes of Ranvier (b) Continuous conduction: unmyelinated axons (2) Comparison
(3) Multiple sclerosis: CNS demyelinating disease C. The synapse: the junction of one neuron to another neuron (or effector) 1. Terminology a. Presynaptic cell: before the synapse b. Postsynaptic cell: just after synapse; can be another neuron, muscle, or gland 2. Electrical synapses: similar to gap junctions in other tissues 3. Chemical synapses: neurotransmitter opens or closes ion gates(channels) a. Events of impulse transmission at synapse 1) Action potential reaches axon terminal a) Opens Ca ++ gates b) Vesicles fuse with the membrane c) NT is released into synaptic cleft 2) NT: diffuses across cleft and binds to Na + gates on postsynaptic neuron 3) Postsynaptic Na + gates open: and cell begins to depolarize
4. Postsynaptic potentials (PSPs) a. EPSP (excitatory): neurotransmitter binding causes depolarization b. IPSP (inhibitory) neurotransmitter binding causes hyperpolarization by making membrane permeable to K+ 5. Synaptic integration: (a single EPSP cannot trigger an AP) a. Summation by a postsynaptic neuron 1) Temporal summation: one neuron rapid-fires 2) Spatial summation: many neurons firing at same time 3) Axon hillock: keeps track of potentials reaching the axon 4) Diagram of summation
a) Synaptic potentiation: Repeated use enhances the neurons ability to fire b. Presynaptic inhibition: an inhibitory neuron synapses with the axon terminal of a presynaptic neuron D. Neurotransmitters: over 100 known types 1. Classification by chemical structure a. ACh b. Biogenic amines c. Amino acids d. Peptides 2. Classification by function
a. Excitatory and inhibitory b. Ionotropic (direct acting) c. Metabotropic (indirect acting) IV. Neural integration A. Neuronal pools: functional groups of neurons B. Types of circuits: 4 basic types 1. Diverging: amplifying circuits - 1-to-many relationship 2. Converging: concentrating circuits - many-to-1 relationship 3. Reverbrating circuits: circular pathways
4. Parallel after discharge: 1-to-several-to-1 relationship