THE NERVOUS SYSTEM
Nervous System
Functions
of the Nervous System
1. Permits
sensory input
Receptors in PNS respond to both external and
internal stimuli
2. Performs
integration
CNS
sums all the input and “decides” what (if anything) to do about it
If
a response is needed the CNS “figures out” how to carry out that response
3. Stimulates
motor output
CNS
sends signal through the PNS to the effectors
The
signal is the “instruction set” to respond to the sensory input
Effectors
are muscles and glands, execute the planned response
Divisions
of the Nervous System
CNS
Brain
Spinal cord
PNS
– cranial and spinal nerves; projections from the CNS to the rest of
the body and inputs from the rest of the body back to the CNS |
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Motor - efferent, nerves from CNS deliver motor information
to effectors
Somatic - motor nerves to skeletal muscle (voluntary)
Autonomic - motor nerves to smooth and cardiac muscle
and glands
Sympathetic division - "fight or flight"
Parasympathetic division - "resting and digesting"
Sensory - afferent; nerves from structures outside
the CNS deliver sensory input to the CNS
Somatic - input to CNS from skin, muscles, joints,
and special senses
Visceral - input to CNS from body organs
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Nervous Tissue
Neurons, transmit nerve impulses
Neuroglial cells - support the function of neurons
Neuron Structure
Dendrites
– receive input and conduct impulses toward cell body (usually)
Cell bodies – contains nucleus and other organelles
Bundles of neuron cell bodies in the CNS are nuclei;
bundles of neuron cell bodies in the PNS are ganglia
Axons
– conduct impulses away from the cell body
Axons and dendrites may also be called processes or
fibers
Bundles of parallel axons in the PNS are nerves;
bundles of parallel axons in the CNS are tracts
Myelin sheath – a covering of lipid material often
found on axons, insulates axons and speeds nerve impulse transmission
Produced in the CNS by neuroglial cells called oligodendroglia,
in the PNS by neuroglial cells called Schwann cells
Neurilemma = Schwann cell cytoplasm and nucleus within
plasma membrane
Myelin sheath is the inner part where the cytoplasm
has been squeezed out
Nodes of Ranvier – gaps between Schwann cells, important
in nerve impulse conduction
Multiple Sclerosis (MS) disease of the myelin sheath,
lesions become fibrotic, interfere with normal nerve impulse conduction
Types of Neurons
Sensory neurons (afferents) – conduct impulses from periphery
to CNS
Motor neurons (efferents) – conduct impulses from CNS
to periphery
Interneurons (association neurons) conduct impulses within
CNS |
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Neuroglial
cells
Supporting
cells
Schwann
cells – PNS, produce myelin sheaths in PNS
Satellite cells - PNS, surround neuron cell bodies, help regulate chemical environment
Microglial
cells – CNS, phagocytic, macrophage like
Astrocytes
– CNS, star shaped, provide support between neurons an capillaries
Oligodendroglial
cells – CNS, produce myelin sheaths in CNS
Ependymal
cells – line cavities in CNS, produce and help circulate CSF
Nerve Signal Conduction
Resting
Membrane Potential
There
is more Na+ outside the cell than inside, and more K+ inside
the cell than outside.
Na+ diffuses in through leakage channels, K+ diffuses
out through leakage channels.
K+ diffuses out faster than Na+ diffuses
in, so more positive charges end up on the outside of the cell.
The negatively
charged internal proteins and the unequal Na+ and K+ distribution
across the membrane contribute to making the cell membrane kind of like a battery,
with a positive pole on the outside and a negative pole on the inside.
The
membrane is said to be “polarized”.
The
difference between the positively charged outside part of the membrane and the
negatively charged inside of the membrane represents potential energy – since
opposite charges attract and like charges repel a positively charged particle
would shoot straight through the membrane from outside to inside if it could
get through.
This
potential energy can be measured: It is called a membrane potential and the amount of potential energy (the difference in charge between the inside and outside of the plasma membrane) is measured in volts (or millivolts at the level of a cell).
At equilibrium the membrane potential is called a resting membrane
potential.
(The Na+ and K+ distributions are maintained
by the Na+ - K+ pump; this protein pumps Na+ back
out of the cell and K+ back into the cell to maintain the uneven
charge distribution; this is the equilibrium that produces the resting membrane
potential.
If Na+ and K+ diffused until the concentration
of each was the same on both sides of the membrane they would be in equilibrium
with regard to distribution but then the cell wouldn't have a resting membrane
potential, which turns out to be pretty useful.)
