12_Sensory Systeoms Intro_Q and A_Jonathan

Question 1

What three groups can sensation be classified into?

Answer 1

• Exteroception – perception of stimuli originating outside of the body (receptors are found at or near the body surface in skin and special sense organs).
• Interoception – perception of stimuli originating inside of the body (receptors are found within the body in viscera, blood vessels, glands, serous membranes, etc.).
• Proprioception – perception of body position and movement (receptors are found within muscles, tendons, joints and the vestibular apparatus).

Question 2

What is the general scheme of sensation?

Answer 2

• a sensory receptor in the periphery
• a chain of neurons that transmits the information (first order, second order, etc.)
• a final target in the CNS (e.g. cerebral cortex, cerebellum, brainstem, spinal cord)

Question 3

Note: axons can branch. Branches are called collaterals.

Answer 3

Note: axons can branch. Branches are called collaterals.

Question 4

What is the only sense that does not need to pass through the thalamus before reaching the cerebral cortex?

Answer 4

• All sensory information, with the exception of olfactory information, must pass through the thalamus in order to reach the cerebral cortex.
• Thus the thalamus is sometimes called the “gateway to the cerebral cortex”.

Question 5

What is the term adequate stimulus?

Answer 5

• Receptors in our body generally are sensitive to only one type of stimulus, termed the adequate stimulus for that receptor.

Question 6

What is modality?

Answer 6

• When a particular receptor is stimulated, a specific region of the brain receives the information and interprets the nature (modality) of the stimulus (e.g. touch, smell, sight, etc.). For example, when the auditory cortex receives sensory information, the brain perceives it as sound.

Question 7

What are first order neurons?

Answer 7

• Neurons that are connected to a sense receptor rather than synapsing with a pre-synaptic neuron.

Question 8

How do the following change membrane permeability?

Photoreceptor
Chemoreceptor
Mechanoreceptor
Thermoreceptor
Nociceptor

Answer 8

• Photoreceptor Absorption of photons by rods and cones in the retina
• Chemoreceptor Ion channels open when specific molecules bind to them, or to a second-messenger G-protein
• Mechanoreceptor Ion channels are linked to the cytoskeleton and open when mechanical force is applied
• Thermoreceptor Not completely clear; recent evidence indicates it involves channels in the TRP (transient receptor potential) family
• Nociceptor Not completely clear; sensory transducers have been discovered tharespond to noxious temperatures, mechanical stress and noxious chemicals. Many of these are in the TRP family.

Question 9

What is the “receptor potential” / “receptor generator potential?

Answer 9

• Changes in the permeability of the receptor membrane results in a local, graded potential that is comparable to an excitatory post-synaptic potential (EPSP).
• must reach a certain threshold to generate an action potential.
• This can be accomplished by summation.

Question 10

In what senses are the sensory receptors (transducer) part of the same neuron in which the action potenial is generated?

Answer 10

• olfactory and somatosensory systems.

Question 11

In what senses are the sensory receptors (transducers) a neighboring cell to the 1st order neuron?

Answer 11

• Auditory, vestibular, and vision
o The auditory and vestibular receptor cells are small enough that their local receptor potentials can trigger release of neurotransmitter and initiate action potentials in the afferent neurons of the vestibulocochlear nerve (CN VIII).
o Vision: receptor potentials are sufficient to transmit information over short distances; the action potential is not generated until the information leaves the retina in the optic nerve (CN II).

Question 12

How does the nervous system distinguish between touch and proprioception?
What is a labeled line?

Answer 12

• When a mechanoreceptor is stimulated, the brain learns that the stimulus contains mechanical energy.
• touch, proprioception, etc… are stimulated by mechanical energy
• chain of neurons that carries information to specific locations in the brain.
• Each chain only carries one type of modality, and is thus a “labeled line”.
• If information comes in on that “line”, the sensation perceived will be the modality the line is carrying.

Question 13

What about the quality of a stimulus within a modality – a soft touch vs. a hard touch for instance?

