Physiology Practical 1: Somatosensation

Question 1

What is somatosensation?

Answer 1

Somatosensation, which includes sensation received from the skin, as well from as the limbs and joints is often described as the sense of touch. The tutor lead session that accompanies this activity explores some of the ways that these sensory modalities can be tested in humans and what happens when these pathways are disrupted.

Question 2

What are the two aims of this practical?

Answer 2

The aim of this practical two fold:

(1) To introduce somatic sensation and test one of these modalities, using the two point discrimination test.
(2) To examine and test changes in sensory perception in a subject when the circulation is occluded and discuss the neurophysiological processes responsible for any changes observed.

Question 3

What are the learning objectives of this practical?

Answer 3

Recall the different types of sensory receptors in the skin that respond to pressure and vibration,
Explain how to perform a two-point discrimination test and its use in measuring spatial discrimination of skin mechanoreceptors
Describe tests for sensory (and motor) functions and the changes in sensory perception in a subject when circulation is occluded in one arm.

Question 4

What is the stimulus in the eye and what receives this stimulus?

Answer 4

Light photons

Stimulus for retinal photoreceptors

Question 5

What is the stimulus in the ear and what receives this stimulus?

Answer 5

Sound

Stimulus for cochlear hair cells

Question 6

What is the stimulus in the inner ear and what receives this stimulus?

Answer 6

Head angle/velocity and acceleration

Stimulus for vestibular hair cells

Question 7

What is the stimulus in the skin and what receives this stimulus?

Answer 7

Heat

Stimulus for thermoreceptors

Question 8

What are the special senses?

Answer 8

There are the special senses, i.e. vision, taste, hearing, smell and equilibrium

Question 9

How can the somatic senses be divided?

Answer 9

The somatic senses can be divided into a combination of at least three types:

Mechanoreceptive
Thermoreceptive
Pain

Question 10

What is sensory transduction and how is it carried out? How can sensory receptors be classified?

Answer 10

Sensory transduction (the process by which properties of the environment become encoded as nerve impulses) is carried out via specialised receptors (sensory receptors). Sensory receptors can be classified on the basis of the environmental properties to which they are sensitive.

Question 11

What are mechanoreceptors, what do they respond to and what are some examples?

Answer 11

Mechanoreceptors: There are various types of mechanoreceptors found in skin that respond to mechanical forces (various modalities of touch) e.g. light touch, pressure and vibration.

In smooth (glabrous skin) there are four main types of sensory receptors : Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs, and Ruffini endings (see diagram).

Smooth and hairy skin also contain free nerve endings these can respond to both touch but also in some cases temperature.

Question 12

What are sensory receptors and what are some examples? to

Answer 12

Sensory receptors are either ‘free nerve endings’ where the sensory nerve branches profusely and ends up lying in the extracellular space between tissue cells, or ‘encapsulated nerve endings’ where the nerve ending is surrounded by a specialised connective tissue ‘capsule’.

Encapsulated nerve endings in skin include Pacinian corpuscles, Meissner corpuscles, Merkels disks and Ruffini endings.

Question 13

How do sensory receptor capsules adapt to different forms of mechanical stimulation? Why are some faster to adapt than others?

Answer 13

The capsule ‘tunes’ the terminal nerve fibre to respond to different forms of mechanical stimulation. Some receptors only respond transiently to a stimulus and are called rapidly adapting (Pacinian corpuscles and Meissner corpuscles): others respond for longer and are slowly adapting (Merkel’s disks and Ruffini endings). Rapidly adapting receptors only respond at the beginning of a stimulus: they ‘fatigue’ after a second or so to a sustained steady stimulus. Slowly adapting receptors will continue firing to a sustained stimulus but at a gradually reducing rate.

Question 14

What is two-point discrimination testing and what is it used for?

Answer 14

Clinical neurosensory testing is performed to evaluate sensory abnormalities. Two-point discrimination (TPD) testing is used for measuring the sensitivity of mechanoreceptors and, due to its simplicity, is still widely used in clinical contexts.

TPD measures tactile spatial acuity. Calipers are used to assess the ability of an individual to determine between two distinct points rather than one (as shown in the video). The minimum distance at which a subject can make this distinction is referred to as the threshold.

Question 15

Which sense is inactivated during TPD and how?

Answer 15

Vision (using opaque goggles)

Question 16

Which parts of the body have the smallest discrimination threshold?

Answer 16

Fingertips, lips, tongue
The receptive fields for touch are particularly small at the tips of the fingers and the lips (about 1mm or less for two point discrimination) where fine tactile discrimination is required. Whereas in other areas such as neck, thigh, back or calf, the receptive fields can be much larger.

