Sensory Pathways - Anatomy & Physiology
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Introduction
Sensory information from the peripheries of the animal ascend through the spinal cord and enters the higher levels of the brain. There are numerous pathways which allow different types of information to be passed to the brain. Types of general somatic sensation include pain, touch, temperature and kinaesthesia (conscious proprioception). This sensory information is sent to one of two destinations; the cerebral cortex or the cerebellum.
When sensory information is relayed to the cerebral cortex, information first passes via the thalamus. The signal may be relayed one or more times by the thalamus en route to the cortex.
This sensory information reaches higher levels within the brain and is therefore reaches consciousness. The cerebellum is involved in co-ordination and in the main this sensory information does not reach consciousness.
Pathways
Ascending Pathways
Sensory information enters the spinal cord on the same side of the body as the stimulus. Ascending tracts cross over the midline of the body to the contralateral side of the thalamus. The thalamus directs the signal to the cerebral cortex for conscious perception. The pathway is direct with very few neurones involved.
1. Dorsal Columns
Dorsal columns transmit information from touch and kinaesthesia; these are both classified as low threshold information. There are two major dorsal columns; the gracile fasiculus situated medially which conveys information from the hindlimbs and caudal trunk and the cuneate fasciculus which is situated more laterally and conveys information from the forelimbs and cranial trunk.
2. Spinothalamic Tracts
Spinothalamic tracts transmit information from temperature and "pin prick" pain; these senses are classified as fast, initial pain sensations. These tracts compare with the ascending reticular formation.
3. Spinocervicothalamic Tracts
The spinocervicothalamic tracts transmit information from touch and kinaesthesia, although these are absent in man.
Spinocerebellar Tracts
These tracts transmit information from proprioception receptors, including information from muscle receptors, joint receptors and golgi tendon organs. Most sensory information enters the spinal cord on the ipsilateral side to the stimulus but some do cross to the contralateral side of the body. Contralateral signals pass back to the ipsilateral side of the body in the brain. Information is processed in the cerebellum and is therefore processed unconsciously.
Dorsal Spinocerebellar Tract
The dorsal spinocerebellar tract relays muscle spindle and golgi tendon organ information from the hindlimbs to the cerebellum.
Cuneo-cerebellar Tract
The cuneo-cerebellar tract serves the same purpose for the forelimbs as the dorsal spinocerebellar tract does for the hind limbs, but is much smaller.
Ventral Spinocerebellar Tract
The ventral spinocerebellar tract is similar to the dorsal spinocerebellar tract but it takes a less direct route to the cerebellum. The forelimb equivalent is called the rostral spinocerebellar tract.
Ascending Reticular Formation (Spinoreticular Tract)
The ascending reticular formation is thought of as the true pain sensation as the pain lasts longer. Sensory information enters the spinal cord on the ipsilateral side of the stimulus. Some signals cross to the contralateral side of the body. The tract consists of several short neurones. Therefore the ascending reticular formation is bilateral and multineuronal, although this pain pathway is thought to be more primitive than the spinothalmic tract. In humans, the ascending reticular formation is superceeded by the spinothalamic tract. In animals, the ascending reticular formation is the main pathway for pain to reach the cerebral cortex.
Pain
Painful stimuli can result in responses ranging from itches, to nausea, to simply being in agony. Painful stimuli may be transmitted to the brain by one of two pathways: the Spinothalamic Tract or the the Ascending Reticular Formation.
Spinothalamic Tract
This fast, initial pinprick is detected by free nerve endings and causes an impulse along large, myelinated fibres. The pain sensation is localised, and ends quickly.
Ascending Reticular Formation
This pain sensation is detected by free nerve endings which causes an impulse along small, unmyelinated fibres. This results in a delayed perception of the sensation of pain, and that pain is often less localised but more persistent.
Clinical Relevance
During trauma the small unmyelinated fibres are the last fibres to fail as they are close to the spinal cord. If deep pain sensation is lost in a case of trauma, then the prognosis is poor. Hyperalgesia is an increased pain sensation. This occurs when tissue is damaged because chemicals are released which increase the sensitivity of nociceptors, so that even light pressure can cause pain. Hyperalgesia may have evolved to aid the healing of injuries.
Visceral Pain
Visceral pain refers to pain related to the internal organs. Pain may be extreme, especially with distension, but the body's ability to localise the pain is poor. Skeletal muscle spasms may be observed during visceral pain. Commonly, the pain is referred to a different part of the body that is unharmed. This is because the areas are served by the same part of the spinal cord e.g. in angina (pain in the heart), pain can be felt in the inner left arm or jaw. This is called referred pain.
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