Answer 13

• intensity, duration, and location.

* this information must be encoded by the action potential (neural code)

Question 14

How is intensity registered?

Answer 14

• the stronger the stimulus, the greater the amplitude of the receptor potential.
• receptor potential amplitude is coded as neural action potential frequency (higher amplitude = greater frequency).
• Simply stated, a strong stimulus causes more nerve impulses to be generated in a specific time interval than a weak stimulus.

Question 15

Why are some sensations more intense for some individuals than others?

Answer 15

• receptors have different thresholds.
• low threshold (high sensitivity) or high threshold (low sensitivity) receptors
• People have varying numbers of low and high receptors (partially due to genetics) and the ratio between these can be altered (by desensitizing or destroying receptors).

Question 16

How do receptors detect duration? What is adaptation?

Answer 16

• Receptors are able to extract information about the duration of a stimulus using a process called adaptation.
• Adaptation is when the response of a receptor to a constantly applied stimulus diminishes with time – the receptor “adapts” to the stimulus and stops generating receptor potentials. Receptors are classified by their rate of adaptation.
• Rapidly adapting receptors respond maximally when the stimulus is first applied, but their response quickly decreases if the stimulus is maintained (in other words, if the stimulus does not change).
• Slowly adapting receptors generate receptor potentials as long as the stimulus is applied.

Question 17

Does adaptation occur at the receptor or the neural level?

Answer 17

• Adaptation that occurs at the receptor level is often related to the structure of the receptor itself.
o For example, one type of somatosensory receptor, the Pacinian corpuscle, consists of a nerve ending surrounded by a multi-layered capsule (it resembles an onion). There is fluid between the layers of the capsule that effectively absorbs small disturbances in the stimulus – thus “filtering them out” and preventing them from reaching the nerve ending. Therefore, the Pacinian corpuscle only responds to significant changes in the stimulus – it is a rapidly adapting receptor.

Question 18

How do receptors convey information about location?

Answer 18

• stimulus through their receptive fields.

Question 19

What is a receptive field?

Answer 19

• A receptive field is the region within which a stimulus will cause a response in a particular neuron.

Question 20

What are examples of receptive fields in touch, sight, and hearing?

Answer 20

• For example, the receptive field of a cutaneous receptor is the area of skin that is innervated by the neuron attached to that receptor (the area of skin that you can stimulate to cause a response in the neuron).
• In the visual system, the receptive field of a photoreceptor is the portion of the visual field that is projected onto a part of the retina where that particular photoreceptor is located.
• In the auditory system, the receptive field of a hair cell refers to a particular range of frequencies of sound that will stimulate that hair cell – so in this example, the receptive field is not a location in space, but a “location” within a range of frequencies.
o Not the best example. Location of sound is registered by the delay of sensory input between one ear vs the other. The combination of input determines where the sound is coming from.

Question 21

What are the two classification schemes to categorize axons?

Answer 21

• conduction velocity

* diameter

Question 22

What is the classification scheme using conduction velocity?

Is this used for efferent or afferent axons?

Answer 22

• type A, B, and C
• A is the fastest fiber and C is the slowest.
• The “A category” is further subdivided into Aα, Aβ, Aγ and Aδ, again listed in order from fastest to slowest.
• Typically this classification is used for efferent axons.

Question 23

What is the classification scheme using diameter?

Is this used for efferent or afferent axons?

Answer 23

• I, II, III, and IV
• group I being the largest myelinated axons, and group IV being the smallest unmyelinated axons (the axons in the middle two groups are myelinated).
• Group I is further subdivided into Ia and Ib.
• Typically this classification is used for afferent axons.

Question 24

The diameter of a fiber correlates with a particular function
the largest axons conduct… (hint name 1)
the smallest axons conduct… (hint name 3)

Answer 24

• the largest axons conduct ==> proprioceptive sensations from muscle spindles
• the smallest axons conduct ==> pain and temperature sensations, and postganglionic motor impulses in the autonomic system.