Download The spinal cord is a complex cable of nerves that connects the brain

Hwang, Lauren
Wissmann
Anatomy 1 #1012
Extra Credit
Student ID: 1255727
The Spinal Cord
INRODUCTION
The spinal cord is a complex cable of nerves that connects the brain to most of
the rest of the body. It is made up of bundles of long nerve fibers and has two basic
functions: to permit some reflex movements and to carry messages to and from the brain.
Simple reflexes are controlled by the spinal cord. The message travels from the sense
receptors near the skin through the afferent nerve fibers to the spinal cord. In the spinal
cord, the messages are relayed through association neurons to the efferent nerve fibers,
which carry them to the muscle cells that cause the reflex movement; spinal cord reflex.
The anatomy of the spinal cord itself consists of nerve fibers which transmit electrical
information to and from the limbs, trunk and organs of the body, back to and from the
brain. The brain and spinal cord are referred to as the Central Nervous System (CNS),
while the nerves connecting the spinal cord to the body are referred to as the Peripheral
Nervous System (PNS).
The gray matter (cell bodies) are interior, exterior is white matter (axons going
mostly to and from the brain). The spinal cord (CNS) is covered by bone (vertebrae),
meninges (connective tissue), and cerebrospinal fluids (CSF) that acts as a cushion to
protect the delicate nerve tissues against damage from banging against the inside of the
vertebrae. The bone is in the vertebral canal in all the vertebral foramina in all of the
vertebrae.
MENINGES
The meninges, singular is menix, in made up of 3 connective tissue coverings
that surround the spinal cord and the brain. The spinal meninges surround the spinal cord
and the cranial meninges surround the brain. The dura mater is the outermost meninx
which is dense, irregular connective tissue which forms a continuous membrane over the
whole brain and spinal cord from foramen magnum thru S2 vertebra. The epidural space
is the outermost space between the dura mater and vertebra and is filled with fat and
connective tissue. The leptomeninges include the arachnoid and pia mater.The subdural
space is between arachnoid mater and dura mater, filled with interstitial fluid. The
arachnoid mater is the middle later of meninges, delicate web of collagen fibers and
some elastic fibers. The subarachnoid space is the space between pia and arachnoid
mater filled with spinal fluid. The pia mater is the innermost sheet of meninges, actually
adheres to surface of spinal cord, thin, transparent layer of collagen fibers, elastic fibers
and blood vessels. The denticulate ligaments are the extensions from pia mater through
subarachnoid to dura which keeps the spinal cord centered in the vertebral foramen.
SPINAL CORD
Two enlargements of the spinal cord are evident: (a.) a cervical enlargement (C5
through T1), which provides nerve supply for the upper extremity and (b.) a lumbar
enlargement (L1 through S2) which supplies the lower extremity.
The peripheral nervous system is also covered by connective tissue that carries blood
vessels to the nerve, present in both myelinated and unmyelinated axons. Nerve fibers,
which are axons, organize into bundles known as fascicles.The epineurium covers the
entire nerve, the perineurium covers the fascicles and the endonereurium covers the
individual axons.
The spinal nerves carry information to and from different levels (segments) in the
spinal cord. Both the nerves and the segments in the spinal cord are numbered in a similar
way to the vertebrae. The point at which the spinal cord ends is called the conus
medullaris and the terminal end of the segments in the spinal cord are numbered in a
similar way to the vertebrae. The point which the spinal nerves continue as a bundle of
nerves called the cauda equina. The upper end of the conus medullaris is usually not
well defined.
The spinal cord is the connection center for the reflexes as well as the afferent
(sensory) and efferent (motor) pathways for most of the body below the head and neck.
The spinal cord begins at the brainstem and ends at about the second lumbar vertebra.
The sensory, motor, and interneurons discussed previously are found in specific parts of
the spinal cord and nearby structures. Sensory neurons have their cell bodies in the spinal
(dorsal root) ganglion. Their axons travel through the dorsal root into the gray matter
of the cord. Within the gray matter are interneurons with which the sensory neurons
may connect. Also located in the gray matter are the motor neurons whose axons travel
out of the cord through the ventral root. The white matter surrounds the gray matter. It
contains the spinal tracts which ascend and descend the spinal cord. The gray matter
forms three pairs of horns throughout most of the spinal cord: (1) the posterior dorsal
gray horns, composed of sensory neurons; (2) the lateral gray horns, well defined in
thoracic segments and composed of visceral neurons; and (3) the anterior ventral gray
horns, especially large in the cord enlargements and composed of motor neurons.
