Sensory Physiology

Question 1

Describe action potential in a Pacinian corpuscle

Answer 1

Pacinian corpuscle is comprised of alternating layers of membrane with fluid between them, surrounding the nerve ending.
When we touch something, all the layers of the membrane are deformed
This leads to the opening of mechanosensitive Na+ channels on the membrane and influx of sodium
And the generation of action potentials back to CNS

Question 2

Describe adaptation and the pacinian corpuscle

Answer 2

If the stimulus is maintained, the action potentials gradually die away as adaption occurs
Adaptation is due to redistribution of the fluid in the corpuscle so that the force is no longer transmitted to the nerve ending
Much of the adaptation that occurs is the result of changes in the periphery (directly altering the afferent).

Question 3

Describe afterdischarge

Answer 3

In some cases of receptor adaptation, the removal of the stimulus triggers action potentials as the ending reforms
Afterdischarge is associated with the persistence of the sensation after the stimulus eliciting the discharge has been removed

Question 4

Define sensory unit and receptive field

Answer 4

Sensory unit: sensory nerve and all its branches

Receptive field: area from which stimulation produces activation of the neuron

Question 5

Describe the coding of the stimulus intensity

Answer 5

The number of action potentials: With greater intensity, there are more action potentials. With further increases, there may be patterned discharges (doublets or triplets, etc)

The number of receptors firing: Different receptors in the area with different thresholds. Activation of neighboring receptors.

Question 6

Describe the relationship between perceived strength and actual (measured) intensity

Answer 6

Perceived intensity = K(measured intensity)^A
K and A are constants and depend on the type of sensory receptor
Muscle senses: both are close to 1: Perceived intensity matches actual intensity very closely
Cutaneous senses: more variability. Perceived intensity diverges from actual rather substantially.

Question 7

Describe the dorsal columns

Answer 7

Relay proprioceptive and discriminative (fine touch) information
Synapses in nucleus gracilis/cuneatus, then decussation
Additional synapse in ventral-posterior nucleus of thalamus
To somatosensory cortex

Question 8

Describe spinothalamic tract

Answer 8

*Thermal, nociceptive, coarse touch*
First synapse occurs in dorsal horn
Decussation within a few segments
Nociceptive and thermal cross quickly
Coarse touch may travel before crossing
Also synapse in ventral-posterior nucleus of thalamus
To somatosensory cortex

Question 9

Describe spinoreticulothalamic system

Answer 9

*Nociceptive inputs*
After synapse in dorsal horn
Additional synapse in brainstem reticular formation. 
Then to intralaminar thalamic nuclei
Then widespread distribution to cortex

Question 10

Describe spinocerebellar tract

Answer 10

Because of the existence of this tract, ablation of the somatosensory cortex does not ablate sensation of pain (may change some aspects of it)
Dorsal and ventral tracts
Take muscle afferent and tactile input to cerebellum

Question 11

Describe pre-synaptic inhibition

Answer 11

Special case of inhibition.
Axo-axonal synapse
Post-synaptic cell is a pre-synaptic terminal
End result of pre-synaptic transmission: reduced neurotransmitter release from inhibited pre-synaptic terminal

Axon that is delivering inhibition usually releases GABA in response to how many action potentials are running down
GABA activates chloride entrance into presynaptic neuron, which then becomes hyperpolarized and allows less calcium to enter
This leads to less neurotransmitter release, which reduces probability of the presynaptic cell inducing an action potential on the postsynaptic cell

Presynaptic inhibition occurs between neighboring receptors at the first synapse in their pathway. This increases brain’s ability to localize the signal

Question 12

How is a generator potential made in sensory neurons? Action potential?

Answer 12

Sensory (afferent) neurons have specialized channels that are opened or closed in response to a stimulus
The opening of these channels produces a local response similar to the sub-threshold response or EPSP.
In sensory neurons, this depolarization is called a generator potential
If the stimulus is strong enough or lasts long enough, the generator potentials will cause the afferent neuron to come to threshold and generate an action potential

Question 13

Describe the organization of the somatosensory cortex

Answer 13

It is a neocortex and has six layers
The neurons are arranged in columns
Each column deals with one sensory modality in one part of the body
Neighboring columns receive information from the same part of the body but a different sensory modality
Sensory cortex is arranged somatotopically

Question 14

In what layer does sensory information arrive?

