Textbook Module 7.2 Mechanical Systems -Somatosensory

Somatosensory Systems
The somatosensory system monitors the sensations of the body and its movements and include multiple systems.

STIMULUS
The different types of mechanical stimuli are listed in the text in Table 7.1 (pg 205). The stimuli include pain (generated by cell injury); heat or cold (on either side of physiological cell injury range)
Movement of hairs on the skin surface; sudden displacement of skin; light touch; skin stretch or also stretch or injury to joints and muscles.

 What these all have in common is that they are associated with types of receptors that are sensitive to them.

TRANSDUCTION
Receptors associated with the types of stimuli listed above are also listed in Table 7.1 They are
Free nerve endings; hair follicle receptors Meissner's corpuscles, Pacinian corpuscles, Merkel's disks, Ruffini endings and Krause end bulbs.  These receptors are located in the  skin and in the joints and muscles.  Stimulation of a touch receptor opens sodium channels in the axon and thereby starts an actionl potential.

TRANSMISSION
Somtosensory information from touch receptors on the head enter the CNS through the cranial nerves, such as Cranial Nerve V. . Somatosensory information  from receptors below the head enter the spinal cord via the dermatones.   Dermatomes were discussed previously in Module 2.1- cells of the Nervous System on this blog.

Transmission Pathway for Fine Touch and Vibratory Sense-  Dorsal Column Medial Lemniscus
see unit 9.5-9.6 of the Coloring Book (pg 142-145)

• The transmission pathway for touch is carried into the spinal cord via the dorsal roots (see Unit 1.4 Coloring Book pg 10) and then without synapsing ascend ipsilaterally in the dorsal part of the spinal cord to the dorsal column nuclei.  Sensory input from the legs synapse in the nucleus gracilis and input from the arms synapse in the nucleus cuneatus. 
• Axons from the dorsal column nuclei decussate in the medulla and ascend to the the thalamus via the dorsal column medial lemniscus.
• Axons from the dorsal column medial lemniscus ascend to the thalamus.  Fibers from Cranial Nerve 5 join the medial lemniscus enroute to the thalamus
• Most of the axons of the medial lemniscus synapse on neurons in the ventral posterior nucleus of the thalamus.
• Axons from the VPN project to the primary somatosensory cortex (Coloring book unit 7.6 pg 120).
• The primary somatosensory cortex is located on the post central gyri (see coloring book unit 7.1 pg 119) of the parietal lobes (CB unit 7.2 pg 112).

• The output of the primary somtosensory cortex is mostly projected to the secondary somatosensory cortex located just inferior to the primary somatosensory cortex located in the ventral portion of the postcentral gyrus and hidden by the lateral fissure
• Output from the sencodary somatosensory cortex is sent to the posterior parietal association cortex.

The somatosensory cortex is laid out somatopically, a map of the body is on the brain. (Just like there is a retinotopic representation of the visual image represented on the occiptial lobe and a tonatopic representation of frequency represented in the cochlea and auditory cortex).
The somatosnesory body map is called a homonculus. (CB unit 8.6 pg 144).


Transmission Pathway for Pain, Temperature, Crude Touch -Spinothalamic Tracts
Pain and Temperature (lateral spinothalamic tracts) Crude Touch (anterior spinothalamic tract)
 see Textbook figure 7.15 and 7.16 (pg 208-209).

Why is pain information conveyed so slowly to the brain?
What are the neurotransmitters involved in conveying pain in the spinal cord?
How does the pain transmission pathway differ from the touch transmission pathway?

Pain information crosses to the contralateral side of the spinal cord at once. The pathways then continue to the brain via the spinothalamic tract. This axonal pathway begins in spine then ends in thalamus- thus the name spine -o- thalamic.



 

• The spinothalamic tract conveys pain and temperature (lateral spinothalamic tract) and crude touch (anterior spinothalamic tract).
• Primary sensory nerves that register pain have  cell bodies that lie in the posterior dorsal root ganglion. 
• They synapse in in the posterior horn of the dorsal root in the spinal cord.
• Secondary neurons cross the spinal cord contralaterally and ascend to the brain via the  lateral spinothalamic tract in a somatotopic arrangement. 
• Secondary neurons synapse in the VPN of the thalamus,
• From the VPN of the thalamus, tertiary neurons ascend via the internal capsule to the primary sensory cortex  
• These cells also branch off axon collaterals to the limbic system, hypothalamus, and other portions of the CNS such s the amygdala, hippocampus and prefrontal cortex.  These areas react not to the sensation of pain but  to its emotional associations.

SENSATION PERCEPTION COGNITION

Descending Analgesic Circuit
Textbook Figure 7.18 pg 210 and unit 8.7 of the Coloring Book (pg 146)

A centrifugal pathway is a pathway that exerts either a facilliatory or inhibitory influence on a nerve signal.  One example of a centrifugal pathway is the descending analgesia circuit.  During times of intense emotion it is possible to feel little or no pain, even when there is a grave injury.

What is a centrifugal pathway?

The inhibition of pain information in the brain is due to the activation of natural opioids or endorphins in the midbrain structure, the periaqueductal gray (PAG)- gray area surrounding the cerebral aqueduct in the midbrain.

Axons from the PAG synapse back to the spinal cord  and decreases pain signals in the spinal cord. Therefore, even though pain receptors may be firing the signal to the brain has been blocked and therefore the injured individual does not experience pain.

See figure 7.18 in the textbook

• "Certain kinds of painful and other stimuli" send emotional pain information from the cortex to the PAG via endorphine neurotransmitters.  
• The PAG projects to the "area in the rostral part of the medulla" and excites cells that release more endorphine neurotransmitter into the spinal cord. 
• The endorphine neurotransmitter inhibits the primary sensory nerve which is releasing substance P as a neurotransmitter and therefore stops "areas of the spinal cord that receive pain messages" from sending ascending pain messages to the brain
• 'The injured person experiences no pain

More recent research about this pathway can be found here.