Transcript STRUCTURE AND FUNCTION OF THE NEUROLOGIC SYSTEM

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