Anatomy of the Spinal Cord

The spinal cord is the inferior extension of the brain that extends down into the vertebral column. The two major functions of the spinal cord are
**control reflex activities
**transmit information by way of the ascending and descending tracts to the brain from peripheral nerves

The spine is separated into five sections. The first 8 vertebrae are called the cervical vertebrae. This section has a slight bulge because of the nerves that emerge from it. The next 12 vertebrae are called the thoracic vertebrae, and the next 5 are called the lumbar vertebra. The lumbar section is also enlarged, due to the nerves that emerge there. The next section is called the sacrum, and consists of 5 sacral vertebrae that are fused together into on plate. The final 4 vertebrae make up to coccyx, or the tailbone. These are also fused together.

The spinal cord exits the skull through the foramen magnum, a hole in the occipital lobe of the skull. The spinal cord has an average length of 45 centimeters, although that number varies depending on torso length. The diameter of the spinal cord in about that of a finger. The cord tapers to a smaller point from top to bottom, or from the cranial end to the caudal end. At the caudal end, the spinal cord narrows into a sharp point called the conus medullaris, from which extends a thin filament called the filium terminale.

There are 31 pairs of spinal nerves in total. The nerves in the lower region of the spinal are called the cauda equina, because of their resemblance to a horse's tail.

The spinal cord can be divided into regions by various fissures. The anterior median fissure and the posterior fissure divide the spinal cord longitudinally. Through the very center of the spinal cord is a canal called the central canal, which houses the CSF.

The gray matter of the spinal cord consists of masses of cell bodies, dendrites, efferent neurons, and unmyelinated axons, as well as blood vessels and glial cells. The gray matter creates a rough "H" shape. Each half of the gray matter section has an anterior and posterior column, and is connected in the middle by a bar called the gray commisure.

The white matter of the spinal cord consists of myelinated axons arranged into bundles called tracts. These tracts pass up and down from the brain. In each half of the cord, the white matter is divided into three columns called funiculi: the anterior, posterior, and lateral funiculi. Each funiculus is divided into fasciculi, or tracts, which are bundles of myelinated axons.

Tracts of the Spinal Cord

Ascending tracts carry impulses up towards the brain.
Descending tracts carry impulses back from the brain

Some examples of tracts are:

Reflexes

A reflex arc is the neural pathway that links the receptor, the central nervous system, and the effector. A reflex action is predictable, automatic, and specific. Internal as well as external stimuli have reflex responses. The homeostatic reflex response to high blood pressure is just as much a reflex as the physical drawback from a painful touch. All reflexes, even the simplest, involve reception, transmission, integration, and response. Certain components are present in all reflexes

* receptor to receive the stimulus
** sensory (afferent) neuron to transmit impulse from the receptor to the CNS
*** reflex center in the CNS for integration, including an association neuron
** motor (efferent) neuron to transmit impulse from the CNS to the effector
* effector to carry out the response

A spinal reflex, or a simple reflex, is carried out solely by the spinal cord, without the participation of the brain. Other reflex actions only involve the part of the brain that doesn't involve conscious thought. Still other reflex actions are subject to conscious control. Reflexes that involve the brain are higher reflexes. A simple reflex DOES NOT involve tracts, while a higher reflex DOES.

A stretch reflex is monosynaptic, meaning there is only one junction between neurons. This type of reflex only requires two neurons, and the best example of such a reflex is the patellar reflex. The tendon is tapped, and impulses are sent through sensory neurons to the spinal cord, where they synapse with motor neurons and then transport impulses to the motor units of the leg muscles. The leg, as the response of the reflex, kicks out suddenly.
The patellar reflex consists of
*a receptor that receives the stimulus
*a sensory neuron that transmits the impulse to the spinal cord
*the CNS, where integration of the impulse takes place
*a motor neuron to transmit the decision to the effector'
*an effector, a muscle that carries out the response
(in a muscular reflex such as this, the muscles that extend the leg as a response are allowed to do so by the inhibition of their antagonistic muscles. The muscles required in the reflex carry out their actions, and the muscles that do the opposite action are inhibited. This stimulation of once muscle and inhibition of another is called reciprocal inhibition)

The flexor reflex is polysynaptic, meaning there are multiple junctions between neurons. Three neurons are required for this type of reflex. An example is yanking your hand away from a hot stove. The receptors in the hand sense the heat, and transfer the impulse along a sensory neuron, which goes to the CNS. In the spinal cord, the sensory neuron synapses with an association neuron, which identifies the problem and solution. The association neuron synapses with the motor neuron, which goes to the arm muscles to yank the hand away.
This reflex uses the five basic parts of a reflex, with an association neuron being the extra step in the passage through the CNS.
(In a reflex where one limb is stimulated, the opposite limb extends as well, in a crossed extensor reflex that keeps the body balanced. It results from the contraction of muscles in the opposite limb, and protects the body from further harm.)

Some common reflexes are...

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