Action
potential
A stimulus to a neuron can cause rapid influx of large amounts of Na+ through voltage-regulated Na+ channels, which causes a large change
in polarity - a depolarization as the inside becomes less negative until the polarity is reversed and the inside becomes positive compared to the outside.
After depolarization the voltage-regulated Na+ channels close and voltage-regulated K+ channels open, allowing K+ to diffuse out of the cell and reverse the depolarization (this is called repolarization).
The time required for repolarization to occur is the refractory period, during which the cell can't be restimulated.
Another problem you may run into is this: Having all that Na+ inside the cell and all that K+ outside the cell eventually will lead to ionic imbalance; however the proper distribution of Na+ and K+ is restored by the Na+ - K+ pump.
Conduction of Action Potentials
As
this change in polarity moves down the membrane a nerve impulse is transmitted
along the cell - the wave of depolarization coming down the axon opens
more voltage-regulated Na+ channels which reinforce the action potential and insures that it will reach the axon terminal.
As the wave of depolarization moves down the axon the degree of depolarization is decreased by the presence of leakage channels, allowing Na+ and K+ to move freely across the membrane.
Myelinated axons conduct action potentials much more quickly than unmyelinated axons because in myelinate axons the leakage channels are only found at the nodes
of Ranvier – which is also where the voltage regulated Na+ channels
are located. The depolarization wave can open the cluster of voltage regulated Na+ channels located here and overcome the effects of
ion leakage.
Conduction is slow in small unmyelinated fibers, fast in thick myelinated fibers.
Transmission Across a
Synapse
A synapse is
where the axon terminal (synaptic knob) of a neuron meets a dendrite on another
neuron (or cell membrane of effector).
There is no
physical contact, they are separated by a small space (the synaptic cleft).
Presynaptic
membrane vs. postsynaptic membrane – well, the plasma membrane of the axon terminal
before the synapse would be the presynaptic membrane and the plasma membrane
of the cell on the other side of the synaptic cleft would be the postsynaptic
membrane.
When the action potential reaches the axon terminal voltage-regulated calcium channels open, calcium floods into the axon terminal, and causes the synaptic vesicles to fuse with the presynaptic axon plasma membrane and release neurotransmitters into the synaptic cleft.
Depending on the neurotransmitter and the type of receptor on the postsynaptic membrane the result of binding can be excitatory or inhibitory.
Graded Potentials and Synaptic Integration
When neurotransmitters are released
from the presynaptic membrane they transmit the stimulus across the synaptic cleft to receptors
on the postsynaptic membrane.
The
receptors are linked to (or part of) chemically regulated ion channels; binding causes the channels to open and a graded local potential is generated.
These receptors may be excitatory, like those linked to Na+ channels – when NT binds the channels open, Na+ floods in, and a graded local potential called an excitatory postsynaptic potential is generated.
Some
receptors are inhibitory, that is, they let negatively charged ions like Cl- in to the cell (which hyperpolarizes the membrane) or they may let positively
charged ions like K+ out of the cell (which also hyperpolarizes the
membrane), generating an inhibitory postsynaptic potential.
The sum of all the graded local potentials must reach threshold
(depolarization of 15 - 20 mV or so less negative than the resting membrane
potential) when they reach a voltage-regulated Na+ channel to generate
an action potential.
Neurotransmitter Molecules
Acetylcholine
– active in all parts of the nervous system
Acetylcholine
esterase - breaks acetylcholine down to remove from synapse
Norepinephrine
– adrenergic; can be stimulatory or inhibitory depending on receptor type
present on postsynaptic cell
Serotonin
and dopamine – behavioral states; mood, tension, learning, memory
Neurotransmitters
and neurological disorders
Parkinson's
disease
Imbalance
in dopamine (probably some serotonin too)
Wide-eyed,
unblinking expression
Involuntary
tremor of fingers an thumbs
Muscular
rigidity
Shuffling
gait
Huntington's
disease (chorea)
Progressive
deterioration of nervous system leading
to constant thrashing and writhing (thus "chorea")
Onset is usually mid-30's to early 40's, prognosis
is insanity and death. The cause is genetic and is one of the few genetic diseases caused by a dominant mutation. The tragedy is that an affected parent has a 50% chance of passing the gene to their children but they don't show signs of the disease until after they've given birth (assuming childbirth occurs before mid-30's).
