The Spinal Cord : The Transmitter of Sensations

The Spinal Cord : The Transmitter of Sensations

Dr.Surya Rao Poodipeddi profile Authored by Dr.Surya Rao Poodipeddi on 1 Mar 2014 - 15:10.
The Spinal Cord
The spinal cord begins where the base of the brain ends and tapers at the end into a bundle of nerves near the sacrum of the spine. It is situated inside the vertebral canal in the center, called vertebral foramina consisting of seven cervical, 12 thoracic, five lumbar and five sacral vertebrae which, together form the vertebral column or the spine.
Structure: It extends from a large opening in the base of the skull called foramen magnum (foramen means opening and magnum means large) down to the end of the 1st and 2nd lumbar vertebrae. (At birth it extends down unto 2nd and 3rd lumbar vertebrae).
The entire length of the spinal cord is divided into several segments totaling to 31 segments. They comprise of eight cervical (C), 12 thoracic (T) five lumbar (L) five sacral (S) and one coccygeal (CO).
Each segment has its respective nerve supply and also receives sensory messages from the respective area. For example: the eight segments of the cervical part of the cord emerge out from each foramen in the vertebra on each side (meant for supplying the respective sides) and each nerve joins outside with the other nerve emerging down below and form a plexus called Cervical (Brachial) plexus. The combination of these nerves is specifically meant for supplying a predestinated area.
Spinal Cord functions: They supply the nerves to the muscles in the area as per orders from the brain. For example the biceps muscle in the arm above the elbow receives its nerve supply from C5/C6. This means that the nerve from the cord emerging out from segments C5 and C6 of the cervical portion join together and supply the biceps muscle.
In other words all actions of the biceps muscle depend on these particular segments. To make it easier, supposing your biceps muscle alone has atrophied and worn-out and inactive, it can be immediately pinpointed that there is something wrong with C5/C6 segments of the cervical portion of the spinal cord.
The same is the case on the other side of the body. Similarly all the nerves emerging from the segments of the thoracic portion of the spinal cord on each side join, with each other and form a plexus called Thoracic plexus. There is one lone difference in this portion of the cord. For example the muscles of the fingers are supplied by C8. However, there are only seven cervical vertebrae. Therefore, in this case the C7 and T1 combine and supply the muscles of the fingers. Any problem in the fingers can immediately be referred to C7/T1 as the seat of action.
cross section of spinal cord
The lumbar portion of the spinal cord with its five segments on each side forms the lumbar plexus and each segment is referred to as L1/L2 etc.Similarly the segments in the sacral portion of the spinal cord form the sacral plexus.
To sum up, the motor supply (supply for the action of the muscles) for each segment of the particular portion of the cord is meant to supply the muscles in the respective area, either alone or in combination with the other segment, like C7/T1 which supply to the finger muscles. A combination of S3 and Coccygeal one is responsible for the supply of muscles in the anus.
Similar to the emergence of nerves from the segments for motor supply, spinal nerves for transmission of different types of sensations to the brain enter the spinal cord through each segment from where the motor nerves emerge out. The sensory system transmits messages to the brain.
There are different types of sensation like pain, temperature (cold & heat), besides several mechanisms involved for co-ordination of balance, gait and recognition of shape of an object etc. These sensory nerves form tracts in the spinal cord. Each tract has a specific function to transmit a specific sensation. The messages are received by the brain to order appropriate action.
Sensory transmission: To make the sensory transmission easily understandable let us suppose that an ant has bitten the middle of your leg. Immediately the sensory nerves send a message to the brain which recognizes the exact area where the ant has bitten and sends orders to the particular muscles to perform the action of removing the ant, in fraction of a second.
Similarly to perceive extremes of cold or heat the sensory nerves transmit messages accordingly through the spinal tracts meant for the purpose. If there is loss of sensation in a particular area as in case of a disease called Leprosy the victim can never perceive sensation of pain or heat and therefore damage of the area occurs. In other words the victim can walk on fire without feeling the heat due to sensory loss in the particular area and the brain is helpless to send orders.
Several motor tracts from brain to the periphery and sensory tracts from periphery to the brain travel in the vertebral column along with the cord for various motor and sensory functions. A detailed description of these tracts is beyond the scope of this article.
However, it is important to know that there are two types of path ways arising from the spinal cord. One is the sympathetic and the other is parasympathetic. It is the parasympathetic pathways which innervate the erectile tissue in the penis and the clitoris (in females), smooth muscles and glandular tissues in the prostate, seminal vesicles (only in the males) urethra (in both sexes) vagina & uterus (only in females).
The most studied and researched observation is the mechanism involved in the erection of penis. Some researchers opine that the stimulation of the pelvic nerves leads to penile erection. Penile erection is secondary to vaso dilatation of the vessels supplying the penis which is maintained by the nervous mechanism. Nitric oxide (NO) is considered as the principal mediator of penile erection in humans.
The spinal cord is supplied by the two vertebral arteries and multiple radicular arteries from the segmental vessels. The corresponding veins carry out the venous drainage.
Cerebrospinal fluid maintains lubrication of the cord and is present around it in the vertebral column. Through this fluid doctors administer drugs for anesthesia (spinal anesthesia) and for therapeutic purposes like various types of meningitis etc. The spinal cord is adequately protected by the vertebral column.
Conditions affecting the spinal cord are many. These are far too many which include congenital and acquired conditions. Majority of the acquired conditions are mainly due to injuries to the vertebral column and through it to the spinal cord.
Notable conditions affecting the spinal cord are:
  • Complete spinal cord transaction syndrome (spinal shock) which manifests in different forms according to the site affected.
  • Anterior cord syndrome due to infarction in the anterior spinal artery supplying the cord and manifests with loss of pain and temperature sensation below the level of infarction.
  • Central cord syndrome: This condition is seen in diseases like syringomylia, hydromylia and trauma to the cord, hemorrhage and intramedullary tumors etc. The syndrome is associated with variable sensory loss and loss of reflexes with predominant loss of sensation to pain and temperature.
  • Brown-sequard Syndrome: This is equivalent to hemicordectomy (Cutting the cord halfway through) characterized by paralysis on the same side, loss of sensation to vibrations and positioning below the level of the lesion and loss of pain and temperature in the other side of the body (contra lateral) below the level of the cut.
  • Cauda Equina Syndrome which affects only the cauda equine (last part of the cord ending like horse hair) leading to pain and sensory changes.
  • Hemiplagia, paraplegia and quadriplegia are the other conditions affecting the cord.
Of all the congenital malformations affecting the spinal cord Spina Bifida is an important condition affecting the infants. Spina Bifida is a neural defect,  involving incomplete development of the brain, spinal cord and/or their protective coverings. It develops due to incomplete and improper closing of the fetus’s spine during the first month of pregnancy. It is said to occur in families.
Treatment includes surgery, medications, physiotherapy and assistive devices like braces, crutches and wheel chairs depending on the severity of affection. Other conditions in this category requiring mention are: Syringomylia, Heridetory Spastic Paraplegia etc. Each condition is treated according to the person’s condition and background, best treated on an individual basis.
*Disclaimer: This is not medical advice. The content is for educational purposes only. Please contact your doctor for any health care issues.