66 - Sensory Systems

Question 1

Define proprioceptor

Answer 1

• Proprioceptors provide information about joint angles, muscle length and muscle tension. This information can be integrated by the brain (with vestibular input) to give you a perception of the position of your body in space and the relative position of your limbs.

Question 2

Define adequate stimulus

Answer 2

Adequate Stimulus Under normal circumstances a specific receptor is affected by only one stimulus modality – this is the adequate stimulus.

Question 3

Define nociceptor

Answer 3

pain receptors

Question 4

Define sensory modality

Answer 4

Sensory modalities include five classical senses: vision, hearing, taste, smell and touch; and also pain, temperature, itch, proprioception and vestibular sense

Question 5

Define receptive field

Answer 5

In the somatosensory system the receptive field of a neuron is the region of tissue (e.g. skin) within which a stimulus can evoke a change in the firing rate of the neuron.

Question 6

Define graphesthesia

Answer 6

the sense through which figures or numbers drawn on the skin can be recognized

Question 7

Define agraphesthesia

Answer 7

Loss of graphesthesia is called agraphesthesia

Question 8

Define stereognosis

Answer 8

ability to recognize objects through touch alone

Question 9

Define asterognosis

Answer 9

loss of stereognosis is called astereognosis

Question 10

Be able to compare and contrast a receptor potential and an action potential. What is the relationship of the receptor potential to the action potential?

Answer 10

The receptor potential is a change in the membrane potential produced by a transducer mechanism. It is a local graded potential.

• Most receptor potentials are depolarizations
• If depolarization reaches threshold, generate action potentials
• Rate at which action potentials are generated increases as receptor potential rises higher above threshold.
• If the receptor potential is a hyperpolarization it does not generate an action potential.

Question 11

How does the brain code for “what” the stimulus is?

Answer 11

The labeled line principle – tells you the sensory modality since there is a chain of connected neurons from the sensory receptor which responds to the stimulus to the brain area that perceives the stimulus

(think sensory modalities)

Question 12

How do we know “where” the stimulus is coming from?

Answer 12

In the somatosensory system the receptive field of a neuron is the region of tissue (e.g. skin) within which a stimulus can evoke a change in the firing rate of the neuron.

Question 13

How do we know “duration” of the stimulus?

Answer 13

Maintain continuous signal during stimulus

Have on-signal and off-signal

Question 14

How does the nervous system code for stimulus strength (intensity)?

Answer 14

Signal increasing stimulus strength by increasing frequency of nerve impulses - frequency code.

Signal increasing stimulus strength by increasing the number of fibers activated - population code.

Question 15

Describe adaptation of a receptor

Answer 15

Adaptation - When a maintained stimulus of constant strength is applied to a receptor, the frequency of the action potentials in the receptor’s sensory nerve decreases over time.

• adaptation is a general property of sensory receptors
• different receptors can have different mechanisms of adaptation

Question 16

Phasic receptors

Answer 16

Rapidly adapting receptors

AKA rate receptors

Question 17

Describe phasic receptors

Answer 17

• disadvantage - cannot give continuous signal in response to continuous stimulus
• useful for signaling beginning and end of stimulus
• can signal change in stimulus intensity
• number of action potentials transmitted related to rate at which change is taking place
• extract dynamic information e.g. velocity and acceleration
• knowing rate of change enables the nervous system to make prediction

Question 18

Example of phasic receptors

Answer 18

Pacinian corpuscles

Meissner’s corpuscles

Question 19

Tonic receptors

Answer 19

Slowly adapting receptors

Question 20

Describe tonic receptors

Answer 20

• useful for giving continuous information about stimulus strength and signaling stimulus duration
• not as useful for signaling stimulus duration at low stimulus intensities
• when stimulus intensity low, firing rate low, difficult to decide when stimulus began or ended

Question 21

Example of tonic receptors

Answer 21

Merkel cell ending or Merkel’s disks

Question 22

Which category of receptors is often categorized with slowly adapting receptors?

Answer 22

Non-adapting receptors (never completely adapt)

Nociceptors

Question 23

There are two different classification systems of nerve fibers. What are they?

Answer 23

• Roman number classification system

- Letter classification scheme

Question 24

Describe the Roman system

Answer 24

Types I, II and III are myelinated, type IV is not.

