STRUCTURE AND FUNCTION OF THE NEUROLOGIC SYSTEM Organization

STRUCTURE AND FUNCTION OF THE NEUROLOGIC SYSTEM

Organization • Central nervous system (CNS) – Anatomical structures • Brain enclosed -- cranial vault • Spinal cord enclosed -- bony spine • Peripheral nervous system (PNS) – Anatomical organization (Fig. 12 -26) • Nerves – Cranial – 12 pairs – Spinal -- 31 pairs – Can be afferent or efferent

– Functional organization • Somatic nervous system – Regulates voluntary, motor control – Neurotransmitter = acetyl choline (ACh) • Autonomic nervous system – Regulates internal environment

• Most organs dually innervated: – Sympathetic neurons (Fig. 12 -24, 23) » From thoracic, lumbar spinal regions » Important for “fight or flight” (incr’d heart rate/resp’n, decr’d digestion) » Neurotransmitters: ACh and epinephrine/norepi

– Parasympathetic neurons (Fig. 12 -25, 23) » From other spinal regions » Important to conserve energy and maintain homeostasis (decr’d heart rate, incr’d digestion) » Neurotransmitter: ACh

Neural tissue • Neuron = primary cell of nervous system – About 1011 neurons/body – Each neuron adapted for specific function – Functions of neurons • Detect env’l changes • Initiate body response to changes – Fuel source -- mostly glucose

– Anatomic components (Fig. 12 -1) • Cell body = soma – Most in CNS – Those in PNS grouped together as ganglia • Dendrites – Extensions of cell body – Carry information TOWARD cell body

• Axon – Usually one per neuron – Long projection; carries impulses AWAY from cell body – Myelin – insulating lipid covering » Forms sheath » Allows fast flow of ions in one direction proper impulse conduction (away from cell body) – Interruptions in myelin coating = nodes of Ranvier » Nec for ions from ISF to enter axon for proper impulse

• Supporting cells of neurological system (Fig. 12 -3, Table 12 -1) – Schwann cells – in PNS • Form myelin sheath around axons – Neuroglia -- “nerve glue” • Support CNS neurons • About ½ volume of the brain and spinal cord

– Several types of neuroglial cells: • Astrocytes -- star shape – Form contact between neurons, circulatory system – “Buffer zone” between neurons (delicate) and molecules circulating in blood • Oligodendroglia – Deposit myelin in CNS (similar job as the Schwann cells in PNS) • Microglia – Phagocytic cells; digest debris in CNS • Ependymal cells – Help produce cerebrospinal fluid (csf)

• Nerve injury and regeneration (Fig. 12 -4) – Mature neurons don’t divide, proliferate • Injury can permanent loss of function – Regeneration of some PNS neurons is possible • Axon of neuron (so only myelinated fibers) repaired – Regeneration more optimistic if cell crushed • If cut, scar tissue can form impede ion flux through cell membrane, so impede proper impulses

– Regeneration more optimistic if injury further away from cell body – With regeneration, see: • Swelling distal to injury • Filaments hypertrophy • Myelin sheath and axon begin to degenerate, BUT • Proximal to injury, see projection of new neurofibriles – Neurilemma (membrane that surrounds the myelin sheath) acts as guide – Not in CNS, where myelin somewhat different • Scar tissue forms, and decr’d/no regeneration of neuronal tissue

Nerve impulses • Action potentials generated – Neuron selectively changes electrical potential of its plasma membrane – Influx of Na+ through selective channels (gated Na+ channels) at dendrite or soma • In response to biochemical signal from a neurotransmitter released from an impinging neuron – Changes electrical potential of membrane in that region

– Neurons influence neighboring neurons (Fig. 12 -2) • Release neurotransmitters (biochemicals signal an action potential in a neighboring neuron) • Synapse – region between two nearby neurons – First neuron in a series = “presynaptic” – Second neuron =“postsynaptic” – Presynaptic impinges on postsynaptic • Neurotransmitters synth’d, stored in vesicles near end of presynaptic neuron

