Human Anatomy Ch. 7 I. The Nervous System A. General characteristics 1. body s control & communication center a. 3 overlapping functions 1) sensory input: receptors monitor stimuli 2) integration: processes, interprets & determines response 3) motor output: causes a response in the effectors a) muscles & glands B. Organization 1. Structural classification based on organs a. Central N.S. 1) brain & spinal cord 2) integrating & command centers b. Peripheral N.S. 1) cranial & spinal nerves 2) carry impulses from sensory receptors into CNS 3) carry impulses out to effectors from CNS
2. Functional classification based on jobs of PNS structures only a. Afferent (sensory) division {Afferent = to go toward } 1) send incoming impulses to CNS about events in & outside of body 2) Somatic sensory fibers = come from skin, skeletal muscles & joints {Soma = body} 3) Visceral sensory fibers = come from visceral organs b. Efferent (motor) division {Efferent = to bring about or cause } 1) carries impulses from CNS to effectors (muscles, glands) 2) two subdivisions a) Somatic NS = voluntary control of muscles, skeletal muscle reflexes b) Autonomic NS = regulates involuntary activity 1)) Sympathetic NS = fight or flight responses 2)) Parasympathetic NS = rest & repair responses 3)) Enteric NS = controls gastrointestinal tract, pancreas, gall bladder
II. Nervous Tissue A. Neuroglia (a.k.a. glia, glial cells){neuroglia = nerve glue } 1. non-excitable supporting cells a. support, insulate & protect neurons b. capable of mitosis 2. CNS glia a. astrocytes star-shaped 1) most abundant - about ½ of all neural tissue 2) help form blood-brain barrier between capillaries & neurons to regulate their exchanges 3) help regulate brain s chemical environment b. Microglia spiderlike 1) phagocytic consume cell debris & bacteria 2) monitor health of neurons nearby c. Ependymal cells 1) line cavities of brain & spinal cord 2) cilia beat to circulate cerebrospinal fluid d. Oligodendrocytes {Oligo = few ; dendr = tree } 1) wrap around nerve fibers 2) produce fatty insulating myelin sheaths around axons of neurons See p. 228 in textbook for similar descriptions
3. PNS glia a. Schwann cells {named for cell scientist Theodor Schwann} 1) produce insulating myelin sheaths around axons of neurons in the PNS b. satellite cells 1) protect & cushion neuron cell bodies in ganglia & regulate their chemical environment B. Neurons (a.k.a. nerve cells) 1. excitable specialized to transmit impulses 2. most are amiotic (non-dividing) lack centrioles 3. common anatomical structures a. Cell body (a.k.a. soma) 1) carries out normal cell functions & metabolism
b. Processes (a.k.a. fibers) 1) dendrites short, multiple fibers on cell body a) conduct impulses in toward cell body 2) axon single fiber leaving from axon hillock on cell body a) send impulses away from cell body b) terminal end branches into 100s 1000s of axon terminals with sacs of neurotransmitters 1) separated from next neuron by tiny synaptic clefts (a.k.a. synapses) {syn = to clasp or join } c. myelin sheaths 1) whitish, waxy, lipid-based material 2) protects & insulates fibers and speeds impulses 3) in PNS, many Schwann cells make sheath for one axon a) gaps between individual myelin deposits = nodes of Ranvier 4) in CNS, oligodendrocytes form sheaths around up to 60 different axons at once
C. Terminology 1. Nuclei = clusters of neuron cell bodies in CNS a. protected by skull and vertebrae 2. Ganglia = small collections of cell bodies in PNS 3. Tracts = bundles of nerve fibers within the CNS 4. Nerves = bundles of nerve fibers with the PNS 5. White matter = dense collections of myelinated tracts 6. Gray matter = mostly unmyelinated cell bodies & fibers D. Neuron classifications 1. Functional by direction impulse travels a. Sensory (afferent) neurons carry impulses from sensory receptors into CNS 1) cutaneous sense organs in skin sense pain, heat, cold, touch & pressure 2) proprioceptors in muscles & tendons sense tension/stretch 3) more specialized receptors for vision, smell, taste, hearing & balance b. Motor (efferent) neurons carry impulses from CNS to the viscera, muscles & glands 1) cell bodies are found in CNS c. Interneurons (association neurons) connect sensory & motor neurons within the CNS 2. Structural classification by # of processes a. multipolar neurons several processes 1) incl. all motor & association neurons b. bipolar neurons 2 processes:1 axon,1 dendrite 1) in adults, only in eye & nose receptors c. unipolar neurons - 1 process that divides into central & peripheral processes 1) sensory, found in PNS ganglia
III. Nerve Impulses A. Resting Neurons 1. at rest, neuron plasma membranes are polarized a. fewer positive ions inside than out 1) more Na+ outside, fewer K+ inside a) Na+ is prevented from entering, some K+ leaks out 2. will remain inactive in this condition B. Action potentials 1. a stimulus briefly changes a neuron s permeability a. Na+ ions are allowed in, 1) creates a graded potential, depolarizing that point b. if stimulus is strong, & enough Na+ enter, neuron sends an action potential (a.k.a. nerve impulse) c. continues to adjacent membrane areas 2. all-or-none: entire axon length sends impulse or it doesn t happen C. Repolarization 1. permeability immediately changes, allowing K+ to diffuse out rapidly & blocking Na+ entry a. restores the polarized, resting condition 1) happens in same direction as depolarization 2) until this happens, neuron can t send another impulse D. If a fiber is myelinated, impulses only travel in gaps (nodes of Ranvier) 1. called saltatory conduction {saltare = to dance, leap } 2. faster than unmyelinated transmission
E. Transmission at Synapses 1. Two forms of sending impulse to next neuron a. Electrical only in cardiac muscle & some smooth muscle 1) gap junctions form path for electrical currents to travel to neighbor b. Chemical most common 1) when impulses reach axon terminal, neurotransmitters are released into synaptic cleft a) neurotransmitters = 50+ chemical compounds that facilitate, stimulate or inhibit an impulse in a postsynaptic neuron or effector b) ex: endorphins, dopamine, serotonin 2) they travel to the postsynaptic neuron & bind to receptors in its plasma membrane 3) this starts a new impulse, and unbinds neurotransmitters a) are either returned to the axon terminal or broken down by enzymes F. Reflexes 1. rapid, predictable, involuntary responses to stimuli a. do not require coordination by brain 2. travel over reflex arcs involving both CNS & PNS 3. Somatic reflexes stimulate skeletal muscles a. ex: pulling hand away from hot stove 4. Autonomic reflexes regulate smooth muscles, heart & glands a. ex: salivation, pupillary reflex, blood pressure, digestion, etc.
5. Elements of a reflex arc a. sensory receptor reacts to a stimulus b. sensory neurons send impulses to the CNS c. interneurons integrate info in brain or spinal cord d. motor neurons take response from CNS e. effector organs carry out a response 1) muscles contract or glands secrete