No. 26 Sensory Pathways (1)

Section 3 The Nervous Pathways Introduction: As known, the basic way of activity of nervous system is reflex. The morphological foundation of completing the reflex is called reflex arc. A simple reflex only consists of two neurons, i.e. afferent (sensory) and efferent (motor) neurons. For example, the reflex arc of knee jerk is merely composed of two neurons.

But a complicated reflex arc includes many neurons But a complicated reflex arc includes many neurons. In view of this, the complicated reflex activity is accomplished by the linkage consisting of afferent, intermediate and efferent neurons. We named the neuronal linkage as the nervous pathway. It has characteristic with long connected course, including afferent and efferent portions. The nervous pathways are the routes formed by chains of neurons, through which sensory awareness reaches the cerebral cortex and a motor response is initiated.

For convenience of study, the nervous pathways are commonly classified into: sensory (ascending) pathways, motor (descending) pathways.

Ⅰ. The Sensory (ascending) Pathways In most sensory pathways there are three orders of neurons involved: ① The lower (the first order) sensory neurons, located in the ganglia, and their peripheral and central processes; ② The intermediate (the second order) neurons in the spinal cord or brain stem and their processes; ③ The upper (the third order) sensory neurons, which are the cells of the thalamus and the fibers passing from them to the cerebral cortex. The sensory pathways mainly include the deep and superficial sensory, visual and acoustic pathways.

Ⅰ) The Proprioceptive (or Deep) Sensory Pathways It mediate the proprioceptive (deep) sensations (including the sensations of the body posture, movement, vibration) and fine touch sensation (discriminatory sensation).

1. The conscious proprioceptive and fine touch sensory pathway of trunk and limbs It consists of three orders of neurons. (1) The first neurons and their fibers The first neurons (pseudounipolar neurons) are located in the spinal ganglia. Their peripheral processes accompany the corresponding spinal nerve to the proprioceptive receptors of the muscle, tendon, periosteum and joint, and the fine touch sensory receptors.

Their central processes enter the spinal cord as a medial bundle (thick myelinated fibers) through the dorsal roots of the spinal nerve. The longer ascending fibers run upward in the ipsilateral posterior funiculus, where they form the fasciculus gracilis (fibers derived from below the T5 spinal segments) and fasciculus cuneatus (fibers derived from above the T4 spinal segments). Both fasciculi end separately in the gracile and cuneate nuclei, in which the second neurons of this pathway begin.

(2) The second neurons and their fibers The bodies of the second neurons are in the gracile and cuneate nuclei. The majority of the fibers of the second neurons sweep ventrally round the central gray matter as the internal arcuate fibers, and then bend medially to reach the median plane, where they decussate with the corresponding fibers of the opposite side, forming the decussation of medial lemniscus. After decussation, the fibers ascend along the two side of median line, constituting the medial lemniscus.

After that, as the medial lemnisci, they ascend on each side through the medulla oblongata, pons (at the anterior border of tegmentum) and midbrain (lateral to the red nucleus) to the ventroposterior lateral nucleus of the thalamus which contains the third neurons.

(3) The third neurons and their fibers The third neurons locating in the ventroposterior lateral nucleus of the thalamus send out axons to form the thalamocortical tract, running through the posterior limb of the internal capsule to the cortex of the superior and middle parts of the postcentral gyrus and to the posterior part of the paracentral lobule.

Interruption of pathways by lesions below the level of the decussation of medial lemniscus causes hemiplegia on the affected side of the body, including the loss of all the sensations conducted by the path. An interruption by a lesion above the level of the decussation produces hemiplegia on the opposite side of the body, including the loss of all the sensations conducted by the path.

2. The unconscious proprioceptive sensory pathway of trunk and limbs This pathway is only composed of two order neurons. (1) First order neurons The cell bodies of the first order neurons are located in the spinal ganglia. Their peripheral fibers end in the proprioceptive sensory receptors of the muscles, tendon, periosteum and joint, and their central processes enter the spinal cord and terminate in the thoracic nucleus of segments C8-L2 and the lateral part of layers Ⅴ--Ⅶ in the cervical and lumbosacral enlargements of the spinal cord.

(2) Second order neurons The second order neurons are in the thoracic nucleus and Ⅴ--Ⅶ in the cervical and lumbosacral enlargements. The axons of the secondary neurons ascend in the lateral funiculi as the anterior or posterior spinocerebellar tracts which enter the cerebellum by way of the superior and inferior cerebellar peduncles respectively.

The connections are made with neurons of the cortex of the cerebellum, where motor impulses are returned to the segmental levels of the spinal cord through part of the extrapyramidal tracts to maintain the coordination movement and equilibrium and appropriate posture of the body.

Ⅱ) The Pain, Thermal and Rude tactile and Pressure (or Superficial) Sensory Pathways They include three orders of neurons and convey the sensation of pain, temperature and rude tactility and pressure from the skin and mucosa to the centers.

1. The pain, thermal and rude tactile and pressure (superficial) sensory pathway of trunk and limbs (1) First neurons The first neurons of the route are pseudounipolar and located in the spinal ganglia. Their peripheral processes join the spinal nerve to the cutaneous exteroceptors of the trunk and limbs.

Their central processes enter the spinal cord. The fine fibers (in the lateral part of posterior root) conducting pain and thermal sensation enter the spinal cord through the dorsolateral fasciculus and terminate in the second neurons. The rude fibers (in the medial part of posterior root) conducting rude tactile and pressure enter the posterior funiculus and then terminate in the second neurons.

(2) Second neurons The cell bodies of the second neurons are mainly in the layersⅠand from Ⅳ to Ⅶ of gray matter of spinal cord. The second order fibers from the second neurons run upward within 1–2 segments of spinal cord and cross to the lateral funiculus and anterior funiculus of the opposite side through the anterior white commissure, forming the lateral spinothalamic tract (conducting the pain and thermal sensations) and anterior spinothalamic tract (conducting the tactile and pressure sensations).

The spinothalamic tracts go upward lying dorsolaterally to the inferior olivary nucleus in the medulla oblongata, and, dorsolaterally to the medial lemniscus in the pons and midbrain, and terminate in the ventral posterolateral nucleus.

(3) Third neurons The cell bodies of the third neurons are located in the ventral posterolateral nucleus. The third neurons send out axons to join the central thalamic radiation (or thalamocortical tract) and pass through the posterior limb of the internal capsule to the upper and middle parts of the postcentral gyrus, and the posterior part of the paracentral lobule.

2. The pain, thermal and rude tactile and pressure (superficial) sensory pathway of the head and face (1) First neurons The cell bodies of the first neurons are situated in the trigeminal ganglion. Their peripheral processes join the sensory branches of the trigeminal nerve and terminate in the superficial receptors in the skin of the head and face and mucosa of the oral and nasal cavities.

Their central processes enter the pons via the sensory root of the trigeminal nerve. The fibers conducting the impulses associated with pain and thermal sensation descends to form the spinal tract of trigeminal nerve and terminate in the spinal nucleus of trigeminal nerve. The fibers being concerned with tactile and pressure sensation terminate in the pontine nucleus of trigeminal nerve.

(2) Second neurons The second neurons, situated in the spinal and pontine nuclei of the trigeminal nerve, give rise to axons to opposite side to join the trigeminal lemniscus running through the pons and the midbrain to the ventral posteromedial nucleus of the thalamus.

(3) Third neurons The cell bodies of the third neurons are in the ventral posteromedial nucleus of the thalamus. They send out fibers to form the central thalamic radiation (thalamocortical tract) coursing through the posterior limb of the internal capsule to the inferior part of the postcentral gyrus. here.