A&P Ch 9 Spinal Cord

While tracts of the spinal cord carry information between the brain and the spinal cord, not all functions of the central nervous system require this communication. The gray matter of the spinal cord can also coordinate many involuntary spinal reflexes, in which we can respond quickly to stimuli without information needing to pass to or from the brain.

Afferent (sensory) signals enter through spinal nerves, are processed by the spinal cord, and efferent (motor) signals leave through spinal nerves. Since the brain is not involved in spinal reflexes, they remain even if the spinal cord is completely severed in an accident or in the case of severe brain injury.

Reflexes combine the spinal sensory and motor components with a sensory input that directly generates a motor response. The reflexes that are tested in the neurological exam are classified into two groups:

1. Deep tendon reflex - This is commonly referred to as a stretch reflex, and is elicited by a strong tap to a tendon, such as in the knee-jerk reflex. The tendon for each of these muscles is struck with a rubber mallet. The muscle is quickly stretched, resulting in activation of the muscle spindle that sends a signal into the spinal cord through the dorsal root. The fiber synapses directly on the ventral horn motor neuron that activates the muscle, causing contraction. The knee jerk reflex is called a monosynaptic reflex because there is only one synapse in the circuit needed to complete the reflex. It only takes about 50 milliseconds between the tap and the start of the leg kick. That is fast! The tap below the knee causes the thigh muscle to stretch. Information is then sent to the spinal cord. After one synapse in the ventral horn of the spinal cord, the information is sent back out to the thigh muscle that then contracts. 2. Superficial reflex - This is elicited through gentle stimulation of the skin and causes contraction of the associated muscles. The most common superficial reflex in the neurological exam is the plantar reflex that tests for the Babinski sign on the basis of the extension or flexion of the toes at the plantar surface of the foot. An infant would present a positive Babinski sign, meaning the foot dorsaflexion and the toes extend and splay out. As a person learns to walk, the plantar reflex changes to cause curling of the toes and a moderate plantar flexion. If superficial stimulation of the sole of the foot caused extension of the foot, keeping one's balance would be harder. This is used as an important neurological test for spinal cord damage because it gives an easily detectable abnormal response if the spinal cord has been injured.

There are five parts of a reflex pathway, or reflex arc:

1.A receptor - which detects some stimulus (pain, temperature, muscle stretch,etc.) 2.Afferent neurons - which carry sensory information toward the central nervous system. Cell bodies are located in the dorsal root ganglia. 3.The integration center - consisting of one or more synapses in the gray matter of the spinal cord. These may invoke interneurons (i.e. association neurons) 4.Efferent neurons - which carry motor information away from the central nervous system. Their cell bodies (like all spinal efferent neurons) are located in anterior or lateral horns of the gray matter. Some of these efferent neurons are stimulated in the integration center while others are inhibited. 5.The effector - consisting of the myocytes, glands, etc..which produce the physical response. Most of these will be on the same side of the body as the stimulus was received (called an ipsilateral response) but some may be on the opposite side (a contralateral response).

The spinal cord is surrounded by coverings called meninges. There are three meninges: the dura mater, the arachnoid mater and the pia mater.

1.The dura mater - this stands for 'tough mother' and is the thick outer covering of the spinal cord. Outside of the dura, by the vertebrae, is the epidural space which is the site where a person would receive an epidural injection. This space is not in the meninges or the spinal cord at all. Beneath the dura mater is a space called the subdural space. This space is a potential space that could occur between the dura mater and the arachnoid mater. It only occurs when there is some type of injury and there is leakage into this area. A common problem from trauma is what is called a subdural hematoma. This is where an injury has occurred and there is bleeding in this space. 2.The arachnoid mater - this is the middle layer of the meninges. It resembles a bunch of cobwebs and is a very thin delicate layer. The space beneath the arachnoid mater is the subarachnoid space which contains cerebrospinal fluid and blood vessels. This is the area where cerebrospinal fluid is taken during a spinal (or lumbar) puncture. 3.The pia mater - this is the tightly bound extremely thin inner layer that is in direct contact with the spinal cord.

In cross-section, the spinal cord contains both white matter and gray matter.The white matter contains the myelinated axons and glial cells; it does not contain cell bodies.

1.The white matter consists of myelinated axons in tracts that run up and down the spinal cord. These carry sensory and motor information to and from the higher-level processing centers of the brain. So sensory runs up the posterior part of the spinal cord and motor runs down the anterior part. Much of the motor and sensory control of the right side of your body is processed by the left side of the brain, and most of the left side of your body is processed by the right side of the brain. When a spinal tract crosses over to the other side, it is called decussation. 2.The gray matter contains neuron cell bodies, unmyelinated axons, dendrites near the cell bodies along with glial cells. The cell bodies in the gray matter coordinate the transfer of information from sensory to the white matter for processing and back to motor. They also send information to the tracts in the white matter that needs to go to the brain to be processed.

