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Nervous System Spinal Cord and Spinal Nerves Nervous System As a Whole All body systems work together to maintain homeostasis Chief coordinating agency for all body systems Nerves carry messages to & from switching centers Conditions change inside & outside Nervous system must detect & respond to changing conditions (stimuli) so body can adapt itself to new conditions Structural (Anatomic) Divisions Central nervous system (CNS) Brain & spinal cord Peripheral nervous system All nerves outside the CNS Cranial nerves – carry impulses to & from brain Spinal nerves – carry messages to & from spinal cord See Figure 8-1, page 138 Central Nervous System Functional Divisions Type of control Voluntary – somatic nervous system Involuntary – autonomic nervous system or visceral nervous system Type of tissue stimulated Effector – muscles or glands that carry out nervous system commands See Table 8-1, page 139 Somatic Nervous System Skeletal muscle control Voluntary control Conscious will All effectors are skeletal muscles Autonomic Nervous System (ANS) Involuntary control – automatic activity Also known as visceral nervous system Controls Make smooth muscle, cardiac muscle & glands up viscera – soft body organs Subdivisions of ANS Sympathetic nervous system Parasympathetic nervous system Neurons & Their Function Functional cells of nervous system Highly specialized with a unique structure Neuron Structures Cell body – main portion Contains Cell nucleus & other organelles fibers (projections out from cell body) Dendrites Axons – some protected by myelin sheath See Figure 8-2, page 139 Dendrites Function as receptors Conduct impulses to cell body Many branches Receptors in nervous system Receive pathway stimulus that begins in neural Neuron Axons Conduct impulses away from cell body Single fiber May be long & branch and branch at end Impulses may be delivered to another neuron, to a muscle or to a gland Myelin Sheath Fatty material (myelin) covers some axons Insulates & protects the fiber Speeds conduction Made by Schwann cells in PNS Neurilemma – outermost layer – aids in axon repair Made by neuroglia in CNS See Figure 8-4 page 140 Myelin Sheath More on Myelin . . . Myelinated tissue White matter Cells of brain & spinal cord Have no neurilemma Permanent damage if injured Unmyelinated tissue – gray matter Not covered by myelin Types of Neurons PNS neurons relay information constantly to & from the CNS - 2 kinds Sensory (afferent) neurons conduct impulses to brain & spinal cord (CNS) Motor (efferent) carry impulses away from CNS Interneurons relay information within CNS (also called central or association neurons) Nerves & Tracts Nerve - bundle of nerve fibers located within peripheral nervous system (PNS) Tract - bundle of nerve fibers located in brain & spinal cord (CNS) Conduct messages to & from brain Nerves or tracts are bound together with connective tissue - fascicles PNS Nerves Sensory nerve contains only fibers that carry impulses to CNS (some cranial) Motor nerve contains only fibers that carry impulses away from CNS (some cranial nerves) Mixed nerve contains both sensory & motor nerve fibers - may travel to & from CNS (most cranial & all spinal nerves) Neuroglia Non-conducting connective tissue nerve cells Protect & support nervous tissue Different types of neuroglia with special functions Continue to reproduce (multiply) during lifetime Most tumors of nervous system are neuroglial tissue Neuroglia More on Neuroglia . . . Aid in cell repair Remove pathogens & impurities Regulate composition of fluids around & between cells Nervous System at Work Electrical impulses are sent along neuron fibers Then transmitted between cells at junctions Nerve Impulse Electrical charge is transmitted along cell membrane of a neuron Plasma membrane of non stimulated (resting) neuron carries an electric charge (potential) Charge is maintained by sodium (NA) & potassium (K) charged particles (ions) on each side of the cell membrane Nerve Impulse Nerve Impulse Plasma (cell) membrane carries the electrical charge (potential) Plasma membrane is polarized (negative charge) Membrane reverses (changes) charge Generates electrical charge (action potential) Polarization At rest, cell membrane is polarized Inside of membrane is negative (-) while outside is positive (+) See Figure 8-8, page 142 Depolarization Nerve impulse starts with local reversal of charge - depolarization Then spreads along the membrane with a sudden electrical change in the membrane (action potential) Depolarization is rapid Followed by immediate return to normal so membrane can be stimulated again Repolarization Return of membrane to resting state is called repolarization Rapid exchange of sodium & potassium ions across cell membrane bring about depolarization & repolarization Stimulus - any force that can start an action potential & spreads along membrane as a nerve impulse Role of Myelin in Conduction In unmyelinated fiber, action potential spreads continuously along cell membrane Myelin fibers conduct faster than unmyelinated fibers Nerve impulse skips from node to node (space between cells) along the myelin sheath – called saltatory conduction See Figure 8-4, page 140 Synapse Junction point for transmitting nerve impulse Point of junction for transmission of the nerve impulse between 2 nerve cells Synaptic cleft - tiny gap between cells Neurotransmitters - chemicals that transmit a nerve impulse across synaptic cleft between nerve cells Contained Axon body in vesicles of axon ending - cell fiber carrying impulses away from cell More on Synapse . . . Axon - presynaptic cell releases the neurotransmitter Neurotransmitter acts as chemical signal to stimulate next cell (post-synaptic cell) Dendrite - postsynaptic (receiving) cell membrane has receptors that receive & respond to specific neurotransmitters See Figure 8-9, page 143 Synapse Neurotransmitters 3 main neurotransmitters function in the autonomic nervous system (ANS) Adrenaline (epinephrine) Norepinephrine (noradrenaline) Acetylcholine (ACh) Released at neuromuscular junction (between nerve & muscle cell) More on