Perception and Sensorimotor Control 2

Question 1

Otolith Organs structure allow them to sense static and translational head movements. How?

Answer 1

Otolithic membrane mass relative to endolymph & physical uncoupling from macula = hair bundle displacement occurs transiently in response to translational head movements (e.g. - head tilting)

Question 2

Otolith Orientation enables them to transmit information about linear forces acting in every direction that the head moves. How?

Answer 2

the utricle (horizontal movement) and the saccule (vertical movement) combine to effectively gauge the linear forces acting on the head at any instant in time, in three directions

Question 3

Vestibular nerves arise from bipolar neurones. What does this mean? (3 points)

Answer 3

• cell bodies reside in the vestibular nerve ganglion
• distal processes innervate semi-lunar canals & otolith organs, while the central processes project via the vestibular portion of the vestibulocochlear nerve to the vestibular nuclei
• large amounts of canal-otolith convergence is found in the vestibular nuclei –> enables the unambiguous encoding of head orientation & movement in the environment

Question 4

central projections of the vestibular system have 2 reflex classes. what are they?

Answer 4

• those responsible for maintaining equilibrium and gaze during movement
• those responsible for maintaining posture

Question 5

what does the ‘vestibulo-occular reflex’ (VOR) do?

Answer 5

the vestibulo-occular reflex produces eye movements that counter head movements, allowing a fixed gaze

Question 6

what occurs in the otolith organs when the head tilts backwards? (2 things)

Answer 6

• inertia of otolith organs push gelatinous membrane down & makes hairs deflect
• sterocellia deflected towards kinocilium –> inc^ firing rate

(opposite when head tilts forwards)

Question 7

what is reflex suppression and reflex potentiation?

Answer 7

reflex suppression: raises the threshold for reflex excitation

reflex potentiation: lowers the threshold for reflex excitation

Question 8

state what it is meant by the key term - muscle synergy

Answer 8

muscle synergy: a group of muscles connected by neural circuitry and act following a coordinated pattern

Question 9

state 2 additional facts about the implications of muscle synergy

Answer 9

• reduces possible degrees of freedom & make movement control easier
• reflexes can work in a feedback control fashion –> the voluntary controller sets the desired value of the controlled variable & reflexes are used to activate the muscles needed to maintain/follow it

Question 10

what type of afferents convey pain and temperature?

Answer 10

smaller afferents

Question 11

what is the function of the receptive field?

Answer 11

receptive field is a function of the branching characteristics of the afferent within the skin; smaller arborizations result in smaller receptive fields

the receptive fields in areas of dense innervation (e.g. - lips) are relatively small compared to those in the forearm that are innervated by a smaller amount of fibres

Question 12

what is the effect of different receptive field sizes?

Answer 12

• regional diff in receptive field size and innervation density limit spatial accuracy with which tactile stimuli can be sensed

fingers = 2 mm

bicep = 40 mm

Question 13

talk about haptics

Answer 13

• active touching (haptics) involves interpretation of complex spatiotemporal patterns of stimuli that are likely to activate many classes of mechanoreceptors
• manipulating an object with your hand can often provide enough information to identify an object (stereogmosis)

Question 14

what are Merkel Cell Afferents?

Answer 14

Merkel Cell Afferents are slow adapting fibres which make up 25% of the mechanosensory in the hand (especially in fingertips)

Question 15

what do merkel cell afferents respond to?

Answer 15

information from the epidermis

Question 16

what are Merkel Cell-Neurite Complexes

Answer 16

merkel cell-neurite complexes lie in the tips of the primal epiderma ridges on finger surfaces

Question 17

what else do Merkel Cells do?

Answer 17

• play a key role in modulating the activity of their afferent axons by releasing neuropeptides on the neutrites at unctions which resemble synapses

Question 18

what is the spatial resolution of Merkel Cells?

Answer 18

0.5 mm (highest of all sensory afferents)

Question 19

What are Merkel Cells receptive to?

Answer 19

points, edges, & curvature –> idead for analysis info on shape and texture

Question 20

what do Meissner Afferents express?

Answer 20

Piezo2 Channels

Question 21

what are Meissner Afferents?

Answer 21

• rrepid adapting fibres innervating skin more densely than merkel cells (40%)

Question 22

where are Meissner Afferents located?

Answer 22

in the Demal Papillae (adjacent to primary ridges and closest to skin surface)

Question 23

what is the structure of Meissner Afferents? (2 points)

Answer 23

• contain flattened lamellar & nerve terminals both suspended from basal epidermis via collagen fibres
• capsule centre contains 2-6 afferent nerves which terminate between & around lamellar cells, contributing to transient responses of these afferents

Question 24

what do Meissner Afferents detect?

Answer 24

with identification of the skin, the dynamic tension transduced by the collagen fibres provides the transient mechanical force that deforms the corpuscle and triggers generator potentials (rapid-responders - on/off quickly)

Question 25

how much more sensitive are Meissner Afferents than Merkel Cells, and what does this mean/cause?

Answer 25

4 x more sensitive

very good at transducing low frequency vibrations (3 - 40 Hz) that occur when textured objects move across hands

Question 26

are pacinian cells fast or slow adapting afferents?

Answer 26

fast

Question 27

what percentage of mechanosensory innervation in the hands are responsible by paccinian afferents?

Answer 27

10 - 15 %

Question 28

where are paccinian afferents located?

Answer 28

deep in the dermis or subcutaneous tissue

Question 29

what is the structure of paccinian afferents?

Answer 29

concentric layers of membranes around a single afferent fibre –> laminar capsule acts as a filter, allowing only transient disturbances at high frequencies (350 Hz) to activate the nerve endings

Question 30

what is the receptive field of paccinian afferents?

Answer 30

10 mm

Question 31

what do paccinian afferents function to do?

Answer 31

well suited to detect vibrations transmitted through objects that contact the hand –> especially when breaking contact –> important for the skilled us e of tools

Question 32

are ruffini afferents fast or slow adapting?

Answer 32

slow adapting

Question 33

what is the structure of ruffini afferents?

Answer 33

elongated, spindle-shaped, capsular specialisations located deep in the skin, as well as in ligaments and tendons

Question 34

what are ruffini afferents sensitive to?

Answer 34

sensitive to cutaneous stretch caused by skin movement

Question 35

what percentage of mechanosensory is made up from ruffini afferents in the hand?

Answer 35

20%