If we exclude the A gamma fiber to the muscle spindles from the I to IV scheme, we can assign an “average” conduction velocity to each fiber type and the average conduction velocity

Question 25

Type I

Answer 25

100 meters per second – A alpha

Question 26

Type II

Answer 26

50 meters per second – A beta

Question 27

Type III

Answer 27

20 meters per second – A gamma

Question 28

Type IV

Answer 28

1 meter per second - C

Question 29

Describe the letter classification scheme

Answer 29

A and B fibers are myelinated. C fibers are unmyelinated

Question 30

A alpha function

Answer 30

Axon of alpha motor neuron; muscle spindle primary ending (Ia);

Golgi tendon organ afferent (Ib)

Question 31

A beta function

Answer 31

Muscle spindle secondary ending (II)

And the axons of cutaneous mechanoreceptors

Question 32

A gamma function

Answer 32

Axon of gamma motor neuron to muscle spindle fibers

Question 33

A delta function

Answer 33

Fast pain, some temperature

receptors (Group III fibers)

Question 34

B function

Answer 34

Sympathetic preganglionic axons

Question 35

C function

Answer 35

Slow pain, some temperature (Group IV)
- and -
Sympathetic , postganglionic axons

Question 36

Be able to describe how you could measure spatial resolution in the somatosensory system. What is “two-point discrimination”? Which areas of the body show the best discrimination? The worst?

Answer 36

Two point discrimination

1. test of tactile acuity
2. good discrimination on lips and fingertips
3. poor on back and calf of leg
4. high density of receptors in areas of good discrimination.
5. more cortical tissue devoted to analyzing signals from areas of good discrimination.

Question 37

Be able to list the receptors involved in proprioception.

Answer 37

Receptors involved in proprioception include:

a. joint receptors
b. muscle spindles and Golgi tendon organs
c. skin tactile receptors

Question 38

Be able to answer the following questions about the dorsal column and anterolateral spinal cord pathways.
a. What kinds of information are carried by each system?

Answer 38

Dorsal column

• This system is responsible for fine tactile discrimination (discriminatory touch is only found in dorsal column)
• Vibratory sense (rapid, repeated touch/pressure)
• Proprioception
• There is also a visceral pain pathway in the dorsal column

Anterolateral column

• Responsible for conveying nociceptive and thermal sensations
• Some tactile information

Question 39

Be able to answer the following questions about the dorsal column and anterolateral spinal cord pathways.
b. Where does information in each system cross the midline?

Answer 39

Dorsal column
- Brain stem

Anterolateral system
- Spinal cord

Question 40

Be able to answer the following questions about the dorsal column and anterolateral spinal cord pathways.
c. What kinds of deficits are associated with damage to each system?

Answer 40

Dorsal column lesions

• Deficits in tactile discrimination
• Vibratory sense
• Proprioception (Rhomberg test is a test for proprioception

Anterolateral column lesions

• Lack of nociciptive and thermal sensations
• Some tactile information is carried, but a lesion does not destroy ability to sense touch (note that light touch is carried by both anterolateral and dorsal columns)

Question 41

Be able to give the location of the somatosensory cortex and the general organization of this area. (Example: Which part of the body is represented medially?)

Answer 41

• Each of the four cortical areas in SI contains a separate and complete representation of the contralateral side of the body.
• Foot area always in medial cortex.
• Plasticity and reorganization

Humonculus on slide 22

Question 42

Be able to predict functional losses associated with damage to different areas of the somatosensory system.

Answer 42

Functions (Do not have to know what each subarea does, just given as examples of how information analyzed in cortex)
• A cortical area could respond primarily to cutaneous stimuli (3b and 1), primarily to proprioceptors (3a) or it could respond to both tactile and proprioceptive stimuli (2). Largest portion of input from thalamus goes to Area 3b and lesions of this area in non-human primates cause profound deficits in all forms of tactile sensation mediated by cutaneous mechanoreceptors.
• Graphesthesia and sterognosis
• Somatosensory area II (SII) - neurons in this area show attention modulated responses, (That is, the responses of the neurons in this area to a stimulus depend on whether the subject is paying attention to the stimulus)

Or the answer could be to just look at the homunculus and know general locations