Nervous System
Protection of CNS
8-6 discuss the roles of gray matter and white matter in the spinal cord
8-5 describe the three meningeal layers that surround the central nervous system
8-3 descibe the events involved in the generation and propagation of an action potential
8-1 List the two major anatomical divisions of the nervous system, and list their general functions
8-2 Distinguish between neurons and neuroganglia on the basis of structure and function
The general function of the nervous system is to coordinate all body systems! This is accomplished by the transmission of (electrochemical) signals from body parts to the brain and back to the body parts
General functions of the nervous system
Nervous System
The organs of the nervous system are divided into two major groups
Central Nervous System
Peripheral Nervous System
brain and spinal cord
nerves that extend from the brain (cranial nerves) and spinal cord (spinal nerves)
Sensory Input Function
Integrative Function
Motor Function
PNS
Sensory receptors (located at the ends of peripheral neurons) detect changes (i.e. are stimulated) occurring in their surroundings
Once stimulated, sensory receptors transmit a sensory impulse to the CNS
a sensory impulse is carried on a sensory neuron
involves interpretation of an incoming sensory impulse (i.e. decision is made concerning what is going to happen next, based on sensory impulse)
integration occurs in interneurons
a motor impulse beings
CNS (brain and/or spinal cord)
involves the response of a body part
motor impulses are carried from CNS to responsive body parts called effectors
a motor impulse is carried on a motor neuron
PNS
effectors
muscles (that contract)
glands (that secrete a hormone)
Central Nervous System (brain and spinal cord)(interneurons)
Peripheral Nervous System (cranial nerves and spinal nerves)
Sensory (input into CNS)(Afferent neurons)
Motor (output from CNS)(efferent neurons)
Somatic (effectors: skeletal muscle)(conscious control)
Autonomic (effectors: smooth muscle; cardiac muscle; glands)(unconscious control)
Parasympathetic (homeostasis)(neurotransmitter: acetylcholine)
Sympathetic (fight-or-flight)(neurotransmitter: norepinephrine)
Neuroganglial cells- accessory cells of the nervous system form supporting network for neurons; "nerve glue"
Classification of Neurons
Neuron- the structural and functional unit of the nervous system; a nerve cell
Regeneration of nerve fibers
Neuron structure
each neuron is composed of a cell body and many extensions from the cells body called neuron processes or nerve fibers
cell body- central portion of a neuron; contains usual organelles, except centrioles
Neuron processes/ nerve fibers- extensions from cell body
nucleus, prominent nucleolus, and many Nissl bodies = RER
Dendrites
Axons
many per neuron
short and branched
receptive portion of a neuron
carry impulses toward cell body
long, thin processes
carry impulses away from cell body
axon branch - axonal terminals, synaptic knobs
one per neuron
Axons in PNS
Axons in CNS
cell body injury - death of neuron
damage to an axon may allow for regeneration
Large axons are surrounded by a myelin sheath produced by many layers of Schwann Cells (neuroganglial cells)
Small axons do not have a myelin sheath
myelin - lipoprotein
interruptions in the myelin sheath between Schwann cells - Nodes of Ranvier
"myelinated nerve fiber)
"unmyelinated nerve fibers"
however all axons (in PNS) are associated with Schwann cells
myelin is produced by an oligodendrocyte rather than Schwann cells
a bundle of myelinated nerve fibers = "white matter"
this is in contrast to CNS "gray matter" = a bundle of cell bodies (or unmyelinated nerve fibers)
PNS = nerve
CNS = tract/column
PNS = ganglia
CNS = nucleus
PNS - Schwann cells produce myelin and satellite cells nourish neurons
CNS - provide bulk of brain and spinal cord tissue
astrocyte
microglia
oligodendrocyte
ependymal cells
looks like eyeball
function - produces myelin
star-shaped
function - nourishes neurons
looks like spider
function - phagocytosis
epithelial-like layer
function - lines spaces in CNS
brain - ventricles
spinal cord - central canal
structural classification
Functional classification
Sensory neurons
Interneurons (association)
Motor Neurons
Multipolar neurons (most common neuron in CNS)
Bipolar neurons (rare, sight, smell, and hearing)
Unipolar neurons (most PNS sensory neurons)
Afferent neurons
carry sensory impulses from sensory receptors to CNS
input information to CNS
location of receptors = skin and sense organs
PNS
visceral receptors (digestive, respiratory, cardiovascular, urinary, reproductive, taste, deep pressure, pain)
link other neurons together (i.e. sensory neuron to interneuron to motor neuron)
CNS
PNS
efferent neurons
carry motor impulses away from CNS and to effectors
output information from CNS
effectors - muscles and glands
many extensions
many dendrites lead toward cell body, one axon leads away from cell body
two extensions
one fused dendrite leads toward cell body, one axon leads away from cell body
one process from cell body
forms central and peripheral processes
only distal ends are dendrites
