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The Nervous System Master controlling and coordinating system of body: Sensory input (incoming) E.g.. seeing, hearing, etc. Integration (processing) CNS (brain & spinal cord) Motor output (outgoing) Responding to stimulus Structural Classification Central Nervous System (CNS) Integrating command center Brain Spinal cord Structural Classification (cont’d) Peripheral Nervous System (PNS) Communication lines 12 pair of cranial nerves 31 pair of spinal nerves Functional Classification Only PNS structures 2 subdivisions Sensory or afferent division Convey impulses to the CNS from sensory receptors Functional Classification (cont’d) Motor or efferent division Carries impulses from CNS to effector organs “effect” a motor response Functional Classification (cont’d) Motor divided into 2 subdivisions Somatic (voluntary) nervous system Allows us to control skeletal muscles Functional Classification (cont’d) Autonomic nervous system (ANS) (involuntary) Sympathetic Parasympathetic Nervous Tissue 2 types: Supporting cells Neurons Neuroglia “nerve glue” Supporting cells in CNS Do not conduct impulses Support, insulate, & protect the neurons Also called glia Never lose ability to divide Types of Neuroglia Astrocytes Star shaped Abundant (half of neural tissue) Anchor the nerve cells in place and connect them to the blood supply Types of Neuroglia (cont’d) Microglia Spiderlike, very small Phagocytes Dispose of dead brain cells, bacteria, etc. Types of Neuroglia (cont’d) Ependymal cells Line the nervous system cavities (in brain and inside spinal cord) By ciliary action, they move CSF Types of Neuroglia (cont’d) Oligodendrocytes (oligodendroglia) Produces a fatty insulating covering called myelin sheath Wraps around nerve fibers in CNS Supporting Cells in PNS Schwann Cells Form the myelin sheath around nerve fibers in PNS Satellite Cells Covering, protective, cushioning cells Neurons Nerve cells Transmit nerve impulses from one part of the body to another Structure of Neuron Cell body Contains nucleus Dendrites Carry impulses toward the cell body Frequently multiple Structure of Neuron (cont’d) Axons Carry impulse away from cell body Usually singular Structure of Neuron (cont’d) Axonal terminals (knobs, boutons) Releases a chemical (neurotransmitter) into a space (synaptic cleft or synapse) between neurons Structure of Neuron (cont’d) Myelin Whitish, fatty material surrounding long nerve fibers Protects and insulates the fibers Increases transmission rate of nerve impulses Structure of Neuron (cont’d) Myelin (cont’d) Axons in PNS myelinated by Schwann cells Myelin sheath Neurilemma – cytoplasm Axons in CNS myelinated by oligodendrocytes Lack neurilemma Structure of Neuron (cont’d) Nodes of Ranvier Gap (nonmyelinated) from one Schwann cell to next Structure of Neuron (cont’d) Clusters of neurons cell bodies and collections of nerve fibers named differently when in CNS or PNS: Cell bodies: CNS – nuclei PNS - ganglia Structure of Neuron (cont’d) Bundles CNS of nerve fibers: – tracts PNS - nerves White & Gray Matter Refer to CNS White matter Dense collections of myelinated fibers (tracts) Gray Matter Mostly unmyelinated fibers and cell bodies Functional Classification Which way information is traveling in relation to CNS Functional Classification (cont’d) Sensory (afferent) neurons Carry info to CNS Cell bodies located in ganglia in PNS Dendrites are associated with receptors (heat, pain, sight, smell, etc.) Functional Classification (cont’d) Motor (efferent) neurons Carry impulses from CNS to viscera, muscles, and glands Cell bodies located in CNS Functional Classification (cont’d) Association (interneurons) In-between Connect motor and sensory neurons Cell bodies in CNS Structural Classification Multipolar Several processes extend from cell body One axon and many dendrites Ex. Motor neurons, interneurons Structural Classification (cont’d) Bipolar One axon and one dendrite off the cell body Rare Located in nose, retina of eye Sensory receptor cells Structural Classification (cont’d) Unipolar Single extension from cell body that divides into 2 Axon carries info to and from cell body Ex. Sensory neurons in PNS Regeneration Not very much Ability to replace neurons not possible In PNS – a little bit of regeneration – neurilemma plays a role in fiber regeneration In CNS - none Nerve Impulses Movement of ions (Na+, K+) across a cell membrane of neuron Resting Membrane Resting, inactive neuron – plasma membrane is polarized. Na+ more concentrated outside & K+ more inside Inside more negative than outside Resting membrane potential -70mv Resting Membrane (cont’d) As long as inside remains more negative than outside - neuron will stay inactive Depolarization & Action Potential Neurons respond to changes in environment – excitability If stimulus is above minimum level, the membrane permeability changes for Na+ ions. -50mv to –55mv threshold – now action potential will happen Depolarization & Action Potential (cont’d) Opens Na+ gates on cell – Na+ ions move inward Makes inside positive and outside negative +30 mv Depolarization & Action Potential (cont’d) Depolarizes membrane down entire length of neuron Action potential are “all or none” Repolarization K+ gates open K+ moves out Inside – negative Outside – positive More K+ outside and Na+ inside Ions in wrong place Repolarization(cont’d) Na+-K+ pump puts Na+ outside and K+ inside Return to –70 mv Refractory period No nerve impulses conducted until ions are in proper place Conduction of a Nerve Impulse Action potential spreads along the length of the neuron Causes a wave of negative charges