SENSORY FIBERS OF TRIGEMINAL NERVE

SENSORY FIBERS OF TRIGEMINAL NERVE

Trigeminal nerve carries somatosensory information from face, teeth, periodontal tissues (tissues around teeth), oral cavity, nasal cavity, cranial dura mater and major part of scalp to sensory cortex. It also conveys proprioceptive impulses from the extrinsic muscles of the eyeball.

Origin

Sensory fibers of trigeminal nerve arise from the trigeminal ganglion situated near temporal bone.

Peripheral processes of neurons in this ganglion form three divisions of trigeminal nerve, namely ophthalmic, mandibular and maxillary divisions. Cutaneous distribution of the three divisions of trigeminal nerve. Central processes from neurons of trigeminal ganglion enter pons in the form of sensory root.

Termination

After reaching the pons, fibers of sensory root divide into two groups, namely descending fibers and ascending fibers. Descending fibers terminate on primary sensory nucleus and spinal nucleus of trigeminal nerve. Primary sensory nucleus is situated in pons. Spinal nucleus of trigeminal nerve is situated below the primary sensory nucleus and extends up to the upper segments of spinal cord. Ascending fibers of sensory root terminate in the mesencephalic nucleus of trigeminal nerve, situated in brainstem above the level of primary sensory nucleus .

Central Connections

Majority of fibers from the primary sensory nucleus and spinal nucleus of trigeminal nerve ascend in the

form of trigeminal lemniscus and terminate in ventral posteromedial nucleus of thalamus in the opposite side. Remaining fibers from these two nuclei terminate on the thalamic nucleus of same side. From thalamus, the fibers pass via superior thalamic radiation and reach the somatosensory areas of cerebral cortex. Primary sensory nucleus and spinal nucleus of trigeminal nerve relay the sensations of touch, pressure, pain and temperature from the regions mentioned above. Fibers from mesencephalic nucleus form the trigeminocerebellar tract that enters spinocerebellum via the superior cerebellar peduncle of the same side. This nucleus conveys proprioceptive impulses from facial muscles, muscles of mastication and ocular muscles.

LEMNISCUS

Lemniscus or fillet is the prominent bundle of sensory nerves in brain.

Lemniscus is of four types:

1. Spinal lemniscus formed by spinothalamic tracts in medulla oblongata

2. Lateral lemniscus formed by the fibers carrying sensation of hearing from cochlear nuclei to inferior

colliculus and medial geniculate body nucleus cuneatus and nucleus gracilis

4. Trigeminal lemniscus formed by fibers from sensory nuclei of trigeminal nerve. This lemniscus carries general senses from head, neck, face, mouth, eyeballs and ears.

APPLIED PHYSIOLOGY

Lesions or other nervous disorders in sensory pathway affect the sensory functions of the body.

SOMATOMOTOR SYSTEM

MOTOR ACTIVITIES OF THE BODY

Motor activities of the body depend upon different groups of tissues of the body.

Motor activities are divided into two types:

1. Activities of skeletal muscles, which are involved in posture and movement

2. Activities of smooth muscles, cardiac muscles and other tissues, which are involved in the functions of

various visceral organs. Activities of skeletal muscles (voluntary functions) are controlled by somatomotor system, which is constituted by the somatic motor nerve fibers. Activities of tissues or visceral organs (involuntary functions) are controlled by visceral or autonomic nervous system, which is constituted by the sympathetic and parasympathetic systems

SOMATOMOTOR SYSTEM

Movements of the body depend upon different groups of skeletal muscles. Various types of movements or motor activities brought about by these muscles are:

1. Execution of smooth, precise and accurate voluntary movements

2. Coordination of movements responsible for skilled activities

3. Coordination of movements responsible for the maintenance of posture and equilibrium.

Voluntary actions and postural movements are carried out by not only the simple contraction and

relaxation of skeletal muscles but also the adjustments of tone in these muscles. The execution, planning, coordination and adjustments of movements of the body are under the influence of different parts of nervous system, which are together called motor system. Sensory system of the body also plays a vital role in the control of movements.