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Thursday, September 27, 2007

Fundamentals Of The Nervous System And Nervous Tissue

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FUNDAMENTALS OF THE NERVOUS SYSTEM AND NERVOUS TISSUE

Characteristics & Functions
Rapid control and communication
Functions:
Sensory—monitors change through receptors that detect stimuli
Integration—analyses sensory information decides what should be done.
Motor—initiates response by activating effectors (muscles, glands or
organs that respond)
Organization of the Nervous System
There are two branches or principle divisions:
Central Nervous System (CNS)
Composed of the brain and spinal cord
It's function is integration and command.
Peripheral Nervous System (PNS)
Composed of:
Cranial nerves—brain
Spinal nerves—spinal cord
It's function is that it provides the network
Body parts are connected to the CNS by the PNS


LEVELS OF ORGANIZATION IN THE NERVOUS SYSTEM:

Peripheral nervous system: is divided into two divisions
Sensory or Afferent (towards) division—function is to carry messages
from receptors to the CNS.
Motor or Efferent (away from) division—function is to carry messages
from the CNS to effectors. It is divided into two systems:
Autonomic Nervous System (Involuntary)—It controls the heart rate,
digestion rate, breathing rate, hormone production, etc. It is
composed of two divisions:
Sympathetic division—your fight or flight division—prepares you for
emergency response.
Parasympathetic division—rest and digest division—conserves energy,
promotes non-emergency functions.
Somatic Nervous System (Neuromuscular junction)—It controls body
parts that are voluntary such as skeletal muscle.


TISSUES OF THE NERVOUS SYSTEM

Nervous tissue is composed of two types of cells:
Neurons (nerve cells) that transmit the electrical messages or
impulses.
Supporting cells or neuroglia or glial cells (nerve glue) that
supports and protects the nervous system.
Types of supporting cells
Cells in the CNS
Astrocytes:
Star shaped and very abundant.
There function is to form barriers between the blood and the neurons.
The blood brain barrier slows the movement of unwanted materials from
the blood into the brain.
Microglia:
Their function is to engulf and destroy microbes and dead or foreign
materials.

Ependymal cells:
They line the cavities in the brain and spinal cord.
Their function is that they posses cilia that circulate cerebro
spinal fluid (CSF)
Oligodendrocytes form a protective coating around the nerve fibers.
They wrap cell extensions around the nerve fiber.
Their function is to produce a myelin sheath. Myelin sheath is a
fatty layer for electrical insulation.
Cells in the PNS
Schwann cells
The whole cell wraps around the nerve fibers.
Their function is also to form a myelin sheath.
Satellite cells:
Their function is they may play a role in controlling the chemical
environment.


NEURONS

General Characteristics
They are specialized to receive and transmit nerve messages
They live a long time
They do not divide
They need oxygen and glucose constantly
The cell body contains the nucleus, cytoplasm and organelles (Peri
Karyon {area around nucleus} and Soma)
Nissi bodies—the rough ER
Neurofibrils—filaments for intracellular transport
Gray Matter—refers to the clusters of cell bodies
Nuclei—clusters in the CNS
Ganglia—clusters in the PNS
Cell processes (extensions)—long, thin bundles going to and from the
cell
Dendrites
Receive impulses from other neurons
Their numbers are a lot
Axons
Initiate and carry impulses away from the cell
There is only one per cell but it may branch. NOTE: A neuron is
excited by other neurons when their axons release neurotransmitters.
Axon hillock—region of the cell body where the axon leaves.
Axon terminals—synptic knobs—at the end of the axon, stores
neurotransmitters.
Synaptic cleft—gap between the cells
Myelinated fibers—axons covered with a myelin sheath.


MOST NERVE FIBERS ARE MYELINATED

In PNS Schwann cells wrap tightly around the fiber to form the
sheath. This pushes the nucleus and the cytoplasm to the outside.

Neurilemma—the external portion of the Schwann cell containing
the

Nucleus and cytoplasm.

Myelin—inner layer that has a high fat content.

Myelin = white matter

In CNS Ogligodendrocyte the whole cell does NOT wrap around the axon.

NOTE: These lack a neurilemma. The neurilemma is important for
regrowth of injured cells. Injury to CNS fibers has o regeneration.

White matter—is from the axon

Gray matter—cell bodies


STRUCTURAL CLASSIFICATIONS OF NEURONS:

Multipolar
Most common type
Major neurons of the CNS, they are found in the brain and spinal cord
Functions as motor neurons in the CNS and to transmit messages to
effectors
Composed of many dendrites and one axon.
Bipolar
Rare—found in the retina and the nose
Composed of one dendrite and one axon
Unipolar
One short process that acts as both a dendrite and an axon
Composed of two branches:
1 functions as a dendrite
The other functions as an axon
Major function is sensory


FUNCTIONAL CLASSIFICATIONS OF NEURONS:

Sensory—afferent—function is to transmit nerve impulses from
receptors to the CNS.
Motor—efferent—function is to transmit nerve impulses from the CNS to
effectors
Interneurons—association—function is to relay between the sensory and
the motor neurons.
NOTE: 99% of all neurons are interneurons.