Neurotransmitters Various paths of removal of neuro-transmitters Diffusion away from synapse Enzyme destruction in synaptic cleft Return to presynaptic cell to be used again (reuptake) Many psychoactive drugs affect the neurotransmitter’s activity Spinal Cord Links peripheral nervous system & brain Located in & protected by vertebral column Continuous tube from occipital bone to coccyx Ends between 1st & 2nd lumbar vertebrae See Figure 8-11, page 145 Spinal Cord Structure of Spinal Cord Unmyelinated gray matter (nerve cell bodies) surrounded by larger area of white matter (nerve cell fibers) See Figure 8-12, page 155 Gray matter is arranged in 2 pairs of columns called dorsal & ventral horns H-shaped appearance on cross section More on Spinal Cord Structure . . . Central canal of spinal cord contains cerebrospinal fluid (CSF) CSF - liquid that circulates around brain & spinal cord Myelinated white matter - consists of thousands of myelinated axons arranged in 3 areas external to (outside) the gray matter on each side Functions of Spinal Cord Links spinal nerves to the brain Relays information to & from brain Center of reflex activities Helps coordinate impulses within CNS Spinal Nerves Link to the Brain White matter divided into tracts that convey impulses to & from the brain Sensory impulses enter dorsal horn of cord & are transmitted up toward brain in ascending tracts of the white matter Motor impulses travel from brain in descending tracts & exit ventral horn of gray matter See Figure 8-13, page 147 Reflex Arc Complete pathway through the nervous system from stimulus to response Order of impulse conduction Receptor - detects stimulus Sensory neuron – transmits impulses to CNS More on Reflex Arc . . . Order of impulse conduction, continued Interneuron – in CNS Coordinates Motor impulses & organizes response neuron - CNS to effector Carries Effector impulse away from CNS - responding muscle or gland Carries out response Reflex Arc Reflex Activities Reflex - simple, rapid, automatic response using few neurons (uncomplicated) Specific - given stimulus always produces same response Spinal reflex - simple reflex arc that passes through spinal cord & does not involve the brain Stretch reflex, eye blink, withdrawal reflex Spinal Nerves 31 pairs of spinal nerves Each nerves is attached to spinal cord by 2 roots -dorsal & ventral roots Connect spinal cord with peripheral tissues All spinal nerves are mixed nerves Motor and sensory Messages go to and from CNS See Figure 8-11, page 145 Spinal Nerve Root Spinal Nerve Roots Dorsal root has a marked swelling of gray matter (ganglion) Contains the cell bodies of sensory neurons Ganglion - collection of nerve cell bodies located outside the CNS Sensory receptor fibers throughout body lead to dorsal root ganglia More on Spinal Nerve Roots . . . Ventral roots of spinal nerves Combination of motor (efferent) fibers Supply effectors (muscles & glands) Cell bodies located in ventral horn gray matter of spinal cord Dorsal (sensory) & ventral (motor) roots are combined to form spinal nerves - mixed nerves Spinal Nerve Root Branches of Spinal Nerves Spinal nerves branch into divisions short distance from spinal cord Small posterior division Larger anterior branches with plexus Plexus - network of nerve branches that distribute branches to body parts See Figure 8-11, page 145 Brachial Plexus More on Spinal Nerve Branches . . . 3 main plexuses Cervical plexus - neck & back of head Phrenic nerve that activates the diaphragm Brachial plexus - radial nerve – shoulders, arms , forearms, wrists & hands Lumbosacral plexus - sciatic nerve – pelvis, legs & feet Spinal nerves affect skin sensations Dermatones – see Fig. 8-15, page 149 Dermatomes Autonomic Nervous System (ANS) Motor (efferent) divisions of the visceral (involuntary) nervous system Regulates action of glands, smooth muscles of hollow organs & vessels & cardiac (heart) muscles Actions occur automatically No conscious awareness of regulatory adjustments & actions Characteristics of ANS ANS sensory (afferent) neurons are grouped with skin & voluntary muscle Motor (efferent) neurons - arranged in a distinct pattern - supply glands & involuntary muscles Autonomic pathway has 2 motor neurons that connect spinal cord with effector organ Synapse in ganglia - relay stations More on ANS Pathways . . . Pre ganglionic neuron - extends from spinal cord to ganglion Post ganlionic neuron - travels from ganglion to effector Autonomic fibers Some are within spinal nerves Some are within cranial nerves See Figure 8-16, page 151 Divisions of ANS Sympathetic nervous system Thoracolumbar - thoracic & lumbar regions of spinal cord Parasympathetic nervous system Craniosacral - brain stem & sacral regions of spinal cord See Table 8-3, page 150 Sympathetic Nervous System Motor neurons originate in spinal cord Cell bodies in thoracic & lumbar area Level of 1st thoracic nerve down to level of 2nd lumbar spinal nerve Nerve fibers extend to ganglia where they synapse with second neurons Fibers extend to glands & involuntary muscle tissues More on Sympathetic NS . . . Sympathetic ganglia form sympathetic chains along spinal column from lower neck to upper abdominal area Nerves that supply organs of pelvic & abdominal cavities synapse in 3 single collateral ganglia Release epinephrine & norepinephrine (adrenaline/noradrenaline) Adrenergic - activated by adrenaline Parasympathetic Nervous System Motor pathways begin in fibers from cell bodies in the brainstem (midbrain & medulla) & the lower part (sacral) of the spinal cord Extend to terminal ganglia in or near the effector organs, then stimulate the involuntary tissues Neurons release acetylcholine Cholinergic Functions of ANS Most organs are supplied by both sympathetic & parasympathetic fibers (the two have somewhat opposite effects on organs) Sympathetic - stimulates fight or flight (stress) response 4 E’s = emergency, excitement, embarrassment, exercise Also brake for systems not involved in stress response Parasympathetic - acts as balance once crisis has passed “rest and digest” SLUDD –salivation, lacrimation, urination, digestion, defecation See Table 8-4, page 152