Two touch discrimination

Question 1

Summarise the sensory map

Answer 1

Sensory input from skin is mapped onto somatosensory cortex
Sensory map is distorted – this distorted image is call the homunculus
Fingers have densely packed receptors with small receptive field compared to legs

Question 2

What is meant by somatotopy

Answer 2

The correspondence of receptors in regions or parts of the body via respective nerve fibres to specific functional areas of the cerebral cortex
 Our brains have a somatotopic layout meaning there is a point-to-point representation of each part of our body in our cerebral cortex – it is upside down though.

Question 3

What is the somatosensory cortex

Answer 3

the map of the body surface on the somatosensory cortex

Question 4

Summarise the touch and pressure receptors

Answer 4

Merkel receptors- sense steady pressure and texture
Meissener’s corpuscle- responds to flutter and stroking movements
Nociceptor- free nerve ending of noxious stimuli
Sensory nerves- carry signals to spinal cord
Ruffini corpuscle- responds to skin stretch
Pacinican corpuscle- senses vibration
Hair root- free nerve ending of hair root senses hair root

Question 5

Summarise where each type of sensory modality fibres decussate

Answer 5

Pain, temperature and coarse touch cross the midline in the spinal cord
Fine touch, vibration and proprioception pathways cross the midline in the medulla

Question 6

Describe the receptive field of neurones

Answer 6

The receptive field of a sensory neuron is a region in which the presence of a stimulus will change firing rate of that neuron

Each neuron’s receptive field is large and overlaps with those of other neurones

Question 7

Why is it good that each neurones receptive field and is large and overlaps with those of other neurones

Answer 7

A single stimulus would be perceived in one receptive field – no indicator of its location within that field

With two overlapping fields, if stimulus lies within overlap area it will stimulate two fibres in different proportions, depending on its exact location. By comparing these the CNS can more accurately determine the location

Damage to one fibre does not cause complete block of sensation (anaesthesia)

Question 8

Describe sensory convergence- two point discrimination

Answer 8

Many primary sensory neurones converging onto a single secondary neurones creates a very large receptive field.
The two stimuli will be perceived as a single point because both stimuli fall within the same respective field.

When fewer neurones converge, secondary receptive fields are much smaller. The two stimuli activate separate pathways and are perceived as distinct stimuli.

Higher density of receptors and smaller receptive fields also give greater discrimination.

Question 9

Summarise sensory awareness- lateral inhibition

Answer 9

Stimulus
Primary neuron response is proportional to stimuli strength
Pathway closest to the stimulus inhibits its neighbours
Inhibition of lateral neurones enhances perception of the stimulus

Question 10

Describe the density of receptors

Answer 10

per square centimetre (approximate):
Palmar surface of the fingertips: 60 pain, 100 touch receptors
Back of finger: 100 pain, 9 touch receptors

Question 11

List the clinical uses of two-touch discrimination

Answer 11

Test for nerve damage (e.g. cut nerve) and recovery.

Test for diseases of the nervous system

Test for nerve recovery after repair

Question 12

Compare the two-point discrimination of a normal subject with that of a patient with diabetic nephropathy

Answer 12

Normal- more sensitive the closer you get to the fingers or the big toe- greater acuity- smaller distances between two point discrimination
Patient with diabetic nephropathy- smaller decreases in TPD- larger receptive fields.

Question 13

Describe age-related differences in TPD

Answer 13

No gender related differences

However, the TPD gets larger as you age for a certain body part.

Question 14

Summarise the dorsal columns

Answer 14

Mid thoracic- 2 dorsal columns- one from each Lower limb

Cervical- 4 dorsal columns- both gracile and cuneate fascicles

Question 15

Describe diabetic nephropathy

Answer 15

Loss of sensory fibres, can’t feel things accurately, hard to function, may not feel temperature- issue in cold weather, or boiling tap- loss of dexterity too- hard to function

Question 16

What is important to remember about the sensory map

Answer 16

• Recognise that the sensory map in the cortex is distorted. Large areas of brain represent small skin areas on the fingertips, while smaller areas of brain represent the large skin areas of the legs. This distorted image of the body in the somatosensory cortex is known as the homunculus - literally little man.