The spinal cord extends from the medulla oblongata of the brain to the superior border of
L2 and extends to L3-4 in newborns, but the spine grows more than cord. The dorsal
groove is posterior median sulcus and the ventral groove is anterior the median
fissure (deep). The spinal cord is widest at the neck and narrowest at the sacrum with
bulges (plexus) at C4-T1 and T9-T12. The Dura mater extends to S2. The filium
terminale is the extension of pia mater all the way to the coccyx that anchors to the
bottom end of the spinal cord.
The spinal nerves exit from the vertebral column thru intervertebral foramina. The
intervertebral foramina are spaces formed on dorsal side where two vertebrae come
together. The nerves are named for the superior vertebra. Except cervical nerves are
named for the inferior vertebra, 8 cervical nerves but only 7 cervical vertebrae. First
cervical spinal nerve exits above atlas. The eight cervical spinal nerve exits between C7
and T1 vertebrae. The first spinal nerve exits between T1 and T2. Each peripheral nerve
contains the nerve fibers of two roots from the spinal cord. The dorsal root is for
incoming impulses on the sensory nerves and its area served is called the dermotome.
The dorsal root has a ganglion that contains the cell bodies of its sensory nerves. The
ventral root is for outgoing impulses on the motor nerves and its area served is called
myotome. The dermotome and myotome of one spinal nerve overlap with each other.
Sensory input comes from the peripheral dendrite to the cell body in dorsal root, then
axon carries impulse to spinal cord and brain via two routes.
TRACTS & COLUMNS
The white matter of the spinal cord contains tracts which travel up and down the
cord. Many of these tracts travel to and from the brain to provide sensory input to the
brain, or bring motor stimuli from the brain to control effectors. Ascending tracts, those
which travel toward the brain are sensory, descending tracts are motor. Figure 12.30
shows the location of the major tracts in the spinal cord. For most the name will indicate
if it is a motor or sensory tract. Most sensory tracts names begin with spino, indicating
origin in the spinal cord, and their name will end with the part of the brain where the tract
leads. For example the spinothalamic tract travels from the spinal cord to the thalamus.
Tracts whose names begin with a part of the brain are motor. For example the
corticospinal tract begins with fibers leaving the cerebral cortex and travels down toward
motor neurons in the cord. The posterior column tracts sensation: proprioception,
discriminative touch, 2-point discrimination, light pressure and vibration. The association
neurons in the spinal cord aid in distributing sensory input to several parts of the brain for
integration with other sensory input. The spinal cord, brain stem, cerebellum and
cerebrum all receive input and each can initiate a motor response. A precise voluntary
movements originate mostly in the outer gray matter of the cerebrum and are coordinated
in the thalamus. The autonomic control of cardiac and smooth muscle and glands
originates in brain stem and hypothalamus. The motor impulse to the skeletal muscle
travels from the brain down the spinal cord via two pathways. The Direct (pyramidal)
pathway for precise voluntary movement and the indirect (extrapyramidal) pathway
for autonomic movements, eye/movement coordination, posture and equilibrium.
SPINAL NERVES
There are 31 pairs of spinal nerves which branch off from the spinal cord. In the
cervical region of the spinal cord, the spinal nerves exit above the vertebrae. A change
occurs with the C7 vertebra however, where the C8 spinal nerve exits the vertebra below
the C7 vertebra. Therefore, there is an 8th cervical spinal nerve even though there is no 8th
cervical vertebra. From the 1st thoracic vertebra downwards, all spinal nerves exit below
their equivalent numbered vertebrae. The spinal nerves, which leave the spinal cord, are
numbered according to the vertebra at which they exit the spinal column. So, the spinal
nerve T4, exits the spinal column through the foramen in the 4th thoracic vertebra. The
spinal nerve L5 leaves the spinal cord from the conus medullaris, and travels along the
cauda equina until it exits the 5th lumbar vertebra. The level of the spinal cord segments
do not relate exactly to the level of the vertebral bodies. For example, damage to the bone
at a particular level for example at the L5 vertebrae does not necessarily mean damage to
the spinal cord at the same spinal nerve level.
