The spinal cord is a long cylindrical portion of the central nervous system. It is suspended in the vertebral column and is enclosed by the meninges and cerebrospinal fluid. The spinal cord is continuous with the medulla oblongata and extends from the upper border of the atlas(C1) to the lower border of L1. It is about 45cm long in an adult male.
Except for the cranial nerves, the spinal cord forms a link between the brain and the rest of the body. Nerves that convey impulses from the brain to the effected body organ passes through the spinal cord. likewise, the sensory impulses also reaches the brain upwards through the spinal cord.
The spinal cord performs some activities which are independent of brain; and are controlled by the levels of spinal cord; known as spinal reflexes. To facilitate these activities, there are neuron connections between the sensory and motor neuron, at the same or at different levels in spinal cord.
The spinal cord is divided in to equal parts.
Anteriorly, divided by a deep median fissure and posteriorly by a narrow septum known as posterior median septum. The spinal cord is composed of grey matter in the center and surrounded by the white matter; supported by the neuroglia.
The arrangement of grey matter i the spinal cord is like letter H; having to posterior, two anterior and two lateral columns. The area of grey matter that lies transversely is the transverse commissure and it is pierced by the central canal. The cell bodies may belong to-
- sensory neurons: receives impulses from the periphery of the body
- motor neurons: transmit impulses to the skeletal muscles
- connector neurons: these are also known as interneurons which links the sensory and motor neurons at the same or different levels and forms spinal reflexes arcs.
Posterior column of grey matter:– These column are composed of cell bodies which are stimulated by the sensory input from the body. The nerve fibers of these cell bodies contribute to the white matter of the cord and transmit sensory impulses to the brain.
Anterior column of grey matter:- These are composed of cell bodies of lower motor neurons; which are stimulated by the upper motor neuron and connector neurons.
The white matter of the spinal cord is arranged in 3 columns or tracts; named as posterior, anterior and lateral. These tracts are formed by the sensory nerve fibers which passes to the brain, motor fibers which are passing from the brain and nerve fibers to connector neurons.
Sensory nerve tracts in the spinal cord:- neurons that transmit impulses towards the brain are sensory The two main sources of sensations transmitted to the brain are
- skin: sensory receptors in the skin are stimulated by pain, light, touch, cold etc.
- tendons, muscles and joints: sensory receptors that lies in these structures are known as proprioceptors. and they are stimulated by stretch. Together with the impulses from eyes and ears, they help in maintaining body posture, balance.
Motor nerve tracts in the spinal cord:– Neurons that transmit impulses from the brain are known as motor. stimulation of motor neurons results in-
- Voluntary muscle movements: The contraction of muscles to move the joints comes under voluntary movements. nerve impulses are transmitted from the brain via tracts of spinal cord. the motor pathway from brain to the muscle is done by two neurons
1. upper motor neurons- this has its cell body in the primary motor area of the cerebrum.
2. lower motor neurons– this has its cell body in the anterior horn of the grey matter in the spinal cord.
- Involuntary muscle movements:
1. upper motor neuron- this has its cell body in the brain at the level below the cerebrum. They helps in maintaining posture, body balance.
2. Spinal reflexes- It has 3 elements: sensory neurons, connector neurons and motor neurons.
3. autonomic reflexes- this has pupillary light reflex, when the pupil constricts immediately during bright light, hence prevents retinal damage.
Multiple choice questions(MCQs)
1. Which of the following is the part of central nervous system?
A. brain B. grey matter
C. spinal cord D. both A and C
2. The spinal cord is continuous with which part of the brain?
A. pons b. medulla oblongata
C. cerebellum D. midbrain
3. Does spinal nerves always form a link between the brain and the rest of the body?
A. yes B. no
C. except cranial nerves D. except spinal nerves
4. the spinal cord consist of which type of cell bodies?
A. sensory neurons B. motor neurons
C. connector neurons D. all of the above
5. Which column of grey matter receives the sensory input for the body?
A. posterior column B. anterior column
C. lateral column D. both B and C
6. Match the following-
a. upper motor neuron (voluntary) 1. has 3 elements
b. upper motor neuron(involuntary) 2. Anterior column of grey matter
c. lower motor neuron 3. Below cerebrum
d. spinal reflexes 4. Primary motor area of cerebrum
7. Where does receptors for pain, temp, touch lies?
A. muscles B. joints
C. tendons D. none of the above
8. Which neuron facilitates the involuntary muscle movements?
A. upper motor neuron B. lower motor neuron
c. connector neuron D. all of the above
9. Which of the following statement is true?
A. spinal reflexes has 4 elements
B. lower motor neurons has cell body in anterior column of grey matter
C. grey matter that lies transversely is lateral commissure
D. spinal cord has 3 equal parts
10.What includes the autonomic reflexes?
A. dilating of pupil B. contracting of pupil
C. dilating of iris D. contracting of iris
- both A and C
- medulla oblongata
- except cranial nerves
- all of the above
- posterior column
- a – 4 b – 3 c – 2 d – 1
- none of the above
- upper motor neuron
- lower motor neuron has its cell body in anterior column
- contracting of pupil
REFRENCE: 1. Ross and Wilson-Anatomy and physiology in health and illness; 12th edition; page no.-: 160-164.