Example: They make up the brain and spinal cord.


NEUROPHYSIOLOGY

Basic physiological properties of neurons:
Irritability—ability to respond to stimuli and convert the stimulus
to a nerve impulse.
Conductivity—ability to transmit the impulse to another cell.
Note: when a neuron is stimulated by a threshold stimulus, an
electrical impulse is conducted down the length of the cell.

Relative distribution of ions (mEq/L) in the intracellular fluid
(ICF) and the extracellular fluid (ECF)
NA+ (sodium) higher concentration outside

K+ (potassium) and A- (other anions) are higher inside

Anions are negative and cations are positive

Resting membrane potential
Resting neuron—a neuron that is not transmitting a nerve impulse.
Physiology of a resting neuron
Polarization (polarized)—electrical charge or difference across the
cell membrane
Cytoplasm—is negative
Fluid just outside the membrane—is positive
Resting membrane potential
Voltage (of the charge) across the membrane is measured in
millivolts.
Measured with microelectrodes.
Negative resting potential—inside is negative compared with outside.
NOTE: this is approximately 70 millivolts (mV) of negative charge.

What creates the resting membrane potential?
There are many large molecules inside the cell which are too big to
diffuse out, these are large Big Fate Anions = A-
The sodium – potassium pump
Active transport mechanism
Pumps
3 NA+ out of the cell
2 K+ in to the cell
NOTE: more + charged ions on the outside

K+ is in higher concentration inside the cell than outside the cell
therefore K+ has a tendency to diffuse out.
NOTE: when these K+ migrate or move out through the membrane, the
negatively charge ions cannot move through the membrane and line up
along the inside

THIS CREATES THE RESTING MEMBRANE POTENTIAL.

Action potential—electrical membrane that is transmitted along a
membrane.
General terminology
Stimulus – causes a nerve signal to be generated
Examples: touch, temperature, light, sound & chemicals

Stimulation – changes the permeability of the plasma membrane by
causing sodium gates to open
Note:

Na+ ions rapidly diffuse into the neuron, increases the number of
positive ions on the inside.
This causes depolarization.
Depolarization – inside the cell membrane becomes less negative and
the outside becomes less positive (depolarized sending the message)
This results in a change in the membrane potential from negative 70
to negative 50.
If this change reaches approximately -50 mV, the membrane potential
has reached threshold.
The stimulus must be strong enough to reach threshold in order for
the message to be sent.
Action potential, nerve impulse, is generated if depolarization
reaches threshold.
NOTE:

If a stimulus is below threshold, depolarization does not reach -50.
NO ACTION POTENTIAL is generated, only local depolarization.
Repolarization
A return to the resting potential occurs as soon as the action
potential has been reached which is positive 30.
Sodium gates close
Potassium gates open, K+ rapidly diffuses out of the neuron
Outside quickly becomes positive again and is repolarized
Hyperpolarization – recovery is so quick that there is a momentary
excess of K+ outside the membrane (temporary overshoot) – inside is
more negative than negative 70.
NOTE: after hyperpolarization, sodium – potassium pump returns ions
to their original place.

Refractory period – a neuron cannot be stimulated again until the
resting potential is restored.
All or none response
Action potential – once generated is self-propagating, spreads down
the entire cell.
All or none means if a stimulus is of threshold strength, the action
potential (nerve impulse) is never partially transmitted.
NOTE:

If stimulus is below threshold no message is sent
Strong stimulus causes nerve impulse to be generated more frequently
Speed of impulse conduction along the neuron
Myelinated fibers – transmit the impulse faster due to the impulse
jumping the nodes.
Salutatory conduction – the impulse jumps from node to node
Multiple sclerosis – when the sheath is destroyed so the impulse is
not sent.
Unmyelinated fibers transmit the impulse slower because the impulse
is transmitted down the entire length with no skipping.
Warm fibers conduct impulses faster than the cooler ones.
NOTE: cold partially blocks pain messages

Large diameter fibers conduct impulses faster than small diameter
fibers because there is less resistance.
Substances/drugs that affect the action potential, many substances
affect nerve transmission by influencing permeability.
Calcium
Acts as "Guard" for sodium channels, required to close Na+ channels
Deficient in Ca+ = Na+ ions may diffuse in repeatedly causing
impulses to be generated again and again and could lead to cramps.
Procaine, cocaine, novacaine
Decrease membrane permeability to sodium
Prevents nerve impulses from being sent
Uses as a local anesthetic
What is their mode of action?

They prevent the sodium gates from opening so a message cannot be
sent.