Four (rami) branches from each spinal nerve after it exits the spinal column:
dorsal ramus (innervates dorsal skin and muscles of back of the trunk, ventral ramus
(innervates arms/legs and lateral/ventral trunk), meningeal branch (reenters foramen and
innervates vertebra, blood vessels, meninges), rami communicantes (form a ganglion of
the autonomic nervous system).
Dermotomes and myotomes of the anterior ventral ramus: cervical nerves from the
cervical plexus (C1-C5) supplies the bottom of the head, neck, top of shoulders, and
phrenic nerve (goes to the diaphragm). The brachial plexus (C4-T2) supplies most of the
shoulders, arms and hands. The axillary nerve innervates the deltoid muscle and
shoulder, along the posterior aspect of the upper arm. The musculocutaneous nerve
innervates the anterior skin of the upper arm and elbow flexors. In from of the elbow:
radial nerve to the thumb, inntervates the dorsal aspect of the arm and extensors of the
elbow, wrist and fingers, median nerve passes thru carpal tunnel to the 1st and 2nd
fingers. Behind the elbow: ulnar nerve to lateral fingers, innervates the medial aspect
wrist and finger flexors. Thoracic nerves: T2-T12 are the intercostals nerves, do not form
plexus, pass directly thru intercostal spaces to final destination. T2 innervates 2nd
intercostals space, axilla. T3-T6 innervate, intercostals muscles, anterior and lateral chest
skin. T7-T12 innervate intercostals muscles, abdominal muscles and skin.
PLEXUSES
Lumbar nerves form lumbar plexus (L1-L5) innervates anteriolateral abdominal
wall, back over the hips, front of the high and crus, and most of the feet. Sacral nerves
form sacral plexus (L4-S4) innervates the buttocks (gluteal muscles), perineum, back of
the thigh (quadratus femoris) and the crus. It supplies sciatic nerve (longest in the body),
sciatic branches into the tibial and peroneal nerves which innervate lower legs and feet.
REFLEX & REFLEX ARCS
Reflex is a complete circuit (sensory nerve to association neuron to motor nerve)
that occurs entirely in the spinal cord. It is a rapid response that does not involve brain
neurons. There are two main types of reflex; somatic and autonomic. Somatic reflex is
the output that goes to the skeletal muscle and the autonomic (visceral) reflex is the
output that goes to the cardiac and smooth muscle and glands.
The four types of reflexes are Stretch, Tendon, Flexor and crossed extensor. The stretch
reflex controls the muscle length by causing contraction.
The characteristics of this reflex are monosynaptic (1 sensory neuron directly synapses
with 1 motor neuron in the spinal cord), ipsilateral: motor impulse leaves from the same
side of the spinal cord that sensory impulse entered, and all monosynaptic reflex arcs are
ipsilateral (no association neuron is involved, therefore there is no way to get to the other
side).
The mechanism is stretching a muscle that sets in motion a reflex that contracts it
back to its normal length. Stretch receptors in a muscle sense the stimulus and the
receptor is a muscle spindle that monitors the muscle length. Stretch produces more than
one impulse (graded potential) along the somatic sensory neuron to the posterior root of
the spinal nerve. In the spinal cord, an excitatory synapse activates a motor neuron in the
anterior gray horn. If excitation is sufficient (exceeds threshold membrane potential),
impulse arises in motor neuron and travels along axon through the anterior root to the
neuromuscular junction. Ach is released at the neuromuscular junction that triggers an
action potential in the skeletal muscle. Contraction occurs in the same muscle as the
original muscle spindle that was stimulated (dermotome and myotome overlap).