A dermatome is an area of skin that is supplied by a single spinal nerve.

Although the general pattern of these dermatomes is similar in all people, the precise areas of innervation are as unique to an individual as fingerprints.

A reflex is an involuntary motor response to an adequate sensory stimulus that occurs subconsciously, without the action of the cerebral cortex, e.g. withdrawal reflex.

An intersegmental reflex means that the reflex arc involves both sides of the spinal segment, in other words the efferent neuron and effector organ are on opposite sides of the receptor organ e.g. the crossed extensor reflex.

The anterior midline of the spinal cord is an area called the ventral median fissure and the posterior midline is an area called the dorsal median sulcus.

Axons enter the posterior side through the dorsal nerve root which ends in a cluster of nerves called the dorsal root ganglion. Since these axons enter the posterior side, they carry the sensory (afferent) neurons. Axons leave the spinal cord through the ventral root leading to the motor (efferent) neurons.

Infants are born with many inborn reflexes. Some examples would be coughing, swallowing or blinking. These behaviors last throughout one's lifetime and are unchanging.

Learned reflexes need to be taught or acquired through learning. Potty training and talking would be considered learned reflexes. These behaviors can be changed.

Dorsal Nerve Root - Axons entering the dorsal horn of the spinal cord. Dorsal Root Ganglion - a sensory ganglion (cluster of nerves) that is attached to the dorsal nerve root of a spinal nerve. Nerve Plexus - Network of nerves not including the neuronal cell bodies.

Ramus - Branches of the spinal nerves. Spinal Nerves - 31 nerves connected to the spinal cord. Ventral Nerve Root - Axons emerging from the anterior or lateral horns of the spinal cord.

The spinal nerves run out to the periphery from the spinal cord through holes in the vertebrae called

intervertebral foramen. There are 31 pairs of spinal nerves but I won't require you to memorize these.

Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level. The cervical plexus is composed of axons from spinal nerves C1 through C5. These branch into nerves in the posterior neck and head, as well as the phrenic nerve which connects to the diaphragm at the base of the thoracic cavity. The other plexus from the cervical level is the brachial plexus. Spinal nerves C4 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. The lumbar plexus arises from all the lumbar spinal nerves and gives rise to nerves innervating the pelvic region and the anterior leg. The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4.

The most significant systemic nerve to come from this plexus is the sciatic nerve. If you have noticed, there is no thoracic plexus. This is because the spinal nerves in this region emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding each spinal nerve.

The gray matter has the appearance of a butterfly with the four points of the wings being the gray horns.

The posterior horns are responsible for sensory processing and the anterior horns are responsible for sending out the motor signals.

The spinal cord goes from the brain down through the vertebrae and ends with the upper lumbar region. It does not go into the sacral region as it stops growing before the vertebrae are finished growing (it ends at L1). This is why the end of the spinal cord at the lower region is made up of a lot of nerves that branch out and appear like a horse's tail. This collection of nerves is thusly called the cauda equina.

The tip of the actual spinal cord before these branches is called the conus medullaris. In order to stabilize the end of the spinal cord so it isn't just floating freely, it is attached to the coccyx by a thin piece of fibrous tissue called the filum terminalis. The spinal cord is visible to the eye and is about the diameter of one's finger. The majority of sensory functions are in the posterior regions of the spinal cord and motor functions are in the anterior regions of the spinal cord.

Spinal nerves extend outward from the vertebral column to innervate (stimulate) the periphery. The nerves in the periphery are combined into a network of nerves that go out to the various areas they serve.

This occurs at four places along the length of the vertebral column, each identified as a nerve plexus, whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies.

Dermatomes have clinical significance, especially in the diagnosis of certain diseases. Symptoms that follow a dermatome, such as pain or a rash, may indicate a pathology that involves the related nerve root. Examples include dysfunction of the spine or a viral infection.

Viruses that remain dormant in nerve ganglia, such as the Varicella zoster virus which causes both chickenpox and shingles can often cause pain, rash or both in a pattern defined by a dermatome. The rash of shingles is almost always restricted to a specific dermatome, such as the chest, leg or arm, which is caused by the residual infection of the nerve that supplies that area of skin with the Varicella zoster virus.

The central canal is in the

center of the spinal cord and contains cerebral spinal fluid.