Possible
malfunction of GABA (another NT)
Alzheimer
disease
Gradual
loss of reason, including memory, personality changes, ability to perform
simple tasks, confusion
Pathological
differences:
AD
neurons exhibit neurofibrillary tangles surrounding the nucleus
Amyloid
plaques surround axon branches
Acetylcholine
appears diminished in AD-affected brains
Neural
abnormalities seen primarily in frontal lobes and limbic system
Susceptibility
16%
liklihood in people with no family history
24%
in people with first degree relative
May
be linked to a genetic defect on chromosome 21
People
with Down’s syndrome (3 copies of chromosome 21) are more likely to develop AD
The
defect on chromosome 21 affects normal production of amyloid precursor protein,
which is though to be the cause of amyloid plaques
Treatments
Drugs: cholinesterase inhibitors allow acetylcholine accumulation; memantine, blocks excitotoxicity (diseased neurons self destruct and cause death of nearby neurons
Future possibilities: autologous induced pluripotent stem cell transplants, gene therapy where applicable
Central
Nervous System
Meninges and Cerebrospinal Fluid
Meninges
– protective membranes
Dura mater – outer layer
Tough, fibrous CT
Lies next to skull and vertebrae
Forms channels (splits into 2 layers; actually consists
of 2 fused layers in most places) called dural sinuses that collect and
return venous blood to circulation.
Epidural hematoma – bleeding between dura and bone.
Subdural hematoma – bleeding below dura, into space
between dura mater and arachnoid.
Arachnoid membrane
Delicate, web-like CT
Attaches to pia mater
Subarachnoid space contains Cerebrospinal fluid (CSF)
Pia mater
Thin, fine follows contours of brain closely
Cerebrospinal Fluid
A clear fluid derived from plasma that forms protective
cushion around CNS, provides nutrients, and collects wastes.
CSF is produced at choroid plexuses, specialized capillary
networks where ependymal cells filter plasma (and circulate the CSF).
CSF circulates through the ventricles, into the central canal
of spinal cord, into the subarachnoid spaces, and returns to veins of the
brain where it drains into the dural sinuses and returns to blood.
Production and drainage are balanced; blockage produces hydrocephalus.
In infants the cranial sutures haven’t fused so the
head can enlarge to accommodate the fluid.
In adults the brain gets mashed against the skull.
The Spinal Cord
Structure of the Spinal Cord
Extends from the base of the brain through the foramen magnum, down the vertebral
canal (middle of vertebrae) and ends between 1st and 2nd vertebrae
External white matter surrounding butterfly shaped gray matter with a central
canal
Gray matter is described as anterior or dorsal horns and posterior or ventral
horns
Gray matter contains cell bodies and short unmyelinated fibers
Sensory neurons enter through the dorsal roots, motor neurons exit through
the ventral roots, roots join to form spinal nerves
White matter contains bundles of myelinated fibers or tracts that form columns
running up and down the spinal cord
Ascending tracts take sensory nerve impulses up to the brain
Descending tracts take motor nerve impulses down and out to effectors
Functions of the Spinal Cord
Center for reflex arcs
Communication between brain and peripheral nerves
Tracts crossover in the medulla; right side of brain controls left side of
body and vice versa
Cauda equina (distal portion of spinal cord - "horse's tail"), spinal nerves “chase” exit point because vertebral
column grows more quickly than cord
Epidural anesthesia – inject it in the epidural space and numb everything
below the level of injection
Spinal
Cord Injuries
Partial
section or transection of cord
Location
and severity of injury determines what part of body is affected
Betwween
T1 ad L2 – paralysis of lower body and legs – paraplegia
Between
C4 and T1 – entire body and 4 limbs – quadriplegia
Unilateral
hemisection (half cut) motor loss on same side as injury (crossover
occurs in medulla)
The Brain
The two lateral ventricles are associated with the cerebrum, the third
ventricle is associated with the diencephalon, and the the fourth ventricle
is associated with the brain stem and cerebellum.
EEG
Measures
electrical activity of the brain
Good
diagnostic tool – irregular brain patterns can indicate pathologic conditions
(epilepsy to brain death for example; although functional MRI studies show blood flow and have
helped elucidate areas of the brain active during specific tasks.)