THE SYNAPSE

Introduction
A nerve impulse is an electrical signal
To be effective, the impulse must be transmitted along the axon and
to another cell.
Cells or structures that may receive the message include:
Muscles
Organs and glands
Other nerve cells
Synapse – the junction between a nerve cell and another cell.
Example: 2 neurons.
Presynaptic neuron – sending neuron, its axon sends impulse to
another cell.
Postsynaptic neuron – receiving neuron, its dendrites transmit
impulse from a cell.
Types of synapses
Electrical
Cytoplasm of adjacent neurons are connected, there is no gap
Ions travel from one cell to the other without a neurotransmitter.
Example: some regions of the brain for stereotyped movements.
Chemical
Release of chemicals relays the message
Most synapses are this type
Transmission depends on chemical neurotransmitters
Structures of the chemical synapse:
Synaptic cleft – narrow gap between the cells
Synaptic vesicles – store the neurotransmitter in the axons.
NOTE: a nerve impulse traveling down the axon causes the vesicles to
release the neurotransmitter when calcium enters.


NEUROTRANSMITTERS – GENERAL INFORMATION

Affects of neurotransmitters vary: some are excitatory and some are
inhibitory

Excitatory neurotransmitters – open Na+ gates, increase the chance of
post-synaptic cell being depolarized
NOTE: causes hypopolarizationn = hyperexcitable – makes it easier for
the nerve to send a message.

Inhibitory neurotransmitters
Open K+ gates, allows K+ to leave the cell
The exiting of K+ decreases the positive ions on the inside
Resting membrane potential becomes more negative
NOTE: causes hyperpolarization = hypoexcitable makes it harder for
the nerve to send messages

Postsynaptic potentials:
excitatory postsynaptic potentials (EPSPs)
caused by excitatory neurotransmitters, cause hypopolarization
excitatory neurotransmitters cause these messages to be sent more
easily
inhibitory postsynaptic potentials (IPSPs)
caused by inhibitory neurotransmitters
inhibitory neurotransmitters cause these messages to be sent more
slowly
NOTE:

a receiving neuron may get input from hundreds of other neurons; some
sending EPSPs and some IPSPs
the sum of all their effects determines what the receiving cell will
do.
Termination of neurotransmitters
Neurotransmitters act only briefly and then they must be terminated

Types of termination mechanisms=

Neurotransmitter is destroyed by enzymes
Neurotransmitter is transported back to the axon
Neurotransmitter diffuses into the receiving neuron
Types of neurotransmitters:
Acetylcholine (Ach)
Best known and studied
Only neurotransmitter at neuromuscular junction causes skeletal
muscles to contract.
Found in many areas of the brain
ACh examples/comments:
Curare – (South American poison arrow frog) – it blocks ACh receptors
on skeletal muscles causing paralysis
Insect poisons – inhibit AChase – can cause seizures or spasms, may
play a roll in Alzheimer's disease.
Alzheimer's disease – some groups of neurons in the brain that
produce ACh may be destroyed
Biogenic amines
Catecholamines: epinephrine (epi = E), norepinephrine (NE), dopamine
(DA)
Dopamine – widely distributed in the brain and affects muscle
activity and good feelings
NOTE: dopamine plays an important role in inhibiting muscle
contraction.

Parkinson's disease – not enough DA

Muscle tremors and rigid muscles are associated with this disease
Certain DA producing neurons degenerate
Schizophrenia too much DA

Epinephrine (E) and norepinephrine (NE) – prepare the body to respond
to stress
NOTE:

NE affects mood and causes good feelings
Drugs that block NE secretion cause depression
Indolamine: serotonin (S) – affects moods and sleep patterns.
NOTE:

Too little could cause anxiety attacks and depression.
Treatment, Prozac.
Antidepressant drugs prolong effects of some neurotransmitters.

Tricyclic block transport of NE & S and remove them from the synapse.
Example: Elavil and Cocaine

MAO (monoamine oxidase) inhibitors inhibit NE
Amino acids are found in the CNS

Glycine and GABA are inhibitory
Tranquilizers bind to GABA receptors and mimic it's effects

Example: Valium

Glutamate – excitatory
NOTE: There are high levels of this in stroke victims.

Peptides

Endorphins and enkephalins reduce the perception of pain (natural
pain killers)
Examples:

Enkephalins increase during labor and delivery.

Endorphins produce a runners second wind.

Narcotics produce euphoria by attaching to the same receptors as our
natural pain killers.
Examples: Morphine, Heroin

Substance P, the neurotransmitter for sending pain signals


THE CENTRAL NERVOUS SYSTEM

Brain
Characteristics:
Pinkish-gray, wrinkled
Texture of cold oatmeal
3 – 3.5 pounds in a typical adult
Intelligence
NOTE:

Size is not related to intelligence
Connections between neurons determines intelligence
100 billion neurons in the brain and each can be connected to as many
as 1000 others.