Conduction of a Nerve Impulse (cont’d) Impulses increase in myelinated fibers Saltatory conduction Jumps from node of Ranvier to node of Ranvier Much faster than on unmyelinated fibers 130m/sec vs 1m/sec Synaptic Transmission Prior to a stimulus, the axonal knob contains ACh Other neurotransmitters Serotonin Dopamine Norepinephrine GABA Glutamate Endorphin Synaptic Transmission (cont’d) Neuron receives a threshold stimulus Impulse moves down the fiber to the axonal knob Knob (vesicle) releases ACh into the synaptic cleft Synaptic Transmission (cont’d) When second neuron is stimulated, AChE released into synaptic cleft digests ACh (in 1/500 sec.) Synaptic stimulus stops Second neuron stops firing 2 molecules – Acetyl and Choline are reabsorbed into vesicle of axonal knob. Restructured into ACh for use again. Synaptic Transmission (cont’d) Neurons can exhibit synaptic fatigue – may stop firing – because of lack of energy Subthreshold Impulse Will do nothing to illicit a response unless there is summation (additive effect) of several subthreshold stimuli Reflexes Rapid, predictable, involuntary responses to stimuli Autonomic reflexes Regulate the activity of smooth muscles, the heart and glands Digestion, elimination, blood pressure, sweating Salivary reflex and pupillary reflex Reflexes (cont’d) Somatic reflexes Stimulate the skeletal muscles Reflexes (cont’d) Reflexes occur over neural pathways called reflex arcs. Reflexes (cont’d) All reflex arcs have a minimum of 5 elements: Sensory receptor (reacts to stimulant) Effector organ (the muscle or gland eventually stimulated) Afferent and efferent neurons to connect the two Synapse between afferent and efferent neurons – the CNS integration center Reflexes (cont’d) Two-neuron reflex arc Simplest type in humans No interneuron Three-neuron reflex arc Has an interneuron Most reflexes are bisynaptic The more synapses in a reflex pathway, the longer the reflex takes to happen Reflexes (cont’d) Spinal reflex – impulse does not have to go to the brain Ex. Knee-jerk reflex Cranial reflex Ex. Pupillary reflex Spinal Cord Elongated extension of CNS Two-way conduction pathway to and from the brain Major reflex center Spinal Cord (cont’d) Protected by meninges Dura mater Arachnoid Pia mater Spinal Cord (cont’d) Cross section of spinal cord Outer white matter (myelinated) Inner gray matter Contains ascending and descending tracts Association neurons Central canal - CSF Spinal Cord (cont’d) Spinal nerves exit between vertebrae Spinal Cord (cont’d) Spinal cord terminates at L1 (1st lumbar vertebra) Swellings at cervical and lumbar regions for the limbs Below L1, region still has nerves – cauda equina PNS Nerves and scattered groups of neuronal cell bodies (ganglia) outside of CNS Structure of a Nerve Nerve - bundle of neuron fibers found outside of CNS Structure of a Nerve (cont’d) Each fiber surrounded by endoneurium Perineurium –wrapping that forms fiber bundles – fascicles Fascicles bound together by epineurium to form nerve Classification of nerves Classified according to direction they transmit impulses Mixed – carry both sensory and motor fibers All spinal are mixed Afferent (sensory) – towards CNS Efferent (motor) – carry only motor fibers away Cranial Nerves 12 pairs Primarily serve head and neck Vagus extends to ventral cavity Numbered in order Named according to function Most are mixed nerves 3 (optic, olfactory, vestibulocochlear) – purely sensory Cranial Nerves (cont’d) Olfactory Nerve (N I) Optic Nerve (N II) Vision Oculomotor Nerve (N III) Smell Eye movement (diameter of pupil) Trochlear Nerve (N IV) Eye movement (looking up) Cranial Nerves (cont’d) Trigeminal Nerve (N V) 3 divisons go to face Opthalmic Maxillary Mandibular Abducens Nerve (N VI) Eye movement (lateral movement) Cranial Nerves (cont’d) Facial Nerve (N VII) Vestibulocochlear Nerve (N VIII) Expressions Balance, equilibrium, hearing Glossopharyngeal Nerve (N IX) Swallowing and taste Cranial Nerves (cont’d) Vagus Nerve (N X) Accessory Nerve (N XI) Widely distributed area Connects brain to viscera Sternocleidomastoid and trapezius Hypoglossal Nerve(N XII) Tongue movements Spinal Nerves Originate from spinal cord 31 pairs Named for region of cord from which they derive Mixed in function Roots of Spinal Nerves Connect the spinal nerves with the spinal cord Lie within the vertebral canal Dorsal roots contain sensory fibers Ventral roots contain motor fibers Rami Branches emerging from the vertebral column Dorsal ramus supplies deep back muscles Ventral ramus supplies the trunk and limbs ANS Also called involuntary nervous system Regulate the activity of smooth & cardiac muscles & glands Purely motor – no sensory nerves Subdivisions of ANS Sympathetic & parasympathetic Both serve same organs, but cause opposite effects Counterbalance each other Sympathetic Mobilizes body during extreme situations (fear, exercise, rage) “Fight-or-flight” Prepares body to cope with some threat Activation results in increased heart rate and blood pressure Parasympathetic “House keeper” system In control most of the time Maintains homeostasis by seeing that normal digestion and elimination occur and that energy is conserved Sympathetic Parasymp. Heartbeat Increase Decrease Resp. rate Increase Decrease Bronchio dilation Dilate Constrict Blood pressure Increase Decrease Glands – salivary, lacrimal Inhibits: dry mouth and dry eye Stimulates: more saliva and tears Digestion Sympathetic Parasymp. Slow down Increases peristalsis & secretions No effect Sweat glands Produces of skin perspiration Prepares for Eyes distant vision Liver Prepares for close vision Releases glucose No effect to blood