Waking
subjects have 2 types of brain waves
Alpha
waves – predominate when eyes are closed; calm, relaxed state of wakefulness
Beta
waves – predominate when eyes are open, indicate mental alertness,
concentration
Sleeping
subjects
Theta
waves – light sleep, not seen normally in awake adults (although common in
children)
Delta
waves – deep sleep
REM
sleep – EEG pattern looks like alph waves, eyes move rapidly beneath eyelids,
dreaming occurs
The Cerebrum
Largest and most superior part of brain
Outer cortex of gray matter
Contains cell bodies and short fibers
Convolutions known as gyri
Shallow grooves known as sulci
Deep grooves known as fissures
Accounts for sensation, voluntary movement, and consciousness
The Cerebral Hemispheres
The right and left cerebral hemispheres are divided by the
longitudinal fissure and joined by a bridge of myelinated fibers, the corpus
callosum.
Each hemisphere contains
a lateral ventricle
Each hemisphere has 4 lobes
Frontal lobe - deeo to the frontal bone, anterior to the parietal
lobe
Central sulcus – divides
frontal lobe from parietal lobe
Parietal lobe - deep to the parietal bone, posterior to the
frontal lobe
Occipital lobe - deep to the occipital bone in the most posterior
area of the cranial vault
Temporal lobe - deep to the temporal bone, separated from the
frontal lobe and parietal lobe by the lateral sulcus
Insula - the fifth lobe, the insula, lies deep to the lateral
sulcus
Motor and Sensory Areas of the Cortex
Primary Motor Area: in the frontal lobe just anterior
to the precentral gyrus; responsible for movement of skeletal muscle
Somatic motor neurons cross over after leaving the primary
motor area so the right primary motor area controls the left side of the body
and the left primary motor area controls the right side.
Primary Somatosensory Area: in the parietal lobe just posterior
to the precentral gyrus; receives input from temperature,
touch, pressure, and pain receptors in the skin.
Like the motor areas, the left hemisphere recieves somatosensory
input from the right side of the body and the right hemisphere recieves input
from the left side of the body.
The figure below, depicting the areas of the cortex that exert
voluntary motor control and recieve sensory input, illustrates that areas with
the greatest amount of control or sensitivity have the greatest amount of cortex
devoted to them.
Primary taste area (gustatory cortex) - located in the insula
just deep to the parietal lobe
Primary visual area - located
in the occipital lobe
Primary auditory area - located in the temporal lobe
Olfactory cortex - located on the medial aspect of the temporal
lobe, part of the rhinencephalon, link to the limbic system
Visceral sensory area - located just posterior tot he gustatory
cortex in the insula, involved in conscious perception of visceral sensation
Vestibular (equilibrium) cortex - conscious awareness of balance,
located in the posterior part of the insula and adjacent parietal cortex
Association Areas
Association areas integrate sensory input and store memories
- interpret sensory experiences and remember visual scenes,
music, and other complex sensory patterns
Premotor area - organizes motor functions for skilled motor
activities and communicates with the primary motor cortex. The primary motor
cortex sends signals to the cerebellum and basal nuclei, which integrate
the information.
Somatosensory association area - posterior to the primary somatosensory
area, process and integrates sensory information from skin and muscles
Visual association area - combines new visual images with memories
of older visual information
Auditory association area - combines new auditory data with
memories of previously encountered auditory stimuli
Processing Areas
Anterior Association Area (Prefrontal cortex)
Most complicated cortical region
Intellect, complex learning, personality
Abstract ideas, judgment, reasoning, persistence, planning concern for
others, and conscience
Linked to limbic system, role in mood
Prefrontal lobotomy – treatment of severe mental illness (‘30s
to ‘50s); reduced anxiety (and judgment, initiative, spurred abnormal
personality changes, caused epilepsy, etc.)
Prefrontal area (Prefrontal cortex; Anterior Association Area) – recieves
input from other association areas, is used for concentration, planning, persistence,
complex problem solving, abstract ideas, judging the consequences of behavior,
concern for others, conscience, and personality.
The anterior association area (prefrontal cortex) is linked
to the limbic system and plays a role in mood.
Damage to these areas can completely change personality traits.
Posterior Association Area (General interpretation area or
gnostic area)
Regionally diffuse
Input from all sensory association areas
Storage site for complex memories associated with sensation
Integrates input into understanding of situation, sends assessment to prefrontal
cortex
Prefrontal cortex adds emotion and decides on response
Damage to this area results in inability to interpret situations (imbecility)
Found in one hemisphere only (usually left)
Language areas – included in posterior association
area
Broca’s area – motor
area for speech
Located at the base of the precentral gyrus in the frontal
lobe
Usually located in left hemisphere
Wernicke's area - recieves inputs
from all other sensory association areas, works with Broca's area in understanding
speech and the use of language to express thoughts and feelings.