REGIONS OF THE BRAIN (4)

Ventricles
Cerebral hemispheres (cerebrum)
Diencephalon:
Thalamus
Hypothalamus
Epithalamus
Brain stem:
Midbrain
Pons
Medulla oblongata
Cerebellum


VENTRICLES

Introduction
NOTE: the brain is not solid
Ventricles are hollow chambers within the brain connected to each
other and the spinal cord canal.
Cerebrospinal fluid (CSF) fills these chambers.
Anatomy of the ventricles
Lateral ventricles, each hemisphere has:
One lateral ventricle
Interventricular foramen which is a small opening in each lateral
ventricle that leads to the third ventricle.
Third ventricle is a slit in the diencephalon
NOTE: the third ventricle is connected to the:

Lateral ventricle via the interventricular foramen
Fourth ventricle via the cerebral aqueduct
Fourth ventricle
Between the brain stem and the cerebellum
Openings in its walls lead to:
Central canal of the spinal cord
Subarachnoid space which is a fluid filled space sorrounding the
brain.


CEREBROSPINAL FLUID

Characteristics:
Liquid cushion in and around the brain and spinal cord
Function is to absorb shocks, float the brain and help nourish the
brain
Composition is 99% water
Source: choroid plexus which is a clust of capillaries in each
ventricle that continuously makes CSF.
Circulation of CSF
Ependymal cells are ciliated to create a current to circulate the
fluid.
Route of CSF is from the ventricles, in and around the spinal cord
and around the brain.
Obstructions would cause the fluid to build up
Hydrocephalus – water on the brain (p. 467)


CEREBRAL HEMISPHERES

Cerebrum – divided into 2 halves

External anatomy:
Gyri (singular is gyrus) are the outward folds
Grooves:
Sulci are shallow grooves
Examples:

Central sulcus which divides the frontal lobe from the porietal lobe.
Lateral sulcus which outlines the temporal lobe
Parieto-occipital sulcus (not visible externally)outlines the
occipital bone or suture.
Fissures are deep grooves
Example – longitudinal fissure seperates the two hemispheres.

Lobes – named after bones they're beneath:
Frontal
Parietal
Occipital
Temporal
Internal anatomy or regions
Composed of 3 general regions:
Cerebral cortex – thin outer layer of gray matter
White matter is on the inside and connects the two hemispheres.
Basal nuclei – island clusters of gray matter scattered throughout
the white matter.
Cerebral cortex
Produces our most distinctive human skills
Examples:

Speech
Emotions
Intelligence
Brodmann (1906) mapped functional areas of the cerebral cortex
3 types of functional areas
Motor – control of voluntary movement
Sensory – receives messges
Association – interpretation, analysis of information
Motor areas
Primary motor area – sends commands to the skeletal muscles
NOTE: The right hemisphere controls the left side of the body

Precentral gyrus (4){Brodman's numbering} contains the primary motor
area
Motor homunculus – little man drawn on gyrus – to represent the body
region controlled by that area of the brain
Broca's area – planning speech and speaking
Overlaps Brodmanns area 44 & 45, present in only one hemisphere,
usually left
Sensory areas
3 neurons minimum:
Receptors to spinal cord
Spinal cord to thalamus
Thalamus to the cortex
Primary somatosensory cortex
Receives signals from body receptors for touch, pain, temperature and
pressure – spatial discrimination.
Postcentral gyrus (1 – 3) behind the central sulcus and contains the
primary somatosensory cortex
Somatosensory homunculus – amount of cortex dedicated to an area is
related to how sensitive the area is.
Somatosensory association area (5 – 7) – interprets the incoming
sensory information.
NOTE: It works on prior knowledge.

Primary visual cortex – receives information (impulses) from eyes
Visual association area, interprets what we see
Occipital lobe
Primary auditory area – hearing
Temporal lobe
Olfactory cortex – smell
Medial part of temporal lobe
Gustatory cortex – taste
Parietal lobe
Wernick's area
Speech area
Sounding unfamiliar words
Probably not the complex language comprehension center
Association area – makes sense of the incoming information

NOTE:

Sites of higher mental activities "thinking" and understanding
abstract ideas
Use the various inputs to make judgements, evaluate consequences,
planning
Necessary for reasoning & concerns
Especially important – prefrontal cortex – sets humans apart because
it gives us intelligence.
Lateralization – each hemisphere has its own unique properties
The 2 cerebral hemispheres differ in structure and function
Their primary motor and sensory areas dominate different sides of the
body
Left and right association areas function differently
Left hemisphere is in charge of language, math, logic, science,
analyzing, reasoning and memorizing.
Right hemisphere has greater control over music, art, poetry and
creative design.
Cerebral white matter—for communication between the cerebral
hemispheres and other CNS centers
Corpus callosum—thick band of nerve fibers that connects the
hemispheres and allows them to function as a whole.

