Sensory Division Of The PNS: The Somatic Senses (Physiology)

Question 1

Describe the various receptor designs

Answer 1

Type 1) The simplest design is a cluster of free nerve endings which can be myelinated or unmyelinated. Pain, heat, and cold are all mediated by this receptor design.

Type 2) Meissner’s corpuscle consists of unmyelinated nerve endings in a connective tissue capsule. These are touch receptors.

Type 3) A pacinian corpuscle consists of an unmyelinated nerve ending surrounded by concentric layers of modified Schwann cells. These are pressure receptors.

Question 2

Explain the process of signal transduction

Answer 2

• This is the process of converting stimuli into action potentials.
• The membrane potential change when a receptor responds to a stimulus is called a receptor potential.
• Receptor potentials occur due to ions crossing the cell membrane, this can be a result of either ion channels opening or a change in cell membrane permeability.
• When the receptor potential reaches a threshold an action potential is evoked.

Question 3

Explain what the phrase receptor adaptation means

Answer 3

• When a continuous stimulus is applied most receptors undergo a process called adaptation when there is a decrease in action potential generation over time.
• Some types of receptors are rapidly adapting such as Meissners corpuscles.
• However, if the steady state changes these receptors are quick to respond.
• Other receptors such as baroreceptors which monitor blood pressure are very slow to adapt and are therefore good at continually monitoring stimuli.
• Pain receptors adapt very little and sometimes become more sensitive to stimuli, responding to non-noxious stimuli.

Question 4

Explain how the labelled line principle and summation allow the CNS to distinguish between different senses and intensities of sensation

Answer 4

• The labelled line principle describes how the sensation felt when the nerve fibre is stimulated, is dictated by the point to which the fibre leads as nerve fibres can only transmit one modality (the action potential, which is all or nothing).
• For the somatic there isn’t a centre in the brain which deals with each sense separately, instead it is organised according to tissue location rather than the type of signal received e.g. the part dealing with the hands is different from the part dealing with the torso.
• Stimuli intensity can be discriminated between through spatial and temporal summation.
• Spatial summation occurs when, as the stimulus intensity increases, so does the number of individual neurones within a nerve generating an action potential.
• Temporal summation occurs when, as the stimulus intensity increases, so does the number of action potentials generated by individual neurones per unit time.

Question 5

Identify the types of receptors involved in mediating the following somatic senses: touch and pressure, temperature, and pain

Answer 5

Touch and pressure

• Grouped together because pressure is sustained touch.
• The stimuli for them is mechanical deformation, therefore the receptors involved are called mechanoreceptors.
• Light touch = tactile (Merkel) discs and Meissners corpuscles
• Heavy touch = Ruffini corpuscles
• Pressure = Pacinian corpuscles and Ruffini corpuscles
• These receptors are most numerous in the skin of fingers and lips.

Temperature

• There are cold and warm receptors (thermoreceptors) and pain receptors which respond to extremes of temperature.
• Warmth receptors are stimulated above 30 degrees celsius.
• Heat-pain receptors are stimulated above 45 degrees celsius.
• Cold receptors are stimulated below 45 degrees celsius.
• Cold-pain receptors are stimulated below 15 degrees celsius.
• Thermoreceptors are found in the skin, cornea, and urinary bladder.
• Cold and warm receptors adapt fair rapidly, therefore thermal senses respond to changes in temperature rather than steady states of temperature.

Pain

• Can be defined as an unpleasant experience associated with actual or potential tissue damage.
• Very important sensation in terns of self-preservation.
• Receptor responsible is called the nociceptor which i simply a free nerve ending.
• Widespread throughout the skin, arterial walls, and joint surfaces.
• Fast pain creates a sharp and immediate sensation which is highly localised. It is conveyed to the spinal cord by thin myelinated nerve fibres (fast conduction velocity). These are referred to as type A fibres.
• Slow pain creates, after a delay, a pain which is hard to locate. It is conveyed to the spinal cord by unmyelinated nerve fibres (slow conduction velocity). These are referred to as type C fibres.

Question 6

Describe the ascending pathways which carry action potentials from receptors to the relevant areas of the brain

Answer 6

• The phrase ‘somesthetic projection pathways’ reefers to the neuronal pathways which carry signals to the parietal lobe of the cerebral cortex.
• Somatic senses originating in the head region travel via the trigeminal nerve to an area of the parietal lobe called the somatosensory cortex.
• Most signals from below the head travel via travel via a tract called the spinothalamic tract. Signals pass up the spinal cord, into the brainstem, the thalamus, and then into 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.

Question 7

Identify the areas of the brain which receive action potentials from the somatic senses

Answer 7

• Stimulation of specific parts of the sensory cortex in conscious patients undergoing brain surgery causes pins and needles in specific areas of the body. This reveals that there are distinct areas of the brain dealing with specific parts of the body.
• Based on this information the sensations have been mapped onto the somatosensory cortex of the parietal lobe (sensory homunculus).
• 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 information.
• In the parietal lobe, somatosensory area one receives action potentials from the thighs, thorax, neck, shoulders, hands, fingers, tongue, and abdomen. Somatosensory area two receives action potentials from the legs, arms, and face.