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