The Nervous System A: Spinal Cord

Objectives:

1. to learn about the location and structure of spinal cord

2. to know about the meningies surrounding the spinal cord

3. to identify the cross section in a spinal cord

4. to identify number of spinal nerves

                                         

THE NERVOUS SYSTEM

The nervous system is divided into two systems:

1. The central nervous system: includes the group of nervous tissue located in the skull and vertebral column, including the integrative function of the nervous tissue. The brain and the spinal cord are the components of the CNS.

2. The peripheral nervous system: includes the group of nervous tissue distributed in the body out of the skull and vertebral column, responsible to transport nervous impulse between the different organ and the central nervous system.

     The elements of the CNS are protected by boney tissue; the brain is housed inside the skull (the cranial bones) and the vertebral column is surrounding the spinal cord carefully. The three layers of meninges enveloping the brain tissue as well as the spinal cord tissue, keeping spaces between them as shock absorption tool.

     The nervous tissue is composed of two types of nerve cell; even the neurons which are responsible for nerve impulse conduction or the neuroglial cells.  The neuron is a polar cell (with two ends) having the cell body, axon and the axon terminals. The cell body contains the nucleus and the other cytoplasmic organells such as lysosomes, mitochondria, golgi complex and Nissl bodies as the rough endoplasmic reticulum. The axon is a thin long, cylindrical shaped structure of the neuron also contains some cytoplasmic organells such as mitochondria, microtubules and neurofibrils, it is responsible for nerve impulse conduction to the other end of the neuron; toward the terminals. The axon may be surrounded by neuroglial cell to become myelenated axon so it can conduct the nerve impulse faster; the nodes of Ranvier are points on axon where the myelin sheath is not found. The axon terminals are the part of neuron the make the connection with other neuron or effector tissue. The neuroglial cells are of many types: astrocyte, oligodendrocyte, microglia and ependymal cell found in CNS and Schwan cell and satellite cell in the PNS. They are the helping cells of the neurons by giving the support, protection and nourishment of the neurons. The neurons of the CNS are myelenated by the oligodendrocytes while the neurons of the PNS are myelenated by the Schwan cells.

THE ANATOMY OF THE SPINAL CORD

     The spinal cord is protected by the vertebral column passing through the vertebral canal of the vertebral column. It is surrounded by the spinal meninges. The outermost meninx is the dura mater which is a dense irregular connective tissue, extends from the foramen magnum of the occipital bone down to the second sacral vertebra. The dura meter is separated from the vertebral canal by the epidural space containing cushion of fat and connective tissue. The arachnoid mater is the middle layer, network structure of collagen and elastic fibers. The subdural space lies between the dura mater and the arachnoid mater filled with interstitial fluid. The pia mater is the innermost meninx richly supplied with blood (O2 and nutrients). The subarachnoid space lies between the arachnoid mater and the pia mater filled with the cerebrospinal fluid (CSF).

     The spinal cord passes through the successive vertebral foramena of the vertebral column. The spinal cord has different diameter measurements in the different regions of the vertebral column; at the cervical region the diameter is large and termed as cervical enlargement from which two plexuses of the spinal cord are extended: the cervical plexus and the brachial plexus. The lumber region also has lumber enlargement from which the lumber plexus is extended. The sacral plexus extends at the two lateral sides of the sacrum.

     The spinal cord extends from the vertebral foramen of C1 down to that of L2 due to the faster growth rate of the vertebral column relative to that of the spinal cord. The conus medullaris is the pointed tapered end of the spinal cord at L2. The filum terminale is the inferior pia mater extention from the conus medullaris to anchor the spinal cord to the coccyx. The cauda equina is the group of spinal nerves extends in the lumber puncture. 

  

CROSS SECTION IN THE SPINAL CORD

     The center of the cross section in the spinal cord has a darker colored H- shaped structure called the gray matter which has an opening in its core resembles the central canal for the cerebrospinal fluid circulation. The two sides of the spinal cord are separated by the presence of the dorsal median sulcus posteriorly and the ventral median fissure anteriorly. The gray matter has two horns at each side of the section; the dorsal (posterior) gray horn and the ventral (anterior) gray horn. The continuation of the dorsal gray horn out of the spinal cord is known as the dorsal root of the spinal cord containing number of sensory nerves aggregating their cell bodies in the dorsal root ganglion. The ventral gray horn continues as the ventral root of the spinal nerve containing number of motor neurons transporting the motor response to the effector muscle or organ. The lateral gray horn is the connection piece between the dorsal and the ventral gray horns transversely in the thoracic, upper lumber and sacral spinal pieces, containing cell bodies of autonomic neurons. 

     The outermost region of the spinal cord is the white matter, contains the dorsal white column, the ventral white column and the lateral white column. Each white column contain number of neuron axons called tract, the sensory (ascending) tract passes the sensory impulse to the brain while the motor (descending)  tract passes the motor response from the brain to the cord.

THE SPINAL NERVES

     There are 31 pairs of the spinal nerves extended from the two lateral sides of the spinal cord through the intervertebral foramena. There are 8 pairs from the cervical region     (C1-C8), 12 pairs from the thoracic (T1-T12), 5 pairs from the lumber (L1-L5), 5 pairs from the sacrum (S1-S5) and 1 pair from the coccyx.

The following spinal nerves are selected to be examined in the lab.:

1. radial nerve: C5-C8,T1

2. ulnar nerve: C8-T1

3. median nerve: C5-T1

4. axillary nerve: C5-C8

5. musculocutaneous nerve: C5-C7

6. intercostal nerve:

7. sciatic nerve: L4-S3

8. femoral nerve: L2-L4

9. obturator nerve: L2-L4

10. tibial nerve: L4-S3

11. common peroneal nerve: L4-S2

12. deep peroneal nerve:   =

13. superficial peroneal nerve: =