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

• 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)

• 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