Usually located in the left hemisphere
Geschwind's territory
Connects Broca's and Wernicke's areas via a region of the parietal lobe
of the cortex, and may be important for the acquisition of language in
childhood.
Apparently the last area in the brain to mature, the completion of its
maturation coinciding with the development of reading and writing skills.
Affective language areas
Present in hemisphere opposite Broca’s and Wernicke’s
areas
Affect – tone, both to impart and to interpret
Add emotional input to speech, interpret emotional content of speech
The
Left and Right Brain
Sperry
and Gazzaniga’s experiments - severed
corpus callosum in epileptic patients
Found that the hemispheres are pretty much structurally the same but functionally
different
Dominant
hemisphere is defined as the hemisphere containing language abilities (
Broca’s
area, Wernicke’s
area)
90%
of people have dominant left hemispheres
Left
brain (usually) controls language, logic, math skills
Right
brain controls spatial discrimination, musical and artistic ability, intuition
and emotion
Central White Matter
Projection Fibers (vertical): Myelinated axons carry information from CNS to the body (descending tracts), from the body and lower brain centers to the CNS (ascending tracts)
Horizontal Fibers: Association fibers carry information between different areas of the cerebrum within the same hemisphere and commissural fibers link corresponding areas in opposite hemispheres (corpus callosum, anterior commissure).
Basal Nuclei
Masses of gray matter deep within the white matter of the cerebrum
Part of limbic system – not really, this statement reflects the fact
that the caudate nucleus is usually included with the basal nuclei and is really
a part (functionally) of the limbic system
Receive inputs from entire cerebral cortex, other nuclei, and
each other
Project to the premotor and prefrontal cortices through thalamic
relays, influence voluntary muscle movements directed by the primary motor
cortex
No direct involvement in motor pathways
Impairment results in disturbances in posture, muscle tone,
involuntary movements including tremors and abnormal slowness of movement
as seen in Parkinson’s
disease
The Diencephalon
Thalamus
Central relay station for sensory impulses traveling from
everywhere else to the cerebrum
Receives all sensory impulses except smell and sends them
to the correct region of the cerebral cortex to be interpreted
Hypothalamus
Maintains homeostasis
Contains centers for regulating hunger, sleep, thirst, body
temperature, water balance, and blood pressure
Controls pituitary gland – links nervous system with endocrine system
Epithalamus - contains the pineal gland, which secretes the
hormone melatonin.
Melatonin regulates sleep-wake patterns (daily rhythyms)
Limbic System
System of pathways connecting parts of the frontal lobes, temporal
lobes, thalamus, and hypothalamus
The emotional brain – responsible for feelings of anger, fear, sorrow,
pleasure, affection, sexual interest
Involved in learning and memory, links emotion to stored memories
Arose from the ancient rhinencephalon or “smell brain”
Our sense of smell has diminished and those structures have
evolved to deal with emotions, memory, and learning
Responsible for the strong link between smell and emotionally
charged memories
Cerebellum
Lies below posterior portion of cerebrum
Two hemispheres joined by narrow medial bridge
Cortex of gray matter
Deeper white matter (some nuclei in white matter)
Functions:
Muscle coordination – integrates impulses from higher centers to produce
smooth and graceful motion
Maintains normal muscle tone and posture
Receives information about body position from inner ear and
sends impulses to muscles to maintain or restore balance
Brain
stem
Medulla
oblongata
Centers
for regulating heartbeat, breathing, and blood pressure
Reflex
centers for vomiting, coughing, sneezing, hiccoughing, and swallowing
Ascending
and descending tracts between higher brain centers and spinal cord
Pons
Bridge;
tracts running between cerebellum and the rest of the CNS
Coordinates
with medulla to regulate breathing
Reflex
centers for head movements in response to visual and auditory stimuli
Midbrain
Encloses
cerebral aqueduct
Relay
station for tracts that run between cerebrum and spinal cord and cerebellum
Reflex
centers for visual, auditory, and tactile responses
Reticular formation
System of loosely clustered neurons extending through brainstem
with projections (all over) to hypothalamus, thalamus, cerebellum, spinal cord
Functions: Arousal of brain
Reticular Activating System
Receives sensory inputs from all ascending sensory tracts
and sends impulses to cerebral cortex through thalamic relays
Maintains cortex in alert conscious state, enhances excitability
Filters out repetitive, familiar or weak signals
Brings attention to unusual, significant, or strong impulses
Role in learning and memory; part of “reward pathway”
Depressed by alcohol, sleep-inducing drugs, tranquilizers;
severe injury results in unconsciousness (permanent = coma)
Damped by sleep centers of hypothalamus, etc.; damping
removed by LSD and other hallucinogens
Motor arm projects to spinal cord
Helps control skeletal muscles during coarse movement of
limbs
Autonomic functions
Include vasomotor, cardiac, and respiratory centers of
the medulla
Peripheral
Nervous System
Afferent, or sensory system
Somatic sensory system includes
all the fibers that innervate the musculoskeletal system, including skin, joints, and tendons (in addition to skeletal muscles); the special senses are also part of the somatic sensory system.
