SOMATOSENSORY SYSTEM

Question 1

Physical stimulation of receptors causes them to depolarize

Answer 1

“Mechanoreceptors” are sensitive to physical distortion:
– E.g., bending and stretching
There are unmyelinated axon branches inside mechanoreceptors:
– These axons contain ion channels sensitive to mechanical force (see slide 5)
The mechanosensitive ion channels open when, e.g., axon membrane is deformed:
– Current flows through the open ion channels leading to depolarization
If the depolarization is sufficient, then the axon generates action potentials:
– The “afferent” axons carry somatosensory information to the spinal cord
We detect different types of physical stimuli:
– E.g., different frequencies of stimulation, different pressures, etc
Different mechanoreceptors have different:
– Sizes of receptive fields
– Responses to pressure, e.g., adaptation
– Preferred stimulus frequencies

Question 2

Different types of mechanosensitive ion channels

Answer 2

Channel opens when membrane stretched
Channel opens when force applied to extracellular proteins
Channel opens when cytoskeleton deformed

Question 3

Receptors with small receptive fields, e.g., a few mm wide:

Answer 3

– Meissner’s corpuscles [most common receptor on glabrous (hairless) skin]
– Merkel’s disks

Question 4

Receptors with large receptive fields, e.g., entire finger or large part of hand:

Answer 4

– Pacinian corpuscles
– Ruffini’s endings

Question 5

Different receptors adapt differently to sustained pressure

Answer 5

“Adaptation” is a reduced response to a physical stimulus over time
– Afferent axon initially fires a number of action potentials, then reduces or stops firing
Meissner’s corpuscles and Pacinian corpuscles are rapidly adapting
Merkel’s disks and Ruffini’s endings are slowly adapting

Question 6

Structure of mechanoreceptor ending contributes to adaptation rate

Answer 6

E.g., Pacinian corpuscle
– Ends with specialized capsule
– Rapidly adapts to stimulus
Capsule is onion-like
– 20-70 concentric layers
– Layers of connective tissue are slick
– Viscous fluid between layers
Adaptation due to capsule
– Less sensitive to low-frequency stimuli
During sustained pressure:
– Layers slide past each other
– No longer deforms axon inside capsule
– Membrane potential back to baseline
– Action potentials stop

Question 7

Different receptors detect different frequencies of stimulation

Answer 7

Pacinian corpuscle
– Detects high frequencies
– E.g., vibrations of 200-300Hz
– E.g., place hand on speaker surface
Meissner’s corpuscle
– Detects low frequencies
– E.g., 50Hz or less
– E.g., move finger over rough texture
Merkel’s disk & Ruffini’s ending
– Detects very low frequencies

Question 8

Two-point discrimination

Answer 8

Minimum distance necessary to differentiate between 2 simultaneous stimuli
– Finger very sensitive
– Back less sensitive
Sensitivity determined by:
– Density of mechanoreceptors
– Size of receptive fields

Question 9

Axons within receptors travel via peripheral nerves to spinal cord

Answer 9

Afferent axons enter spinal cord through dorsal roots
– Cell bodies in the dorsal root ganglia

Question 10

Afferent axons have various sizes and properties

Answer 10

Myelin shown in blue-grey (myelin is fatty sheath insulating axons; myelin
formed from membrane of glial cells)
Diameter of axon and its myelin determines speed of action potential conduction: Larger means faster
Axon types associate with particular receptor types

Question 11

Spinal cord

Answer 11

Spinal cord organized into 30 segments
The 30 segments are divided into 4 groups: cervical, thoracic, lumbar and sacral
Dermatome is area of skin innervated by right & left dorsal roots of a spinal segment
– That is, one-to-one correspondence between spinal segments and dermatomes

Question 12

Touch-sensitive Aβ axons enter dorsal horn of spinal cord and branch

Answer 12

One branch connects with so-called second-order sensory neurons in dorsal horn
– These connections can initiate or influence reflexes
Other branch ascends to brain
– Information carried by this branch enables perception of somatosensory stimuli

Question 13

Dorsal column of spinal cord

Answer 13

Carries tactile information from body
to dorsal column nuclei

Question 14

Dorsal column nuclei

Answer 14

– At border of spinal cord & brainstem
– Represents ipsilateral sensations

Question 15

Medial lemniscus

Answer 15

– Axons leaving dorsal column nuclei cross to contralateral side of brainstem
– These axons form medial lemniscus pathway

Question 16

Ventral posterior (VP) nucleus

Answer 16

– First-order somatosensory thalamus
– Represents contralateral sensations

Question 17

Trigeminal nerve

Answer 17

Carries tactile information from face to principal sensory trigeminal nucleus

Question 18

Trigeminal nucleus

Answer 18

– In brainstem (in pons)
– Represents ipsilateral sensations

Question 19

Ventral posterior (VP) thalamus

Answer 19

– Axons leaving trigeminal nucleus cross to contralateral side of brainstem
– Axons synapse in medial part of VP thalamus
– VP thalamus represents contralateral sensations

Question 20

Area 3b

Answer 20

– Primary somatosensory cortex
– Receives dense input from ventral posterior thalamus

Question 21

Area 3a

Answer 21

– Receives input from muscle spindles for proprioception
– Proprioception is sense of limb position in space

Question 22

Area 1

Answer 22

– Receives mainly texture information from area 3b

Question 23

Area 2

Answer 23

Receives mainly size and shape information from area 3b

Question 24

Columnar organization of primary somatosensory cortex, S1

Answer 24

S1 contains columns (like other cortical areas)
– S1 neurons with similar responses are located in columns
– These vertical columns extend across the cortical layers
E.g., finger (digit) representations in S1
– Each digit (D1, D2, D3, etc) represented in neighboring sites of S1
– Column of rapidly adapting neurons and column of slowly adapting neurons within each digit representation

Question 25

Homunculus

meaning “little man”, is another word for somatotopic map

Answer 25

Disproportionate body representation in somatotopic map
– Large representation of face and hands
– Small representation of trunk, arms and legs
Size of cortex representing body part related to:
– Density of sensory input from that body part
– Importance of sensory input from that body part
– E.g., information from fingers more useful than trunk

Question 26

Pacinian corpuscle

Answer 26

Large receptive field, rapidly adapting, detects high frequencies

Question 27

Meissner’s corpuscle

Answer 27

Small receptive field, rapidly adapting, detects low frequencies

Question 28

Merkel’s disk

Answer 28

Small receptive field, slowly adapting, detects very low frequencies

Question 29

Ruffini’s ending

Answer 29

Large receptive field, slowly adapting, detects very low frequencies