Download Neuroanatomy Part 1

ANSC 2401 Lecture
Nervous System
Nerve Impulses
Resting Membrane Potential: This is the electrical voltage difference across the
the membrane of neurons. It is measured in millivolts. (-70mV)
Voltage Difference: Due to a slight buildup of negative charges in the
cytosol and an equal buildup of positive charges in the
extracellular fluid. This voltage difference is due to the unequal
distribution of ions across the plasma membrane and relative
permeability of the plasma membrane to Na+ and K+.
Unequal Ion Distribution
Extracelluar: Na+ and Cl- on outside (more Na+ ions)
Intracellular: PO43-, Cl-, and amino acids with a
negative charge (smaller concentrations of K+ ions)
Active Transport
Na+ is actively transported to the outside of the membrane
to maintain the resting membrane potential – three
Na+ are transported for each two K+
K+ is actively transported into the neuron
Action Potential (Impulse): A sequence of rapidly occurring events
(either a chemical or physical stimulus) that decrease and
eventually reverse the membrane potential (depolarization) and
then restore the resting membrane potential (repolarization). This
impulse travels along the membrane along the neuron’s cell
membrane.
Depolarization
Stimulation cause channels to open in the membrane at the
site of stimulation allowing Na+ to rush in. This causes the
inside (which was negative) to become positve.
If a stimulus cause the membrane to depolarize to the to a
critical level, called the threshold, the influx of Na+
becomes so large that membrane potential changes from
negative to +30mV. When this occurs the nerve fiber
cannot be stimulated again until repolarization is nearly
complete.
All or None Principle: The stimulus must be
strong enough for the nerve fiber to “fire” or
no impulse will occur. If the stimulus does
not cause depolarization to reach the action
potential then the nerve will not fire.
Repolarization
As the inflow of Na+ slows as the Na+ channel closes, it
cause the K+ channels to open, allowing the K+ to flow out
of the neuron. This reestablishes the resting membrane
potential.
Refractory Period: This is the time during which a cell cannot
generate another action potential. This takes about 1/2500
of a second for large diameter axons and 1/250 of a second
for small diameter axons.
Conduction of Action Potentials
Continuous Conduction: This is a step-by-step
depolarization of adjacent areas of the membrane.
This only occurs in unmyelinated axons.
Saltatory Conduction: This is the conduction in
myelinated axons. In this conduction the pulse
jumps from one Node of Raniver to the next.
Because the pulse jumps, it takes less time to travel
down the axon, therefore myelinated axons carry
the nerve impulse faster than unmyelinated axons.
Transmission at the Synapses: In order for the impulse to travel throughout the
nervous system the pulse not only has to travel along the axon, but it must
also be able to cross the synapses and reach the dendrites of the next
neuron.
Electrical Synapses
Ion current spread directly from one cell to another via gap
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junctions. The gap junctions have a series of tubular proteins
called connexons that form tunnels between the cells that allow the
ion current to flow from one cell to another.
Found in visceral smooth muscle, cardiac muscle, and developing
embryos.
Advantages
They are faster than chemical synapses.
They can synchronize the activity of a group of neurons or
muscles (heart or viscera).
Allow two-way transmission of impulses, unlike chemical
synapses.
Chemical Synapses
The presynaptic and postsynaptic neurons are separated by the
synaptic cleft filled with extracellular fluid.
Pulses cannot jump the cleft so there must be an alternate way to
cross this space. This is done with neurotransmitters that are
released by the presynaptic neuron, diffuse across the synaptic
cleft, and act on receptors in the membrane of the postsynaptic
neuron.
Operation of the Chemical Synapses
1. As the nerve impulse reaches the synaptic bulb of the
presynaptic neuron, depolarization causes Ca2+
channels to open (in addition to the Na+ channels).
2. Because Ca2+ is more concentrated in the extracellular
fluid, the Ca2+ flows into the presynaptic neuron.
3. The increase in Ca2+ cause the synaptic vesicles to
undergo exocytosis and release their neurotransmitters
into the synaptic cleft.
