Somatic Senses

Question 1

What is a receptor?

Answer 1

any structure specialised to detect a stimulus - there are three types

Question 2

Most simple receptor

Answer 2

cluster of free nerve endings, which can be myelinated and non-myelinated

pain, heat and cold are all mediated by this receptor design

Question 3

Meissner’s corpuscle

Answer 3

mediates touch

consists of unmyelinated nerve endings in a connective tissue capsule

Question 4

Pacinian corpuscle

Answer 4

unmyelinated nerve ending is surrounded by concentric layers of modified Schwann cells

Question 5

Merkel disc

Answer 5

the combination of the enlarged myelinated axon terminals and the Merkel cell to which they are closely opposed in the epidermal layer

Question 6

Ruffini’s corpuscle

Answer 6

consists of a bundle of collagen fibrils with a myelinated sensory axon branching between the fibrils, enclosed inside a flattened capsule

Question 7

What is signal transduction?

Answer 7

the process of converting the stimuli into action potentials

Question 8

What is the receptor potential?

Answer 8

The membrane potential charge when a receptor responds to a stimulus is called a receptor potential

Question 9

What is adaptation?

Answer 9

when a continuous stimulus is applied, there is a decrease in action potential generation over time

Question 10

Spatial summation?

Answer 10

occurs when, as the stimulus intensity increases, so does the number of individual neurones within a nerve generating an action potential

Question 11

Temporal summation?

Answer 11

occurs when, as the stimulus intensity increases, so does the number of action potentials generated by individual neurones per unit time

Question 12

Somatic Sense 1+2 (explain)

Answer 12

Touch and Pressure

• pressure is sustained touch
• stimuli is mechanical deformation, therefore receptors involved are referred to as mechanoreceptors
• light touch mediated by tactile (Merkel) discs and Meissner’s corpuscles
• Ruffini corpuscles detect heavy touch
• Pacinian and Ruffini corpuscles detect pressure
• receptors are found throughout the skin and in subcutaneous tissue, but are most numerous in skin of the fingers and lips

Question 13

Somatic Sense 3 (explain)

Answer 13

Temperature

three types of receptors:
- cold receptors and warm receptors (thermoreceptors)
- pain receptors responding to extremes of temperature

warmth receptors are stimulated at temperatures greater than 30oC

heat-pain receptors are stimulated at above 45oC

cold receptors are stimulated at temperatures less than 45oC

cold-pain receptors are stimulated at less than 15oC

thermoreceptors are found in skin and other tissues including the cornea and urinary bladder

Question 14

Somatic Sense 4 (explain)

Answer 14

Pain

two types of pain are often felt together e.g. pricking finger with needle

Fast pain:
creates a sharp, seemingly immediate sensation which is highly localised. It is conveyed to the spinal cord by thin, myelinated nerve fibres which exhibit a rapid conduction velocity (6-30m/s). These fibres are generally referred to as type Aδ fibres

Slow pain:
This type of pain creates, after a delay, a dull pain that’s hard to locate. It is conveyed to the spinal cord by unmyelinated nerve fibres with a slow conduction velocity (0.5-
2m/s). These fibres are generally referred to as type C fibres

K+ ions (released from damaged cells) excite nociceptors, and the intensity of pain felt can be directly related to the local increase in potassium ion concentration
- other chemicals exciting nociceptors are bradykinin and serotonin

Question 15

Somesthetic Projection Pathways (explain)

Answer 15

Neuronal pathways which carry somatic signals to the parietal lobe of cerebral cortex

Somatic senses originating in the head region travel via the trigeminal nerve to an area of the parietal lobe referred to as the somatosensory cortex.

From areas below the head most signals travel via a tract referred to as the spinothalamic tract, signals
pass up the spinal cord, into the brainstem, the thalamus and from there to the somatosensory cortex.

Sensory fibres travelling via the spinothalamic tract cross over in the spinal cord, and as a result the cortex receives sensory information from the opposite side of the body.

The dorsal columns of the spinal cord carry proprioceptive signals and the sensation of fine touch.

Question 16

Somatosensory Cortex (explain)

Answer 16

There are distinct areas of the somatosensory cortex of the parietal lobe that deal with specific parts of the body.

the more sensitive the body part, the greater the area of cortex allocated for that part.

This is because the more sensitive the body part, the greater the number of sensory
receptors are located there, and essentially more brain tissue is required to deal with the greater amount of incoming data.