Modulators of Stretch Reflex are muscle tone (partial contraction at rest) where the brain
adjusts sensitivity in order to allow for voluntary stretching and to set level of the muscle
tone. The polysynaptic collateral axon from the muscle spindle sensory neuron can use an
ascending pathway to the brain for perception and higher level coordination. Reciprocal
innervation, one stimulus causes both contraction of agonist and relaxation of antagonist
muscle, and the mechanism is the collateral axon from a sensory neuron synapses with
inhibitory association neurons in the spinal cord and causes relaxation of antagonist
muscles. Tendon Reflex controls muscle tension by causing relaxation, therefore it
protects against excess tension on tendons and muscles. Ipsilateral and polysynaptic
reflex. The receptor is called the Golgi tendon organ which is located in a tendon near its
connection with its muscle. It detects muscle tension from either passive stretching or
contraction. If tension exceeds threshold depolarization, then impulse is sent via sensory
neuron to the spinal cord. The sensory impulse activates an inhibitory association neuron
which hyper-polarizes (inhibits) the motor neuron so the muscle governed by that motor
neuron relaxes. More tension causes more inhibitory impulses which causes more
relaxation. Reciprocal innervation is when the Golgi tendon organ also synapses with an
excitatory association neuron that excites the motor neuron that governs the antagonistic
muscle and causes it to contract. It also collateral axons from the sensory neuron go to the
brain for perception and higher level coordination.
Flexor (withdrawal) reflex is the intersegmental reflex arc, which is also
polysynaptic and ipsilateral. The pain receptor impulse has collateral axons that connect
with several association neurons in the spinal cord. Association neurons extend to the
adjacent vertebral segments that are above and below (ascending and descending
association neurons). It activates several motor neurons which leads several groups of
flexor muscles to contract which makes it a divergent circuit. Produces large response
area when stimulus is only small area for example the sensory and motor areas don’t
overlap.
Crossed Extensor Reflex is a contralateral reflex arc. Collateral axons from the
sensory nerve, connects with association neuron that crosses to the other side of the
spinal cord. Then the association neuron stimulates other ascending and descending
association neurons on the other side too, which activates motor neurons of antagonistic
muscles on the other side.
Examples of impaired reflexes: patellar reflex has its receptor in the patellar
ligament. Mechanism: afferent impulse goes to the spinal nerves L2-L4 and causes
contraction of the quadriceps femoris which causes extension of the knee joint. Reflex is
absent in neurosyphilis and diabetes mellitis. Reflex is excessive in the spinal cord
pathology that involves motor tracts that descend from the brain.
Achilles reflex: mechanism: receptor is in the Achilles tendon which goes to the
lumbarsacral spinal nerves which causes contraction of the gastrocnemius and soleus
which is the planta flexion. Reflex is absent in chronic diabetes mellitus, neurosyphilis,
alchoholism, subarachnoid hemorrhage.
Baginski sign (normal is “absent”): mechanism: stroke receptors are located in the
lateral margin of the sole of the foot. Normal is clenched toes which is the absent
Babinski sign. Abnormal is dorsiflexion of the big toe which is present in the Babinski
sign. This is found in the lesion in the upper corticospinal tract, and normal babies up to 1
½ years due to incomplete myelination.
Abdominal reflex: mechanism: receptors on the side of the abdomen detect touch
(stroking). Normal is contralateral muscle contraction which makes the umbilicus to be
pulled to the opposite side. Abnormal is the absence of contraction found in the lesion of
corticospinal tract or of peripheral nerves or in multiple sclerosis.
Examples of spinal nerve pathology:
Neuritis is the inflammation of the peripheral nerve. It causes blow, fracture, stab,
thiamine deficiency and some poisons. Some examples are sciatica which is the neuritis
of the sciatic nerve (radiates down the leg), shingles which is called herpes zoster
(chicken pox virus) infection. The virus lies dormant in peripheral ganglion then
exacerbation spreads the virus by fast axonal transport to the peripheral dermotome.
Poliomyelitis viral infection of CNS neurons which includes motor neurons in anterior
horns and nuclei of cranial nerves.
Nerves control the body’s functions including the vital organs, sensation, and
movement. The nervous system receives information and initiates an approximate
response. It is affected by internal and external factors. Nerves follow tracts and cross
over junctions called synapses. Simplified, it is a complex communicative process
between nerves conducted by chemical and/or electrical changes.