4. The neurotransmitters diffuse across the synaptic cleft
and bind to the neurotransmitter receptors in the
postsynaptic membrane.
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The neurotransmitters can either cause the postsynaptic neuron to
be excited and therefore bring the postsynaptic neuron’s membrane
closer to the threshold, or they can inhibit the postsynaptic
neuron’s membrane from reaching its threshold.
Neurotransmitters can be recycled by actively being transported
back into the neuron that released them (norepinephrine), or they
can linger in the synaptic cleft and produce excessive stimulation
(dopamine).
Neurotransmitters
PNS
Acetylcholine (ACh) can be either an excitatory or
inhibitory. It is also found in the CNS.
CNS
Glutamate and Aspartate are amino acids that act as
neurotransmitters that have an excitatory effect.
Gamma aminobutyric acid (GABA) and Glycline are
important inhibitory amino acid neurotransmitters. GABA
is used by ~ a third of the synapses in the brain, while
glycine is more prevalent in the spinal cord.
Norepinephrine, Epinephrine, and Dopamine are
catacholamine neurotransmitters and are both excitatory
and inhibitory.
Some simple gases such as Nitric Oxide (NO) and Carbon
Monoxide (CO) can function as neurotransmitters.
There are ~ 20 peptide neurotransmitters called
Neuropeptides that act as neurotransmitters. They are
found throughout the PNS and CNS and can be excitatory
or inhibitory. Many of these serve as hormones (Oxytocin,
Melatonin, ect.).
Autonomic Nervous System:
Regulates the activity of smooth muscles,
cardiac muscles and some glands.
The sensory components of the ANS consist of interoceptors that monitor body
functions such as carbon dioxide levels in the blood or stretching of organs and
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blood vessels. These signals are not usually perceived as conscious unless they
give rise to sensations such as nausea, a full urinary bladder or angina pectoris
(chest pain).
The motor part of the ANS is divided into the Sympathetic and Parasympathetic
divisions.
The first neuron as it leaves the CNS is called the preganglion. The second one
is called the postganglion.
Sympathetic Division
The nerve cell bodies of this division lie within the gray matter of the
thoracic and lumbar divisions of the spinal cord. Their axons leave the
spinal cord through the anterior root of a spinal nerve. Depending on
where they are located they are then distributed to glands, smooth
muscles, cardiac muscle, organs, ect.
Impulses from this division increase the activity of the area that they
innervate, preparing for fight or flight
Parasympathetic Division
The nerve cell bodies for this division are found within the brain stem or
the sacrum. Their axons leave as either cranial nerves or as the anterior
root of the spinal cord as a spinal nerve. They innervate most of the same
areas as the sympathetic division.
Impulses from this division decrease the activities of the area that they
innervate, slowing down after fight or flight.
Somatic Nervous System
Includes both sensory and motor neurons.
Sensory neurons convey information from the special senses (vision, hearing,
taste, smell, and equilibrium) via proprioceptors (muscle and joint position) and
general somatic receptors. All of these sensations are consciously percieved.
The motor neurons of the SNS innervate the skeletal muscles and produce
conscious, voluntary movements. These neurons are always excitatory.
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Cerebrospinal Fluid:
A clear fluid produced by the Choroid Plexuses (a
capillary network that extends into the ventricles). It flows through the ventricles,
central canal, and subarachnoid space. It surrounds the brain and spinal cord to
act as a shock absorber.
Meninges:
The coverings of the brain and spinal cord.
Dura Mater: The outer covering of the brain and spinal cord.
Periosteal (Outer) Layer is fuses to the periosteum of the skull.
Menigeal (Inner) Layer is the inner layer that comes in contact with the
Arachnoid layer.
Epidural Space is the space above the dura mater around the spinal cord,
between it and the vertebral canal.
Subdural Space is the space below the dura mater between the it and the
arachnoid layer filled with interstitial fluid.
Arachnoid is the middle layer of the meninges.