Basal nuclei
Islands of gray metter in the white matter
Important in motor coordination; like starting and stopping.
Parkinson's disease—could result from problems with the basal nuclei.


DIENCEPHALON: THALAMUS, HYPOTHALAMUS, EPITHALMUS

Thalamus
Forms the side walls of the third ventricle
Two masses of gray matter joined by a bridge of gray matter called
the gray commissure
Function—relays all sensory information (except smell) to the
cerebral cortex.
NOTE: 3 neurons:

Receptor --- spinal cord --- thalamus --- primary somatosensory
cortex

Hypothalamus
Forms the floor of the third ventricle
Small but important
Function helps regulate homeostasis
Controls hormone secretion by the pituitary gland
Regulates organ systems to maintain homeostasis
Influences eating, drinking, sleeping and body temperature
Helps us experience emotions, pleasure, fear, anger
NOTE: connects the nervouse and the endocrine systems

Epithalamus
Forms the roof of the third ventricle
Contains pineal gland and associated with
Melatonin
Chemical that regulates our biological clock


BRAIN STEM: MIDBRAIN, PONS, MEDULLA OBLONGATA

NOTE: houses all the sensory and motor neurons between spinal cord
and upper brain regions.

Midbrain—top of the brain stem
Cerebral peduncles (paired & ventral)—contain motor nerves going down
to the spinal cord.
Corpora quadrigemina (4 & dorsal)
Visual reflex centers –moving eyes and head when you visually follow
a moving object or if you reflexively turn to look at something.
Auditory reflex centers—moving head towards sound
Pons—the enlarged region of the brainstem
NOTE:

Mainly fibers that connect the various parts of the brain to the
spinal cord
Respiratory centers: help control breathing
Medulla oblongata
Lowest part of brainstem, blends into the spinal cord
Pyramids—2 ridges that contain motor nerve tracts that cross to
opposite sides of the brain.
Decussation of pyramids:

Point where most of these fibers cross
Each hemisphere controls the opposite side of the body
Three vital reflex centers:
Cardiac center—controls the heart
Vasomotor center—regulates blood pressure by controlling the diameter
of the blood vessels
Respiratory center—affects breathing
Non-vital reflexes—vomiting, hiccuping, coughing and sneezing.
NOTE: a blow to the medulla can be fatal


CEREBELLUM

Anatomy of the cerebellum
Two hemispheres—have fine parallel gyri
Vermis—worm like connection between the two hemispheres
Cerebellar cortex—the outer layer of the cerebellum
Arbor vitae—when cut, inside surface looks like a branching tree
Functions—planning center for subconscious events
Makes movements that are complex & smooth. EX: driving a car, riding
a bicycle, typing and skating.
Helps maintain posture
Maintains balance and equilibrium (information from inner ear)
Receives information about muscle tension


FUNCTIONAL BRAIN SYSTEMS

Work together but are not localized in a specific region

Limbic system
Location—structures in cerebrum and diencephalon that encircle the
upper brain stem
Function—responsible for emotion and memory
NOTE:

Origin in primitive smell are of the brain
Smells cause emotions and memories
Reticular system
Location—extends throughout the brain stem, connections to all areas
of the brain.
Function—regulates the reticular activating system (RAS)
Sends impulses to cerebral cortex which keeps it alert and conscious
Inhibited by sleep centers, alcohol and tranquilizers
Damge can produce unconsciousness or coma


PROTECTION OF THE BRAIN

Protective structures or fluids
Bones of the skull
Meninges which are membranes wrapped around the brain
Cerebrospinal fluid
Blood-brain barrier
Meninges—3 layers of membranes
Dura mater—"tough mother"
Outer layer (membrane)—double layered
Lines cranial cavity and is attached to the skull
Inward folds—to anchor the brain
Subdural space—containing serous fluid
Arachnoid mater—arachnoid is "spider web"
Thin net like covering
Subarachnoid space—filled with CSF
Pia mater—"gentle mother"
Delicate
Adheres to the surface of the brain
Meningitis
Inflammation of meninges
Caused by viruses or bacteria
May spread to the brain
Swelling around brain—a stiff neck, fever and headache
Encephalitis—inflammation of the brain
Blood-brain barrier
Brain capillaries (tiny blood vessels) differ from others in the body
How? Less permiable

Capillaries allow nutrients (glucose) in but keep others out
Excludes non-essential chemicals: drugs
Fat-soluble material can diffuse through. EX: anesthetics, nicatine
and alcohol.