Question 17

What determines the distance between two points on the skin which can be detected as separate stimuli?

Answer 17

This is closely allied to the density of touch receptors and size of their receptive fields (two point discrimination threshold).
In the skin, the higher the density of receptors and the smaller the receptive field, the finer the discrimination for mechanoreception (and touch).

Question 18

Why do we need better touch discrimination in the fingers and lips?

Answer 18

Tips of fingers:
from an evolutionary perspective: contributes and improves the ability to use tools, hunt, obtain meat, scavenge, protect offspring; also aids in acquiring a mate
Lips are important for:
the sucking reflex (to maintain a tight seal for swallowing), eating (to avoid choking on food or accidently swallowing toxics), communication (speech; smiling; kissing –which is also used for choosing sexual partners)

Question 19

How is touch across different parts of the body represented in the brain?

Answer 19

The primary somatosensory cortex is a strip of cortex running medio-laterally just posterior to the central sulcus.
The primary somatosensory cortex has a complete map of the body surface but with a gross distortion of the relative size of different body areas.

Areas of skin with the highest density of touch and proprioceptive receptors occupy the largest areas of the cortical map.

The parts of our body that we use to discriminate shape and texture (hands and lips) have the largest representations whereas shoulder and back have a much smaller area in the somatosensory cortex.

Question 20

What is the method for nerve conduction block practical? What are the results?

Answer 20

See tutor powerpoint

Question 21

What are the general observations from the nerve conduction block practical?

Answer 21

Students reported feeling “pins and needles” in their arms and fingers after 5 mins.
Motor movements became “weaker”.
Discoloration in arm as experiment progresses, skin noticeable paler in some students.
When cuff released students report feeling prickly sometimes uncomfortable sensations in their arm accompanied by a reddening of skin in some circumstances.

Question 22

What was the sequence in perceptual changes during the practical?

Answer 22

Joint sense is being affected from 10 min

Touch and two-point discrimination are being affected within 10-15 min

Detection of vibration is lost within approx. 15 min
Peripheral sensory axons are more vulnerable than motor axon: Muscle weakness is progressing over time, but motor function is relatively preserved until late (15-20 min)

The sensation of pain is preserved throughout.

Question 23

Which different sensory receptors are innervated by different types of nerve fibres?

Answer 23

Ia and Ib innervate proprioceptors
II and III innervate mechanoreceptors
III and IV innervate noiceptors and thermoreceptors
(see table on powerpoint)

Question 24

How does local pressure on a nerve affect axonal conduction?

Answer 24

External pressure -> Reduced blood flow in the vessels supplying the nerve
(vasa nervorum)
-> Local ischaemia -> Immediate effect on the ability of axons to transmit action potentials

With time will lead to focal demyelination, axonal damage and scarring (e.g. carpal tunnel syndrome etc)

There is an initial reduction in epidural blood flow, followed by a reduction in the endoneural blood flow, and an oedema

Question 25

What is paraesthesia?

Answer 25

“an abnormal sensation, typically tingling or pricking (‘pins and needles’), caused by pressure on or damage to peripheral nerves”
2 phases:
ischemic
post-ischemic
Results from the aberrant activity of specific subtypes of mechanosensitive fibres (light-touch mechanoreceptors; i.e., mainly non-noxious receptors)

Question 26

What is ischemic paraesthesia?

Answer 26

All excitable tissues depend upon having the correct distribution of ions across the membrane, notably monovalent cations, Na+ and K+
The resting membrane potential depends on the functioning of an enzyme, the Na+/K+ ATPase, or Na+ pump
The Na+/K+ ATPase utilises ATP to move Na+ and K+ around
In the ischemic phase, ATP decreases, the Na+ pumps fails, resulting in depolarization (anoxic depolarization)
During anoxic depolarization, some fibres become active during brief periods of time (due to the activity of Na+ channels), generating the ischemic paraesthesia

Question 27

What is post-ischaemic paraesthesia?

Answer 27

When the cuff is released, ATP is regenerated by the mitochondria
The axon has been loaded with Na+ during the ischemic phase, therefore the Na+ pump can work again
The membrane potential is unstable during this phase, switching between phases of repolarization and depolarization, generating ectopic impulses when flipping between these two states
These impulses are felt as intense paraesthesia (sensory axons), or muscle cramps or contractions (motor axons)

Question 28

What is reactive hyperaemia?

Answer 28

During the period of occlusion, tissue hypoxia and a build up of vasodilator metabolites (e.g., adenosine) dilate arterioles and decrease vascular resistance
When the occlusion is released, the flow is drastically increased
The tissue becomes reoxygenated and vasodilator metabolites are washed out