Subarachnoid Space is the space below the arachnoid and above the pia
matter.
Pia Mater is the inner layer of the meninges that adheres to the brain and spinal
cord.
Denticulate Ligaments are thickened extensions of the pia matter that
project laterally and fuse to the arachnoid and inner surface of the dura
mater along the length of the spinal cord between the dorsal and ventral
roots of the nerve roots of the spinal cord.
Filum terminale is an extension of the pia mater that anchors the spinal
cord.
Peripheral Nervous System:
The part of the nervous system that lies outside
of the CNS (nerves and ganglia).
Cranial Nerves: The first two originate outside the brain. The other ten
nerves originate from the brain stem and all exit the skull through the
foramina of the skull.
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Olfactory (CN I)
Location: Originates in the olfactory mucosa, pass through the
olfactory foramina and terminates in the olfactory bulb of
the frontal lobe of the cerebrum.
Function: Smell
Optic (CN II)
Location: Originates in the retina, passes through the optic
foramen, forms the optic chiasm, passes through the optic
tracts and terminates in the thalamus. From the thalamus
projections extend into the visual areas in the occipital lobe
of the cerebral cortex.
Function: Vision
Oculomotor (CN III)
Location: Motor – originates in the mesencephalon (midbrain),
pass through the superior orbital fissure, and is
distributed to the superior rectus, inferior rectus,
medial retus, inferior oblique, ciliary muscles and
pupilary sphincter.
Sensory – proprioceptors in the eye muscles.
Function: Motor - eyeball movement, constriction of pupil, and
accommodation of lens for near vision.
Sensory – muscle sense.
Trochlear (CN IV)
Location: Motor – originates in the mesencephalon (midbrain),
passes through the superior orbital fissure, and is
distributed to the superior oblique.
Sensory – proprioceptors in the superior oblique muscle
pass through the superior orbital fissure and
terminate in the mesencephalon.
Function: Motor- eyeball movement.
Sensory – muscle sense.
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Trigeminal (CN V)
Location: Motor – Mandibular branch originates in the pons,
passes through the foramen ovale, and terminate in
the mastication muscles.
Sensory – Opthalamic branch originates in the skin over
the upper eyelid, eyeball, lacrimal glands, nasal
cavity, side of nose, forehead, and anterior half
of scalp, pass through the superior orbital fissure
and terminate in the pons. Maxillary branch
originates in the mocosa of the nose, palate,
pharynx, upper teeth, upper lip, and lower
eyelid, pass the foramen rotundlum and
terminate in the pons. Mandibular branch
contains somatic sensory fibers (not taste) from
the anterior two-thirds of tongue, lower teeth,
skin over mandible, cheek and mucosa under it,
and the side of the head in front of the ear.
Function: Motor – chewing.
Sensory – sensations of touch, pain, and temperature
from the areas that are innervated.
Abducent (CN VI)
Location: Motor – originates in the pons, passes through the
superior orbital fissure, and is distributed to the
lateral rectus muscles.
Sensory – proprioceptors in the lateral rectus muscle,
pass through the superior orbital fissure, and
terminate in the pons.
Facial (CN VII)
Location: Motor – originates in the pons, passes through the
stylomastoid foramen, and is distributed to the
facial, scalp, and neck muscles, the lacrimal,
sublingual, submandibular, nasal, and palatine
glands.
Sensory – originates from the taste buds on the anterior
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two-thirds of the tongue, passes through the
stylomastoid foramen, and terminates in the
pons. The pons sends fibers that pass through
the thalmus and on to the parietal lobe of the
cerebrum.
Function: Motor – facial expression and secretion of saliva and
tears.
Sensory – muscle sense and taste.
Vestibulocochlear (CN VIII)
Location: Cochlear Branch – originates in the spiral organ (organ
of Corti), forms the spiral ganglion, passes through
the internal auditory meatus, and terminates in the
thalamus. Fibers from the thalamus relay impulses
to the auditory areas of the temporal lobe of the
cerebrum.
