Functional Organization of the Central Nervous System III - The Sensory System

The sensations can be broadly divided into:

1. Superficial sensation (touch, pain and temperature)

2. Deep sensation (deep pain, pressure, sens of position, sense of movement, joint sensation and vibration); and

3. Cortical sensations (tactile localization, tactile discrimination and stereognosis).

At the periphery, there are receptors to appreciate different sensory modalities.

I. Touch receptors- Meissner's corpuscles, Merckel's discs and free nerve endings.

II. Pressure- Pacinian corpuscles

III. Heat- Ruffini's Corpuscles

IV. Cold- Krause's end bulbs

V. Pain- Free nerve endings.

The modern view is that the specificity of these receptors may not be absolute. The same receptor may be subserving different sensations under different conditions. When the receptors are stimulated, afferent impulses pass in the dendrites of the first sensory neuron into the spinal cord. This afferent neuron is situated in the posterior root ganglion. The axons of this cell pass through the posterior root into the spinal cord and ascend up as the sensory tracts. Fibers subserving different sensations take different paths.

Fibers subserving proprioception, vibration and a portion of touch column (colum of Goll and Burdach) to reach the lower part of the medulla, where they synapse with the cells in the gracile and cuneate nuclei. Lateral fibers of the posterior column carry sensation from the upper limbs whereas the medial fibers carry sensation from the lower limbs. From here the second order neurons corss the mid-line in the medulla and Pons and pass up in the medial lemniscus to reach the main sensory nucleus of the thalamus.

Fibers subserving the remaining part of touch, pain, heat and cold synapse with the cells in the posterior horn of the spinal cord soon after entry. The second order neurons arising from these cross to the opposite side at different levels (pain and temperature fibers more obliquely one or two segments above) and pass up on the other side as the anterior spinothalamic tract (touch) and lateral spinothalamic tract (pain and temperature). In the lateral spinothalamic tract fibers from the lower limbs are placed laterally and fibers from the upper limbs are medial. Some fibers do nto cross and they pass up the ipsilaterla spinothalamic tracts. In the brainstem, the spinothalamic tracts pass up lateral to the medial lemniscus to reach the thalamus. Third order neurons arise from the thalamus. Third order neurons arise from the thalamus and fibers which maintain their functional specificity pass up through the internal capsule to be relayed to the sensory area of the ortex int he postcentral gyrus.

In the sensory cortex, the body image is arranged similar to that in the motor area from above downwards. From the postecentral gyrus fibers are projected to other cortical areas. In the pareital lobe, the sensory information derived from superficial and deep sensations is integrated to give the impression of size, shape, texture, weight and pattern of the objects (stereognosis). The mental picture of the body (body image) is obtained by integration of the sensory information with information obtained from the special senses. This function of "body image" is mainly carried out by the nondominant parietal lobe. The corresponding portion of the dominant hemisphere carries out the function of receptive and interpretative components of speech. This part of the parietal lobe has connection with the ipsilateral motor cortex. This arrangement helps in producing patterns of movements in the lips, tongue, fingers, and respiratory muscles which form motor symbols to represent speech (gesture speech).

The thalamus appreciate crude (protopathic) sensation and pain. The fine sensations (epicritic) are appreciated in the cortex. Among the afferent impulses reaching the CNS, most do not reach consciousness. Some subseve spinal reflexes. These afferent fibers make contact either directly or through interneurosn with the spinal motor nuclei. Other fibers which carry muscle and joint sensations end in the base of the posterior horn of the spinal cord and synapse with the second order of neurons. These neurons form tracts which ascend up on the anterior and posterior spinocerebella tracts to reach the cerebellar cortex of the same side. Most of the fibers ascend on the same side but some fibers of tis tract cross to the opposite side to ascend in the anterior spinocerebellar tracts. These tracts carry propioceptive impulses which enable the cerebellum to coordinate activity.

Some other sensory fibres which do not bring sensation to consciousness and collateral branches of the main spinothalamic pathways and of the special sensory paths join the upper part of the reticular formation in the midbrain. In the reticular formation, there is an intercommunicating system of short neurons which also receive fibers from ost parts of the cerebral cortex. This system is important for maintaining consciousness.

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