Somatosensory system worksheet
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The somatosensory system handles what we call colloquially the “sense of touch”, but its scope is wider than we normally understand by “touch”: it also gives us information about muscle stretch, muscle force and joint position, a sense that it technically known as proprioception, and in addition, receptors called nociceptors detect actual or potential tissue damage and give us the sensation of pain.
The discovery of sensory “spots” in the skin led to the search for specific receptors and several types of receptor have been described. These all have several features in common. They have long axons (nerve fibres) terminating at one end in the skin and at the other end in the spinal cord, and the cell body is found in the dorsal root ganglion.
Information is transmitted by way of electrical signals known as action potentials, or more colloquially as “nerve impulses”, and the intensity of the stimulus determines the frequency of these electrical signals. Nerve fibres at rest are negative inside, relative to the extracellular space, and when an action potential is being conducted along a nerve fibre it becomes briefly positive inside.
Muscle stretching (lengthening) is detected by receptors that are made up of specialised muscle fibres with nerve endings wrapped around them; the whole receptor is known as a muscle spindle. Muscle force is detected by receptors that are located between muscle and tendon, called Golgi tendon organs.
Nociceptors do not generate action potentials unless tissue damage has taken place, or a stimulus is being applied that would be likely to cause tissue damage. They can be activated by three classes of stimulus:
1. stimuli causing tissue damage (such as heat, low pH, mechanical stimuli)
2. substances released from or present around damaged cells (such as ATP, potassium, bradykinin)
3. inflammatory mediators (substances present in inflamed tissue, which control the process of inflammation).
Nociceptive nerve fibres conduct at different speeds; those with myelin (an insulating layer around the nerve fibre) conduct faster than those without myelin. This results in two distinct pain sensations: “fast pain” and “slow pain”. Fast pain is useful to us in situations like the following: dropping an object that is too hot to hold while slow pain is involved in situations like putting something down when it is slightly too hot
Referred pain is the phenomenon of pain from internal organs (visceral pain) appearing to come from an area on the body surface. This is because sensory neurones from internal organs and from the skin form synapses onto the same second-order neurone in the spinal cord (explain in your own words). As an example, pain from the heart is often referred to the following areas: chest, left arm, left shoulder
The somatosensory cortex in the brain gives us our conscious somatosensory sensation. The area of somatosensory cortex dedicated to each part of the body surface is not constant; some areas of the skin have a much larger sensory cortical representation than others. In humans, the following body areas have a particularly large cortical representation: lips, tongue, face, hands (especially fingertips and thumbs)
This unequal cortical representation means that certain areas of the body can resolve much finer detail of an object than others. This can be tested quantitatively with a pair of dividers from a geometry set, using the two point discrimination test.