TRAUMATIC INJURIES TO THE BRAIN

Can cause damage

Concussion—slight injury with mild symptoms but no permanent damage.
Contusion—marked tissue destruction with variety of symptoms
including a coma


DEGENERATIVE DISEASES

Cerebrovascular accident (CVA) "stroke"—occurs when a local region of
the brain has neuron death from ischemia
Ischemia—a lack of blood

Alzheimer's disease—it could result from a gene that causes neurons
to die
SPINAL CORD

Characteristics:
Located within the vertebrae
Continuous with the medulla oblongata
Extends from foramen magnum to bottom of 1st lumbar vertebra (17
inches long; thumb width wide)
Functions:
Transmits messages to and from the brain
Forms reflexes
Protection—bone, CSF & meninges
Meninges:

Dura mater forms sheath around the spinal cord
Epidural space between the dura mater and the bone
NOTE:

The epidural space is filled with fat and blood vessels
"saddle block" is given to block pain messages in this space
Arachnoid
Subarachnoid space
Filled with CSF
Below L3, is site for spinal taps (lumbar punctures)
Pia mater—the inner layer
Anatomy of the spinal cord and associated structures
4 general regions—cervical, thoracic, lumbar and sacral
Specific regions posterior to the lumbar region:
Conus medullaris—the cone shaped bottom of the spinal cord
Cauda equina—the horse tail, nerve fibers below the spinal cord
Filum terminale—the fibrous anchors for the spinal cord
Spinal nerves – their exit from the spinal cord
31 pair of spinal nerves exit via intervertebral foramina – go to the
body parts.
cervical enlargement goes to the arms
lumbar enlargement goes to the legs
cauda equina
spinal nerves below L1 angle down before exiting the foramina.
Collection of nerves at the bottom
cross-section anatomy – the spinal cord is a flat oval
two grooves divide the spinal cord into a right and a left half
anterior median fissure which is deep
posterior median sulcus which is shallow
gray matter and spinal roots
gray matter: H-shaped – butterfly shape
gray commissure – is the median cross bar
posterior (dorsal) horns are cell bodies of sensory neurons coming in
anterior (ventral) horns are cell bodies of motor neurons going out
to muscles
later horns are cell bodies of motor-neurons going out to organs
ventral root is where axons of all motor neurons leave
dorsal root is where sensory neurons enter
dorsal root ganglion – enlargement containing cell bodies of sensory
nerves

NOTE: dorsal and ventral roots are short and fuse to form the spinal
nerve at the ganglion.

white matter is mostly myelinated nerve fibers
funiculi – columns of white matter on each side of the spinal cord
examples – posterior, anterior, lateral

NOTE: each column contains several fiber tracts which are axons with
similar destinations.

ascending tracts conduct sensory messages up
descending tracts deliver motor impulses down.
THE PERIPHERAL NERVOUS SYSTEM

PNS overview – links the central nervous system to the various parts
of the body.
NOTE: consists of nerves running to and from the CNS

Nerve – cord like bundle of axons
Connective tissue associated with nerves: (see diagram below)
endoneurium – connective tissue around each axon
fascicles – a bundle of axons

perineurium – connective tissue around each fascicle
epineurium – connective tissue around all the fascicles
Functional divisions of nerves:
sensory – carry messages to the CNS
motor – carry messages away
*Most nerves are mixed, have both sensory and motor

CRANIAL NERVES

Function and Location:
connect the brain with receptors, muscles and glands
mainly in the head, face and neck
12 pairs – viewed from ventral (under) side of the brain
names of the cranial nerves
NOTE: Roman numerals indicate their order anterior to posterior

I – olfactory

II – optic

III – oculomotor

IV – trochlear

V – trigeminal

VI – abducens

VII – facial

VIII – vestibulocochlear (auditory)

IX – glossopharyngeal

X – vagus

XI – accessory (spinal accessory)

XII – hypoglossal

Mnemonics: "Oh Oh Oh, To Touch And Feel A Girl's Vagina Ahh Heaven"

functions of the cranial nerves:

I – olfactory (sensory) transmits sensory (smell) impulses from the
nasal cavity through the cribriform plate of the ethmoid bone.

II – optic (sensory)

transmits sensory impulses for vision
lead from retina through optic foramen into cranial cavity
some fibers cross over to the other side at the X shaped optic
chiasma
NOTE: damage to the optic nerve would result in blindness

III – oculomotor (motor)

control most of the muscles that help move "focus" the eye.
Internal muscles that move the eyeball in its orbit
Ptosis – upper eyelid droops because the nerve is damaged
IV – trochlear (motor) pulley

only ones to emerge from dorsal side
control eye muscles
V – trigeminal (both) the largest nerve with three branches

- motor portions to muscles involved in chewing

- sensory the major sensory nerve of the face

- 3 divisions

(1) opthalmic – transmit sensory impulses from the scalp,

The eye and nose.