Visceral sensory system supplies internal organs.
Efferent, or motor system
Somatic
motor system innervates skeletal muscles.
Autonomic motor system innervates smooth and cardiac muscle and glands.
Types of Nerves
Cranial nerves are attached to the brain, spinal nerves are attached to the spinal cord.
Cranial
Nerves
Twelve
pairs attached to the brain
Consist
of motor, sensory, and mixed nerves
Innervate
head, neck and facial regions, except the vagus nerve (internal organs)
Spinal
Nerves
Thirty
one pairs
8 cervical, 12 thoracic, 5 lumbar, 5 saccral, 1
coccygeal
Dorsal
and ventral roots leave cord, fuse to form spinal nerves just before exiting
the vertebral column
Dorsal
roots contain sensory neurons
Dorsal
roots have enlargements that contain the cell bodies of the sensory neurons
(dorsal root ganglia)
Ventral
roots contain axons of motor neurons
Spinal
nerves contain sensory “dendrites” and motor axons; they are mixed nerves
Dermatomes
– segments of skin supplied by a particular spinal nerve
Knowledge
of which spinal innervates a particular dermatome allows interpretation of
sensory aberrations in a dermatome as an indicator of damage to that particular
spinal nerve
Somatic Motor Nerves and Reflexes
Reflexes and the Reflex Arc
Automatic, involuntary responses to changes occuring either
inside or outside the body
Cranial reflexes involve the brain, spinal reflexes involve only the spinal cord although the brain is usually notified
Reflex Arc
Structure
Receptor
Receives sensory input and transduces that information
into a nerve impulse
Impulses are transmitted to the sensory neuron dendrites
Receptors may be just free dendritic endings of the sensory
neuron
Sensory neuron fiber
Unipolar
– dendritic ending, long axon on both sides of cell body– draw
it and explain it
Axon located in a spinal nerve
Cell bodies of sensory neurons are located in the dorsal-root
ganglion (ganglia –
groups of cells bodies in the PNS)
Axon continues into the gray matter of the dorsal horn
where they form synapses with interneurons
Interneuron
Located completely within the gray matter of the spinal
cord
Motor neuron
Dendrites and cell body located within the gray matter’s
ventral horn
Axon located in the spinal nerve
Function
Information travels along the sensory neuron to the interneuron
and back through the motor neuron to elicit a response
In the case of skeletal muscle the muscle contracts and
moves the body part out of harm’s way or moves to maintain posture
Knee-jerk reflex
Ankle-jerk reflex
Autonomic
Motor Nervous System and Visceral Reflexes
Innervates
smooth and cardiac muscle and glands (all internal organs)
Involuntary
– doesn’t require voluntary control; automatic
Utilizes
2 motor neurons and one ganglion for each impulse
First
neuron has cell body in CNS and preganglionic axon
Second
neuron has it’s cell body within the ganglion and it’s axon is postganglionic
Visceral reflexes
Occur in response to normal physiological fluctuations
Breathing
Heart rate
Body temperature
Food digestion and waste elimination
Swallowing, coughing, sneezing, vomiting
Sympathetic
Division
Fight
or flight division
Fibers
arise from thoracic and lumbar regions of the spinal cord (also called the
thoracolumbar division)
Ganglia
lie very near the spinal cord; short preganglionic fibers, long post ganglionic
fibers
Postganglionic
axons release NE
Parasympathetic
Division
Resting
and digesting division
Fibers
arise from cranial and sacral nerves (craniosacral division)
Ganglia
lie near the organ that is innervated; long preganglionic fibers, short post
ganglionic fibers
Postganglionic
axons release acetylcholine
Effects
Of Aging
Begin
to lose thousands of neurons a day after age 60
By
age 80 brain weighs about 10% less
Cerebral
cortex loses as much as 45% of its cells
Learning,
memory, and reasoning decline
Homeostasis