Vestibular Branch – originates in the semicircular
canals, saccule, and utricle, forms the vestibular
ganglion, and terminates in the pons and
cerebellum.
Function: Cochlear branch – hearing.
Vestibular branch – equilibrium.
Glossopharyngeal (CN IX)
Location: Motor – originates in the medulla, passes through the
jugular foramen, and is distributed to the
stylopharyngeus muscle and parathryroid gland.
Sensory – originates in the taste buds on the posterior
one – third of the tongue and from the cartoid
sinuses, passes through the jugular foramen, and
terminates in the medulla. There are also
proprioceptors in posterior one – third of the tongue
and swallowing muscles.
Function: Motor – secretion of saliva.
Sensory – taste, regulation of blood pressure, and
muscle sense.
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Vagus (CN X)
Location: Motor – originates in the medulla, passes through the
jugular foramen, and terminates in the muscle of the
airways, lungs, esophagus, heart, stomach small
intestine, most of the large intestine, gallbladder,
and glands of the gastrointestinal tract.
Sensory – originates from the same structures, passes
through the jugular foramen, and terminate in the
medulla and pons.
Function: Motor – smooth muscle contraction and relaxation and
secretion of digestive fluids.
Sensory – sensations from visceral organs and muscle
sense.
Accessory (CN XI)
Location: Motor – Cranial portion originates from the medulla,
passes through the jugular foramen, and terminates
in the voluntary muscles of the pharynx, larynx, and
soft palate. Spinal portion originates from the
anterior gray mater of the first five cervical
segments of the spinal cord, pass through the
jugular foramen, and terminate in the
sternocleidomastoid and trapezius muscles.
Sensory – proprioceptors in the muscle pass through the
jugular foramen.
Function: Motor – Cranial portion mediates swallowing. Spinal
portion mediates movement of head.
Sensory – muscle sense.
Hypoglossal (CN XII)
Location: Motor – originates from the medulla, passes through the
hypoglossal canal, and terminates in the muscles of
the tongue.
Sensory – proprioceptors in the tongue muscles pass
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through the hypoglossal canal and terminate in the
medulla.
Function: Motor – movement of the tongue during speech and
swallowing.
Sensory – muscle sense.
Spinal Nerves: Nerves that communicate between the spinal cord and most of
the body.
Dorsal Root: Contains sensory nerve fiber and carries information from
the periphery to the spinal cord.
Dorsal Root Ganglion: A swelling of the dorsal root that contains
the cells bodies of the sensory neurons.
Ventral Root: Contains the motor neuron axons that carry impulses from
the spinal cord to the periphery.
Reflex Arc: The most basic nerve pathway in the body that connects a
receptor and effector. It consists of a receptor, a sensory neuron,
an integrating center in the CNS, a motor neuron, and effector.
Receptors are the distal end of the sensory neurons and respond
to a stimulus.
The nerve impulse travels to axon terminals in the CNS.
The impulse travels to an integrating center. This can be as
simple as a synapses between the sensory and motor neuron
(monosynaptic) or involve one or more interneurons
(polysynaptic).
The impulse triggered by the integrating center travels along a
motor neuron to an effector.
The effector is the part of the body that responds to the impulse. If
the effector is a skeletal muscle the reflex is a somatic reflex. If the
effector is a smooth muscle, gland, or cardiac muscle it is an
autonomic reflex.
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Cervical Plexus: A network of nerve fibers formed by the ventral rami
(branches of spinal nerves that occur soon after leaving the
vertebral formen) of the first four cervical nerves.
Phrenic Nerve: Arises from the cervical plexus and
supplies the motor nerves to the diaphragm.
Brachial Plexus: The ventral rami of the spinal nerves from C5 – C8 and
T1.
Dorsal Scapular: Originates at C5 and innervates the levator
scapulae, rhomboideus major, rhomboideus minor muscles.
Long Thoracic: Orignates at C5 - C7 and innervates the
serratus anterior muscle.