Testing – corneal reflex – is that anything
touching

Cornea will cause blinking and tearing.

maxillary – transmit sensory impulses from the middle of the face.
Mandibular – transmit sensory impulses from the bottom of the face
NOTE: inflammation – trigeminal neuralgia – tic douloureux –
excruciating pain of unknown cause

VI – abducens (both) supply nerves to muscles which cause
lateral eye

Movements

VII – facial (both)

sensory from the taste buds on the front of the tongue
motor main motor nerve of the face – control facial expressions and
glands
5 branches:

(1) temporal

(2) zygomatic

(3) buccal

(4) mandibular

(5) cervical

NOTE: inflammation – Bell's palsy paralysis of facial
muscles on 1 side

Results in drooping eye lids, sagging mouth and dripping
tears

VIII – vestibulocochlear (sensory) – sensory nerves from
the inner ear

vestibular branch for balance
cochlear branch sound
IX – glossopharyngeal (both)

motor – controls swallowing
sensory – transmits taste & sensation from the back of the mouth
X – vagus (both)

NOTE: only cranial nerve that goes down to chest and abdomen,

Called the wanderer.

motor fibers
go to the throat and larynx (swallowing & speech) – if it is damaged
you would have problems swallowing or speaking
regulate – breathing, heart rate and digestion
sensory portion – transmits sensory impulses from the same organs
NOTE: damage – you would die; heart could not beat and you could not
breath.

XI – accessory (motor)

motor fibers to throat & to muscles involved in turning the
head and shrugging the shoulders

XII – hypoglossal (both)

motor fibers to the tongue
SPINAL NERVES

Distribution of spinal nerves

31 pairs – all mixed – both motor and sensory
Categories:
cervical nerves – 8 pairs – exit above each vertebrae
thoracic nerves – 12 pairs – exit below
lumbar nerves – 5 pairs
sacral nerves – 5 pairs
coccygeal nerves – 1 pair
Nerve roots – emerge from the spinal cord
ventral root – contains motor fibers
dorsal root – contains sensory fibers
NOTE:

these 2 roots fuse to form a spinal nerve
the spinal nerve exits the vertebral column through the foramina
once outside the intervertebral foramen each spinal nerve branches to
form rami
rami:
dorsal ramus – is small and serves the back
ventral ramus – serves rest of the body
meningeal ramus – reenters vertebral canal to innervate the meninges
Innervation of the Anterolateral Thorax and Abdominal Wall
Intercostal nerve is a branch of the thoracic nerve

NOTE: the ventral rami of all other spinal nerves except thoracic
intertwine and criss-cross to form a plexus

Plexuses Serving the Neck and Limbs
cervical plexus – neck
located deep in neck under sternocleidomastoid
phrenic nerve – motor fibers to the diaphragm
NOTE:

irritation causes hiccups – no cure
both phrenic nerves destroyed – you cannot breath on your own.
brachial plexus – arms
location – in shoulder between the neck and arm pit
supplies – most of the nerves of the arm
major nerves:
axillary – is in the arm pits
musculocutaneous – muscles that flex the forearm
median – muscles that pronate the forearm and flex the wrist and
fingers
ulnar – behind medial epicondyle of the humerus
radial
triceps brachii
compression would result in the inability to extend the hand at the
wrist
lumbar plexus – is in the small of the back
serves most nerves of the thigh
femoral nerve serves the quadriceps
NOTE: if slipped disc compresses lumbar plexus you could have trouble
walking

sacral plexus above the sacrum
nerves that serve the buttocks and legs
sciatic nerve – the thickest, longest nerve in the body
its branches supply all the muscles of the leg
injury
impairs the lower limbs
severed you could be paralyzed
sciatica – stinging pain in you leg
Innervation of the Skin, Dermatomes and Referred Pain
general statements
cranial nerves send branches to the skin of the face and scalp.
all spinal nerves except C1 send branches to the skin of the rest of
the body
dermatomes area of skin served by a spinal nerve
referred pain
nerves serving certain internal organs and dermatomes to the same
region of the spinal cord

NOTE: it is not understood why, but the brain may
interpret pain

as coming from a dermatome or skin area

example – heart pain – is felt as pain in the left arm


REFLEX ACTIVITY

Spinal cord has 2 main functions:
carry information to and from the brain
form reflexes
reflexes – rapid, predictable, involuntary responses to stimuli
types of reflexes:
autonomic reflexes – not conscious of these – they control visceral
activities (digestion, urine, eggs, hormones, sperm, etc.)
somatic reflexes
involves stimulation of somatic sensory receptors
aware of these
example to touch a hot stove
reflex arcs – the neural pathways that reflexes travel
components:
receptors – is the site of the stimulus
sensory neuron transmits the impulse to the spinal cord
integration center within the spinal cord relays the information to
the motor neuron
motor neuron transmits the impulse from the integration to the
effector
effector – the muscle, gland or organ that responds
types of reflexes:
monosynaptic – only one synapse
simplest type
sensory neuron synapses directly with motor neuron in spinal cord
patellar reflex or knee jerk is an example
NOTE: most reflexes are more complicated and include one or more
inter-neurons in the spinal cord.