somatic receptors
external receptors (touch, pressure, temp, sight, smell, hearing, and touch)
proprioceptors (position and movement)
somatic motor neurons (skeletal muscles)
visceral motor neurons (muscle and glands)
sympathetic (adrenergic)
parasympathetic (cholinergic)
characteristics of a nerve impulse
synaptic (chemical) transmission
nerve impulse transmission
scheme of synaptic transmission
summary
neurotransmitters
action potential
neuropeptides
Membrane potentials
disorders associated with neurotransmitter imbalances
Resting nerve cells
the resting membrane potential (RMP) of a neuron is results from the distribution of ions across the cell membrane
the RMP of a nerve cell is measured to be -70mV (inside/outside)
DEF: potential difference = the difference in electrical charge between 2 points (across a cell membrane)
as long as the RMP in a nerve cell is undistributed, it remains polarized. However, in order for a nerve impulse to be started or propagated in a nerve cell, this resting potential must be distributed
a resting neurons cell membrane is said to be polarized = electrically charged (charge inside the cell is different than the charge outside)
more negative = hyperpolarization
less negative (i.e. towards 0) = depolarization
when the resting membrane potential of a neuron is depolarized to -55mV, threshold potential is reached
when threshold potential is reached, the rapid opening of Na+ channels results in rapid depolarization (and even reversal of the membrane potential to +30mV)
then K+ channels open, (while Na+ channels close), and repolarization occurs = recovery of the RMP to -70mV
this all occurs very quickly = 1/1000 sec
RPM of neuron = -70mV
MP of neuron falls to -55mV = threshold potential
MP of neuron reaches +30mV = reversal of MP
RMP of neuron returns to -70mV
DEF: nerve impulse = the propagation of action potentials along a nerve fiber (i.e. the entire length of the neuron)
Refractory Period = the period following NI when
All of nothing response = if a nerve cell responds at all, it responds completely
Summation = many subthreshold stimuli received one after another may allow threshold potential to be reached, trigger an AP and begin a NI on a neuron
Conduction = the manner in which the NI runs down the neuron/nerve fiber
synapse - the junction between two neurons where a nerve impulse is transmitted
NI reaches axonal terminal of pre-synaptic neuron causing depolarization of synaptic knob
Ca++ channels open and calcium ions rush into axonal terminal causing
synaptic vesicles (filled with neurotransmitter/NT) to release NT via exocytosis into the synaptic cleft
NT diffuses across synaptic cleft and depolarizes the post-synaptic neuron's membrane
an action potential is triggered and a NI begins in the post-synaptic neuron
some neurons produce/release only one while release many
most typical NT is acetylcholine (ACh)
at least 30 different produced by CNS
other NRs include
Fate of neurotransmitter in synaptic cleft
act as NTs or neuromodulators that either
include enkephalins
synthesized by CNS neurons
include endorphines
Alzheimers = deficient ACh
Clinical depression = deficient norepinephrine/ seratonin
Epilepsy = excess GABA leads to excess norepinephrine and dopamine
Huntingtons disease = deficient GABA
Hypersomnia = excess serotonin
Mania = excess norepinephrine
Myasthenia gravis = deficient ACh receptors at NMJs
Parkinsons disease = deficient dopamine
Schizophrenia = deficient GABA leads to excess dopamine
SIDS = excess dopamine
Insomnia = deficient serotonin
Tardative dyskinesia (uncontrollable movements of facial muscles) = deficient dopamine
K+ = high outside
Na+ = high outside
Cl = high outside
negatively charged proteins or Anions = high outside
the cell membrane of a neuron must be depolarized (to approximately -55mV) in order for a certain ion channels to open and therefore start a nerve impulse
Therefore, a threshold stimulus = +15mV
the threshold potential for a neuron is -55mV
this event is called the action potential
the action potential represents the start of the nerve impulse on a neuron
(threshold stimulus)
+15mV stimulus
Na+ channels open (rapid depolarization)
action potential (nerve impulse) is produced
K+ channels open
(repolarization)
Na+ channels close
the NI is an electrical impulse
a NI is similar to a row of dominos falling
a nerve impulse begins on a dendrite (or cell body of a neuron), runs toward the cell body, through the cell body, and then down the axon
threshold stimulus cannot produce another NI
the RMP has to be restored before it can be depolarized again
subthreshold stimulus (5mV) = no AP; no NI
threshold stimuli (15mV) = yes AP; yes NI
threshold stimulus (20mV) = yes AP; yes NI, but no greater intensity than above
+15Mv = threshold = AP = NI
+5, +5, +5, = +15mV = threshold = AP = NI
unmyelinated nerve fibers: NI must travel the length of the nerve fiber; slow
myelinated nerve fiber: "saltatory conduction"
very fast transmission
Ni jumps from node of Ranvier to node of Ranvier
occurs between the axon of one neuron and dendrite or cell body of a second neuron
