Sensory system - Ascending tracts, Synthetic sense

Sensory Tracts

They are group of neurons present in the spinal cord which carry the sensory information to the brain.

They are also called as ascending tracts.

The sensory information is first relayed in the thalamus and finally projected to the sensory cortex, area number 3, 2, 1.

The crude form of sensation and pain which are called protopathic sensation are appreciated at the thalamus.

The finer details of sensations which are called epicritic sensation is perceived by the cerebral cortex.

The important sensory tracts are

1. Dorsal or Posterior column pathway
2. Anterior or ventral column pathway
3. Lateral column pathway Pathway

Dorsal or Posterior column pathway

It is consists of two tracts

1. Tract of Goll (Fasciculus gracilis)
2. Tract of Burdach (Fasciculus cuneatus)

Hence it is also called Tract of Goll and Burdach

They carry sensations of

• Fine touch
• Proprioception (Joint and position sense)
• Vibration
• Pressure

Pathway of Tract of Goll and Burdach

Three orders of neurons are present in this pathway - First, Second and Third order.

First order neuron

• It arises from the respective sensory receptors and enters the spinal cord through the dorsal or posterior root.
• They ascend on the same side of spinal cord in Fasciculus grasalis and Fasciculus cuneatus and relay in the Nucleus gracilis and Nucleus cuneatus present in the medulla.
• Fasciculus gracilis which lies medially in the spinal cord, contains fibers from the lower parts of the body.
• Fasciculus cuneatus which lies laterally in the spinal cord, contains fibers from the upper part of the body.

Second order neurons 

• They cross to the opposite side and relay in the Ventral Posterolateral Nucleus (VPN) of Thalamus.
• The crossing fibers are called Internal Arcuate Fibers.
• The crossings of fibers occur at the level of medulla and not at the level of spinal cord.

Third order neurons 

• They arise from the Ventral Posterolateral Nucleus of Thalamus and project on to the Sensory cortex, area number 3, 2, 1. 

Dorsal column pathway

Lateral column pathways consists of 

• Lateral spinothalamic tracts
• Dorsal spinocerebellar tract
• Ventral spinocerebellar tract
• Spinotectal tract
• Spino-oivary tract
• Spino-vestibular tract
• Spino-pontine tract
• Spinoreticular tract

Anterior and lateral column pathway are together called Anterolateral Spinothalamic pathway

They carry sensatons of

• Pain and temperature by the lateral tract
• Curde touch by the ventral tract

Pathway of Anteriolateral Spinothalamic tract (also known as Pain Pathway)

First order neurons arising from the receptors enter the spinal cord through the dorsal horn and end there. 

Second order neurons arise from the dorsal horn and cross to the opposite side at the same level of spinal card. 

The second order neurons of lower part of the body are present laterally and upper part of the body are present medially.

This arragement is seen oppostite in Tract of Goll and Burdach (Dorsal column pathway)

The fibers ascend in the anterior and lateral part of spinal cord, passes through medulla, pons and midbrain to end in Ventral Posterolateral Nucleus of Thalamus.

Third order neurons arise from the Vertral Posterolateral Nucleus of Thalamus and finally project on to the Sensory cortex, area number 3, 2, 1. 

Anterolateral spinothalamic tract

The other ascending tracts apart from Dorsal column and Anterolateral Spinothalamic tracts do not relay in Thalamus, instead relay in different nucleus of the brain stem and cerebellum.

They are:

• Dorsal spinocerebellar tract
• Ventral spinocerebellar tract
• Spinotectal tract
• Spino-oivary tract
• Spino-vestibular tract
• Spino-pontine tract
• Spinoreticular tract

Important of them are Dorsal and Ventral Spinocereballar Tracts

Dorsal spinocerebeallar tract

It is also called as Flechsig's tract.

The sensations carried by this tract are:

• Unconciuous kinesthetic impusles to cerebellum

Pathway of Dorsal spinocerebellar tract

First order neurons arise from muscle spindle, joints and skin and enter the spiral cord through dorsal root.

They terminate at the Clarke's column situated at the base of the dorsal horn of spinal cord.

Second order neurons arise from Clarke's column and ascend on the same side of the spinal cord till the medulla.

In the medullay it turns posteriorly to enter the cerebellum through the Inferior Cerebellar Peduncle.

Ventral spinocerebellar tract

It is also called as Gower's tract.

The senstions carried by Ventral spinocerebeallar tract are:

• Uncondious kinestehtic impulses to the cerebellum

First order neurons arise from muscle spindle, joints and skin and enter the spiral cord through dorsal root.

They terminate at the Rexed lamina situated in the lateral horn of spinal cord.

Second order neurons cross to the opposite side at the same level and ascend till medulla, pons and midbrain.

In the midbrain it turns posteriorly to enter cerebellum through Supeiror Cerebellar Pedunce.

Dorsal and Ventral Spinocerebellar tracts

Synthetic sense

It is produced by combination of two types of basic sensations.

Examples of Synthetic sensations are:

1. Stereognosis
2. Vibration sense

Stereognosis

It is the ability of a person to appreciate the size, shape, texture and weight of the objects without actually looking at them. This is possible by the past experience of the objects by the person.

Stereognosis is a combination of two sensations - Touch and Pressure.

Any damage to the Parietal lobe posterior to the post central gyrus will lead to impaired Stereognosis.

The inability of a person to identify the commonly used objects by touching them is called as Tactile agnosia.

Vibration sense

It is also a type of synthetic sesantion produced by combination of touch and pressure sensation

It is lost under the following conditions

• Damage to dorsal column tracts
• Uncontrolled/poorly controlled diabetes mellitus
• Vitamin B12 deficiency
• Pernicious anemia
• Tabes dorsalis