– When action potential reaches end of presynaptic neuron: • Signals vesicle holding neurotransmitters to merge with neuron’s plasma membrane in presynaptic area • Neurotransmitters released into synapse • Neurotransmitters travel through synapse, where they encounter postsynaptic neuron • On plasma membrane of postsynaptic neuron is a receptor specific for a particular neurotransmitter

– Neurotransmitter binds the receptor on the postsynaptic neuron • Signals opening of nearby Na+ channels • Membrane potential changes in the postsynaptic neuron • Generation of action potential • Action potential travels through postsynaptic neuron’s dendrite, cell body and axon to axon ending (now presynaptic) • Signals neurotransmitter release to next neuron or muscle fiber on which it impinges, and changes occur within that cell

– Some widely studied neurotransmitters • Norepinephrine, dopamine, ACh, serotonin (and MANY others)(Table 12 -2) – Excitatory neurotransmitters cause Na+ to flood into neuron depolarization and action potential – Inhibitory neurotransmitters dampen Na+ influx into neuron inhibition of depolarization, so no action potential – Different neurotransmitters have different functions (some excitatory, some inhibitory)

Central Nervous System (CNS) • The brain – Allows reasoning, intelligence, personality, mood – Weighs about 3 lb. in average adult – Receives about 20% of cardiac output – Divisions (Table 12 -3; Fig. 12 -6) • Different regions, each associated with different function (Fig. 12 -7) • BUT some functions controlled by more than one region • Ex: cerebrum -- centers for sensory/motor, reasoning, memory, intelligence

– Characteristics/Structures • Gyri – convolutions of tissue along brain surface – Importance: increase surface area of brain • Sulci – grooves between gyri • Gray matter – cerebral cortex – Cell bodies of neurons (so not myelinated) • White matter – myelinated nerve fibers (= axons) – Lies beneath cerebral cortex

• Spinal cord (Fig. 12 -9, 10, 11) – Long nerve cable – Continuous with brain – Lies in vertebral canal • Surrounds, protects spinal cord – Divided into 31 anatomical sections – Gray matter (Fig. 12 -11) • In center of spinal cord • Butterfly shaped • Divided 3 horns • Composed of neuronal cell bodies

– White matter • Surrounds gray matter • Myelinated tissue (so axons) • Forms ascending, descending tracts – Motor neurons (Fig. 12 -12, 13) • Directly influence the muscle cells • Cell bodies of motor neurons lie in gray matter of spinal cord • Axons extend out of spinal cord • Regulate motor activity

• Protective structures of the CNS – Cranium • 8 fused bones; encloses and protects the brain – Epidural space • Lies between cranium and meninges • Site of blood collection ( epidural hematoma) if trauma disruption of blood vessels of scalp/skull

– Meninges – 3 protective membranes (Fig. 12 -14): • Dura mater – 2 layers of tissue • Arachnoid membrane – named for appearance (spider web) • Pia mater – cells to produce cerebrospinal fluid – Spaces between layers -- also sites where blood may collect if hemorrhage

• Cerebrospinal fluid (csf) – Clear, colorless fluid similar to ISF and plasma (Table 12 -4) – Helps cushion CNS – Produced within pia mater (about 600 m. L/day) – Circulates within cranium in cavities, subarachnoid space – Exerts pressure within brain, spinal cord • Forms pressure gradient between arteries, cavities of CNS – Reabsorbed into venous circulation – Valves in arachnoid membrane move fluid into venous circulation (and opposite)

• Vertebral column (Fig. 12 -15, 16) – Vertebrae • 33 – Intervertebral discs • Between vertebrae • Pulpy, absorb shock – Prevent damage to nervous system structures • If rupture back pain

Vertebral circulation • Arises from aortic arch internal carotid arteries and vertebral arteries (Fig. 12 -18; Table 12 -5) – May be conducting ( brain surface), OR – Penetrating ( structures below the cortex) • Healthy brain can regulate its blood supply to maximize oxygen supply – Can increase extraction of oxygen from blood when systemic bp decreases (for awhile) – Can decrease resistance in cerebral vessels when systemic bp decreases (up to a point)

• Blood-brain barrier – Supporting neural cells and blood capillaries have rel. tight junctions • Selectively allow partic blood components from blood brain – Important in brain chemotherapy