Answer 14

Layer IV via the thalamus

Question 15

What layer provides the output?

Answer 15

Layer V

Question 16

Describe somatic sensory area 1 (S1)

Answer 16

Postcentral gyrus
Brodmann’s 1, 2, and 3
First stop for most cutaneous senses
Somatotopic representation: toes medial, head lateral

Question 17

Describe somatic sensory area 2 (S2)

Answer 17

Wall of lateral (sylvian) fissure
Receives input from S1
Somatotopic representation (not as detailed as S1)

Question 18

Describe the processing that occurs in S1

Answer 18

Involved in integration of information for position sense as well as size, shape discrimination
Feeds process information to other places like S2

Question 19

Describe the processing that occurs for S2

Answer 19

“Cognitive touch”
Stereognosis
Comparison between two different tactile sensations (slower/faster vibration)
Determining whether something becomes a memory (prefrontal cortex)

Question 20

Describe effects of lesions in S1 or S2

Answer 20

Damage to S1 will impair S2 because they function in series
Damage in S2 will not affect S1
Lesions in S1 and/or S2 does not affect thermoreceptive (insular cortex) or nociceptive (insular, others) perception

Question 21

What is required for high-level interpretation of sensory inputs?

Answer 21

Parieto-temporal-occipital (PTO) association cortex

It receives input from different sensory cortical areas like S1 and S2

Question 22

What is the function of the PTO in the dominant hemisphere?

Answer 22

Analysis of spatial coordinates of self/surrounding objects
Naming of objects
Language comprehension (Wernicke’s)
Initial processing of visual language (reading)

Question 23

What is the function of the PTO in the non-dominant hemisphere?

Answer 23

Interpreting music, non-verbal visual experiences, and body language

Question 24

Describe plasticity of the cortex related to senses

Answer 24

Early in life, many experiences refine the map that is genetically coded in the cortex. This includes anatomically eliminating synapses as well as strengthening others
If an area of the body is amputated (or otherwise denervated), afferent input from remaining body will reinnervate the cortex
While if an area of cortex is lost, those afferents will innervate neighboring columns. Sense will not be as precise but will still be providing input

Question 25

Describe the doctrine of specific nerve energies

Answer 25

Stimulation of a sensory pathway at any point leads to perception of a sensation that is dictated by the nature of the receptor that started the pathway.
Ex: If I stimulate cortical column that receives input from pacinian corpuslce, you will perceive the sensation as light touch

Question 26

Describe synesthesia

Answer 26

Perception of one stimulus as another
Ex: Musical tones as both pitch and color. Probably results from some afferents from ear being misdirected to visual cortex

Question 27

Describe the law of projection

Answer 27

No matter where along the path we stimulate, the perceived sensation is always referred back to the area of the body in which the receptor is located.
Ex: If the cortical column that I stimulated receives input from a pacinian corpuscle in your left index finger, you perceive touch as occurring on your left index finger

Question 28

Your sense of position of your body in space (proprioception) is an excellent example of a sense that defies classification. What pieces of sensory information do you use to identify your position in space, and how would you classify them?

Answer 28

Proprioceptive (particularly kinesthetic) senses require multiple inputs from our brains to arrive at an adequate picture. These include information from muscle spindles (muscle length) and joint receptors (interoreceptors), receptors of linear and rotational acceleration, and vision (teloreceptors)

Question 29

What is most likely to move and in which direction after GABA binds to its receptor on presynaptic terminal of second axon?

Answer 29

GABA binds to one of two receptors: GABA-A (chloride channel) or GABA-B (second messenger system). Activation of either receptor leads directly (GABA-A) or indirectly (GABA-B) to hyperpolarization of main presynaptic terminal. This moves the terminal farther from threshold and reduces number of presynaptic calcium channels that open

Question 30

Information carried up dorsal columns is highly localized, very precise sensation in contrast to that coming up spinothalamic pathways, despite the fact that at least some of it arose from same afferents (1a and 1b fibers that branched and went both places). Explain how the same sensory input could give rise to different perceptions.

Answer 30

One of the major reasons that this change (from highly localized to very diffuse) may occur is due to onvergence of several fibers onto single neurons at synapse in dorsal horn (synapse that the fibers that entered the dorsal column did not make). By converging onto a single neuron, we relay very well the fact that stimulation occurred, but we lose localization