THE AUTONOMIC NERVOUS SYSTEM

Characteristics:

- Division of PNS

- Involuntary – regulates activities, not under our conscious
control

- Composed of motor neurons serving smooth and cardia muscle

- Important in maintaining homeostasis by regulating activity
of organs

Examples: heart rate, breathing, digestion, blood
pressure

- Functions of reflexes and may be effected by stress

- Composed of 2 sets of neurons with opposite effects

Parasympathetic division – most active under calm
conditions

Examples: lower heart rate and breathing

Sympathetic division "fight or flight" – active during
emergencies

Examples: speeds up heart rate and breathing

NOTE: these are opposite extremes; most organs receive both
parasympathetic and sympathetic signals which adjust the activities
to a suitable level


- Comparison of efferent pathways of the somatic and autonomic
nervous

System

Preganglionic – cell body lies in the brain or spinal
cord and its axon

Synapses with another neuron.

Parasympathetic = lonfiltered= short

Postganglionic – cell body lies in a ganglion outside the
CNS and

Its axon synapses with the effector

Parasympathetic = short sympathetic = long

Diagram & summary/comparison of somatic and autonomic nervous

Systems (see page 534)

PARASYMPATHETIC DIVISION (CRANIOSACRAL)

Neurotransmitter of the parasympathetic division is ACh

Note: both pre and postganglionic neurons release ACh

Cranial outflow = cranial nerves with parasympathetic function:

- Cranial nerve III innervates the eye muscles

- Cranial nerve VII innervates major glands of the face

Examples: the salivary glands

- Cranial nerves IX innervates the parotid salivary glands

- Cranial nerve X sends branches to several plexuses and
innervates many

Organs

Examples:

Cardiac plexuses – will go to the heart

Pulmonary plexuses – will go to the lungs

Esophageal plexuses – will go to the esophagus

Sacral outflow – sacral spinal nerves with parasympathetic function:

Where? S2 – S4 – innervate several visceral organs: to control
digestion

Examples: send preganglionic fibers to innervate several
visceral

Organs of the intestines, the bladder and the genitals.

SYMPATHETIC DIVISION (THORACOLUMBAR)

Neurotransmitters:

- Preganglionic fibers release ACh

- Postganglionic fibers release E and NE

- Norepinephrine (NE) = noradrenalin – the
neurotransmitter released

By the adrenal gland

- Epinephrine is released from the adrenal gland

NOTE: percentage of E:NE = 80:20

Sympathetic Complexity

NOTE: the sympathetic division of the autonomic nervous system
is much

More complex than the parasympathetic

Examples of structures under sympathetic control:
blood vessels,

Sweat glands, arrector pili muscles, heart, the lungs and digestive
organs

Origin, function & ganglia associated with sympathetic fibers

- Preganglionic neurons are found in spinal cord segments T1
through L2

- Sympathetic fibers from these neurons emerge from the
thoracic and

Lumbar regions of the spinal cord to form ganglia

- Chain ganglia – line each side of the vertebral column

- Collateral ganglia – are located in front of the
vertebral column

Chain ganglia fibers supply:

Examples: the salivary glands and the thoracic
glands

- Collateral ganglia and the organs their fibers supply:

Superior mesentric

The upper abdominal
organs:

Liver, gallbladder,
stomach, spleen

Celiac

Inferior mesenteric

The lower digestive
and reproductive

Organs: small
intestine, kidneys,

bladder, ovaries

Hypogastric

RECEPTORS OF THE AUTONOMIC NERVOUS SYSTEM

Cholinergic Receptors (parasympathetic) – receptors are responsive to
ACh

Types:

- Nicotinic – when nicotine binds to these, it produces
same effect

As ACh

ACh stimulates nicotine receptors

Types of nicotinic receptors (examples):

N1 – are found at all post ganglionic sites

N2 – are found at the neuromuscular junctions

- Muscarinic – these bind a poison (muscarine) from
mushrooms

Respond to ACh and produce similar effects

NOTE:

Usually excitatory but can be
inhibitory

Examples: digestive and sweat glands – E

Heart rate and blood pressure – I

Adrenergic "adrenal gland" (sympathetic) Receptors

Two types:

Alpha – is mainly stimulatory

Types:

Alpha 1

Alpha 2

Beta – is usually inhibitory

Types:

B1

B2

NOTE: knowing the location of cholinergic and adrenergic receptors
and their subtypes is useful.

Example: important clinical breakthrough was the discovery of
adrenergic blockers that attach beta blockers; are used to reduce
heart rate and prevent irregular heart beat.

SOURCE:Good Nurse's Club


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