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,Table12-1) 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, epinephrine, dopamine, ACh, serotonin (and MANY others)(Table12-2) 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 Motor neurons (Fig.12-12,13) 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 mL/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)

Selectively allow partic blood components from blood  brain 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