Subclavian: Originates at C5 - C6 and innervates the subclavian
muscle.
Suprascapularis: Originates at C5-C6 and innervates the
supraspinatus and infraspinatus muscles.
Musculocutaneous: Originates at C5-C7 and innervates the
coracobrachialis, biceps brachii, and brachialis muscles.
Lateral Pectoral: Originates at C5 – C7 and innervates the
pectoralis major muscle.
Upper Subscapular: Originates at C5 – C7 and innervates the
subscapularis muscle.
Thoracodorsal: Originates at C6 – C8 and innervates the
latissimus dorsi muscle.
Lower Subscapular: Originates at C5 – C6 and innervates the
subscapularis and teres major muscle.
Axillary Circumflex: Originates C5 – C6 and innervates the
deltoid and teres minor muscles and the skin over deltoid
and superior posterior aspect of arm.
Median: Lateral head originates at C5 – C7 and medial head at
C5 – C8 and T1 and innervates the flexors of forearm
(except the flexor carpi ulnaris) and the skin of the lateral
two-thirds of the hand and fingers.
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Radial: Originates at C5 – C8 and T1 and innervates the triceps
brachii and other extensors of the arm and forearm and the
skin of the posterior arm and forearm, lateral two-thirds of
dorsal surface of hand, and over the proximal and middle
phalanges.
Medial Pectoral: Originates at C8 – T1 and innervates the
pectoralis major and pectoralis minor muscles.
Medial Brachial Cutaneous: Originates at C8 – T1 and
innervates the skin of the medial and posterior aspect of the
distal third of the arm.
Medial Antebrachial Cutaneous: Originates at C8 – T1 and
innervates the skin of the medial and posterior aspects of
the forearm.
Ulnar: Originates at C8 – T1 and innervates the flexor carpi
ulnaris, flexor digitorum profundus, and most of the
muscles of the hand and the skin of the medial side of the
hand, little finger, and medial half of ring finger.
Lumbar Plexus: Ventral rami of the spinal nerves L1 – L4.
Illiohypogastric: Originates at L1 and innervates the muscles of
the anterolateral abdominal wall and the skin of the inferior
abdomen and buttock.
Illioinguinal: Originates at L1 and innervates the muscles of the
anterolateral abdominal wall and the skin of the superior
medial aspect of the thigh, root of the penis and scrotum in
males, and the labia majora and mons pubis in females.
Genitofemoral: Originates at L1 – L2 and innervates the
cremaster muscle and the skin over the anterior surface of
the thigh, scrotum in male, and the labia majora in females.
Lateral Femoral Cutaneous: Originates at L2 –L3 and
innervates the skin over the lateral, anterior, and posterior
aspects of the thigh.
Femoral: Originates at L2 – L4 and innervates the flexor muscles
of the thigh and the extensor muscles of leg and the skin
over the anterior and medial aspects of the thigh and the
medial side of leg and foot.
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Obturator: Originates at L2 – L4 and innervates the adductor
muscles of leg and skin over medial aspect of thigh.
Sacral Plexus: The ventral rami of the nerves L4 – L5 and S1 – S4.
Superior Gluteal: Originates at L4 – L5 and S1 and innervates
the gluteus minimus, gluteus medius, and the tensor fasciae
latae.
Inferior Gluteal: Originates at L5 – S2 and innervates the gluteus
maximus muscle.
Posterior Femoral Cutaneous: Originates at S2 - S3 and
innervates the skin over the inferior medial aspect of
buttock.
Sciatic: Originates at L4 – S3 divided into the Tibial and
Common Peroneal.
Tibial: Innervates the gastronemius, host of other muscles
and the skin on the lateral third of the sole of the
foot.
Common Peroneal: Innervates a host of muscles and the
skin over the anterior aspect of the leg and dorsal
surface of the foot.
Pudendal: Originates at S2 – S4 and innervates the muscles of the
perineum and the skin f the penis and scrotum in males and
the clitoris, labia majora, labia minora, and vagina in
females.
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