note that the two neurons do not touch. there is a gap between them = synaptic cleft
ACh is released by
all motor neurons (stimulate skeletal muscles)
some CNS neurons
monoamines (modified amino acids)
are widely distributed in the brain where they play a role in:
are present in some motor neurons of the ANS
include
unmodified amino acids
circadian rhythm
emotional behavior
epinephrine
norepinephrine
dopamine
serotonin
histamine
glutamate
aspartate
GABA (gamma aminobutyric acid)
glycine
destruction of neurotransmitter
reuptake of neurotransmitter
both of the above processes prevent continual stimulation of the post-synaptic membrane
enzymes that are present in the synaptic cleft destroy NT
for example, acetylcholinesterase
NT is transported back into pre-synaptic knob
alter a neurons response to a NT
block the release of a NT
synthesis is increased during painful stress
bind to the same receptors in the brain as the narcotic morphine
relieve pain
same as above, but with a more potent and longer lasting effect
Bones
the brain is encased by eight skull bones
the spinal cord is encased by approximately 26-30 bones called vertebrae
Meninges - the membranes around the brain and spinal cord
Brain
Spinal cord
Ventricles and Cerebrospinal Fluid
Dura mater
Arachnoid mater
Pia mater
tough, white fibrous CT
contains many blood vessels and nerves
note: DM splits into two layers where it encloses the dural sinuses (that collect venous blood from the brain)
outermost membrane that is attached to the inner periosteum of the skull
middle layer
thin net-like membrane
beneath the arachnoid mater lies a wide space called the sub-arachnoid space
this space is filled with cerebrospinal fluid (CSF) and serves as a cushion for the brain
very thin delicate CT
many nerves and blood vessels = nourishment
dips into grooves and contours
inner layer that clings to brain surface
CSF fills the subarachnoid space and central canal
the space between the dura mater and the bone is called the epidural (subdural) space and is filled with loose CT and fat
note that the dura mater is not attached to bone of the vertebrae (as in the brain where it is attached to the skull)
in addition to filling the subarachnoid space, the CSF fills the ventricles (interconnected cavities) within the cerebral hemispheres and brain stem
ventricles
Secretion and circulation of CSF
CSF
are continuous with central canal of spinal cord
are filled with cerebrospinal fluid
are lined by ependymal cells
CSF is secreted by specialized capillaries in choroid plexuses into the lateral ventricles (1 & 2)
CSF circulates down into the 3rd and 4th ventricle and then into either:
CSF is reabsorbed back into the bloodstream through arachnoid granulations that project into dural sinuses
CSF movement is aided by cilia of ependymal cells
the central canal of spinal cord
the subarachnoid space of meninges
Total volume in above spaces = 150 mL
Functions
about 1 liter is secreted daily to replenish the circulating 150 mL every 3-4 hours
mechanical protection (cushion)
chemical protection (ions, hormones)
cross-sectional anatomy of spinal cord - butterfly wings (gray matter) surrounded by white matter
other important features
Gross structure of spinal cord
note cervical and lumbar enlargements
note: cauda equina (horses tail) in which the lower lumbar and sacral nerves travel downward (lower spinal nerves muse chase their points of exit)
contains 31 segments (and therefore gives rise to 31 pairs of spinal nerves)
note: filum terminale that represents distal portion of the tail (pia mater)
length = about 17 inches
start = foramen magnum
end = tapers to point (conus medullaris) and terminates near the intervertebral disc that separates the 1st - 2nd lumbar vertebra
Gray matter or "butterfly" - bundles of (interneuron) cell bodies
Note location of
White matter = myelinated (interneuron) axons
lateral horns
anterior (ventral) horns
posterior (dorsal) horns
central canal (lined by ependymal cells)
gray commissure
anterior median fissure
posterior median fissure
Locations
the white matter of the spinal cord represents the location of our major nerve pathways called "nerve tracts"
lateral funiculi or white column
anterior (ventral) funiculi or white column
posterior (dorsal) funiculi or white column
provide a 2-way system of communication
in general, ascending tract are located in the posterior (dorsal) columns and conduct sensory (afferent) impulses from body parts to brain
in general, descending tracts are located in the anterior (ventral) columns and conduct motor (efferent) impulses from brain to effectors
general characteristics of nerve tracts
most consist of 2-3 successive neurons
most exhibit somatotropy (i.e. tracts from/to upper body are located on outside, tracts from/to lower body on inside)
most cross over
all pathways are paired (right and left)
ventral root
dorsal route
the fusion of the dorsal and ventral roots designates the beginning of the spinal nerve which then passes through its intervertebral foramen
dorsal root ganglion
ganglion = aq bundle of cell bodies outside the CNS
DRG contains the cells bodies of sensory (afferent) neurons bringing impulses to the CNS