Exam 7 (CH. 12)

Question 1

Know the mechanoreceptor types - what do they detect? Compare and contrast them. What types of skin are they located in/beneath?

Answer 1

1. Meissner’s corpuscle: rapid adaptation (responds to change), small receptive field, high resolution. Located in the ridges of glabrous skin (ex. The raised parts on your fingertips).
2. Pacinian corpuscle: rapid adaptation (responds to change), large receptive field, poor resolution. Located deep in the dermis and can be as long as 2 mm and almost 1 mm in diameter.
3. Ruffini’s ending: slow adaptation, large receptive field, poor resolution. found in hairy and glabrous skin. Slightly smaller than Pacinian corpuscles.
4. Merkel’s disk: slow adaptation, small receptive field, high resolution. Located within the epidermis. Consists of a nerve terminal and a flattened, non neural epithelial cell.
   - they detect touch, pressure, vibration and sound

Question 2

What is the receptive field? How can these be mapped? What are the relative advantages and disadvantages of each?

Answer 2

• the region of a sensory surface (retina, skin) that, when stimulated, changes the membrane potential of a neuron.
• by introducing a micro electrode into the median nerve of the arm, it is possible to record the action potentials from a single sensory axon and map its receptive field on the hand with a fine stimulus probe.
• Small receptive field
  - Advantages: allows us to identify the stimulus location more specifically.
  Disadvantages: doesn’t cover a lot of area
• Large receptive fields
  - Adavnatges: gives great spatial coverage
  - disadvantages:doesn’t have good localizing ability (poor spatial resolution)

Question 3

Describe the pathways form receptor to brain for somatosensation. How is this similar or different from the pathway for pain/temp? Where do these cross over? Is there something about the fibers that affects rate of transmission in these pathways? Which one is fastest/slowest? Why?

Answer 3

• Dorsal root ganglion neurons that carry fine touch and pressure information. Have large myelinated axons whose receptors are located in the skin, muscles, and tendons. As the name implies, the cell body is located in a dorsal root ganglion of the spinal cord. Fine touch and pressure axons ascend in the ipsilateral spinal cord, forming the dorsal spinothalamic tract to the neocortex.
• They differ with respect to their nerve endings in the skin. The touch pathway is characterized by specialized structures in the skin; the pain pathway has only free nerve endings. Second, they differ with respect to the diameter of their axons. The touch pathway is swift, using fat, myelinated, A fibers. The pain pathway is slow using thin, lightly myelinated A fibers C fibers. Third, they differ with respect to their connections and the spinal cord information about touch as ipsy laterally, while information about pain and temperature as sense contralaterally.
• Crossover happens in the medulla.
• Touch is faster. Pain is slower

Question 4

What thalamic nucleus/nuclei is/are most involved in touch/pain processing?

Answer 4

Ventral posterior nucleus

Question 5

What is somatotopy?

Answer 5

The mapping of the body’s surface sensations onto a structure in the brain

Question 6

Why is the somatotopic map on the brain not proportionally accurate to the body surface? Explain in detail.

Answer 6

• the mouth, tongue, and fingers are large while the trunk, arms, and legs are tiny. The relative size of the cortex devoted to each body part is correlated with the density of sensory input received from that part. Size on the map is also related to the importance of the sensory input from that part of the body.

Question 7

What do modifications of the periphery (e.g. amputating a finger) result in, with regard to the somatotopic map on cortex? Why?

Answer 7

No longer have inputs from that part of the body. Get representation of adjoining areas that were previously used for amputated limb, will creep into the area and take over cortical area that were previously used for the amputated limb. Reflects the revealing of dormant synapses.

Question 8

Be able to make predictions about other species regarding the organization of their somatotopic maps in cortex.

Question 9

What Brodmann areas are somatosensory regions? Which are posterior parietal. How are these cortical areas distinct?

Answer 9

• 3a (muscles), 3b (skin, slow), 1 (skin, fast), 2 (joints, pressure)
• ## areas 5 and 7

Question 10

What is hemispatial neglect? What might it suggest if you see a patient with this symptom?

Answer 10

• reduced awareness of stimuli on one side of space, even though there may be no sensory loss.
• a person can see all the objects in a room but neglect the left half of each individual object.