Somatic Senses

Question 1

1. There are x somatic senses: x
2. Proprioception is
3. Nociception detects x, and is perceived as x

Answer 1

1. There are 4 somatic senses: touch, temperature, proprioception, and nociception
2. Proprioception is awareness of the position of body parts relative to each other; e.g. even with your eyes closed you can sense how much your elbow is flexed.
3. Nociception detects tissue damage or the threat of it, and is perceived as pain or itch.

Question 2

1. x are all neurons
   a) Receptors for somatic sensation below the chin have their cell bodies in x. Receptors for the head have their cell bodies x
   b) The parts of these neurons that transduce touch, pressure etc., into electrical signals are in x
2. There are a variety of receptors in the skin
   a) Free nerve endings detect x
   b) Merkel receptors (or Merkel disks) are x in contact with x
   c) Encapsulated receptors, e.g. x, are x sheathed in x

Answer 2

1. Somatosensory receptor cells are all neurons
   a) Receptors for somatic sensation below the chin have their cell bodies in the dorsal root ganglia. Receptors for the head have their cell bodies in the brain.
   b) The parts of these neurons that transduce touch, pressure etc., into electrical signals are in their nerve endings, i.e. in the tips of their fibers, in the skin and viscera
2. There are a variety of receptors in the skin
   a) Free nerve endings detect mechanical stimuli, temperature, and chemicals.
   b) Merkel receptors (or Merkel disks) are mechanoreceptor nerve endings in contact with specialized epithelial cells called Merkel cells.
   c) Encapsulated receptors, e.g. Meissner and Pacinian corpuscles, are mechanoreceptors sheathed in connective tissue

Question 3

1. At the bottom of the epidermis are saucer-shaped x
   a) They are very sensitive to x, and are more x than x, i.e. x
   b) They signal x.
2. Most mechanoreceptors are X
   a) Given a sustained, constant stimulus, the nerve ending’s membrane x but then returns to x in x — i.e. it
   registers x, not x.
   b) So you don’t perceive much unless x: if you run your hand along a surface, x ; after your hand stops, x
3. At the top of the dermis are x
   a) They are mainly in the x and x — x and x
   b) Inside each corpuscle are x, like the coils of a spring mattress. They detect x, as when you x
   c) They are x, so they sense x
4. Deep in the dermis are x
   a) The nerve endings are x. They can sense x if x
   b) They are x, and so they respond strongly to x and other x

Answer 3

1. At the bottom of the epidermis are saucer-shaped Merkel disks
   a) They are very sensitive to deformation of the skin, and are more tonic than phasic, i.e. they send a sustained message as long as the deformation persists.
   b) They signal contact.
2. Most mechanoreceptors are phasic
   a) Given a sustained, constant stimulus, the nerve ending’s membrane depolarizes but then returns to baseline in ~3 ms — i.e. it
   registers changes, not steady levels.
   b) So you don’t perceive much unless the stimulation is changing: if you run your hand along a surface, you get a vivid impression of its texture; after your hand stops, you sense far less
3. At the top of the dermis are egg-shaped
   Meissner corpuscles
   a) They are mainly in the tongue and hairless skin — erogenous zones, palms and fingertips
   b) Inside each corpuscle are many looping endings, like the coils of a spring mattress. They detect sideways shearing, as when you stroke a surface or lift something with your fingertips
   c) They are phasic, so they sense changes in shear.
4. Deep in the dermis are onion-shaped Pacinian corpuscles
   a) The nerve endings are sheathed in many layers. They can sense tiny displacements if the motion is quick
   b) They are phasic, and so they respond strongly to vibration and other fast-changing stimuli

Question 4

1. Distrubution of receptors over the body’s surface
   a) 3x are the foveas of the somatosensory system: they have x, and therefore x
   b) The test of acuity is x : which is
   c) On your lips and fingertips you can distinguish points x, but on your calves you need x
2. Thermoreceptors are x
   a) Cold receptors respond maximally at x (which is x) .; warm receptors at x. Both are x, which is why we x
   b) Above 45°C, x. Cold fibers also respond briefly x, causing x: which is
   c) We have more x than x, and few x in total — as few as x fibers may carry temperature
   information up the x to the x (x isn’t crucial for temperature).
3. Nociceptors are x that respond to x
   a) Some respond to x, others to x
   b) Some respond to x (3x) or to x released by x in response to x

Answer 4

1. Receptors are not uniformly distributed over the body surface
   a) Palms, fingertips, and lips are the foveas of the somatosensory system: they have more densely packed receptors, and therefore higher acuity, than other areas.
   b) The test of acuity is 2-point discrimination: if your skin is touched at 2 places simultaneously, can you tell whether there are one or 2 contact points?
   c) On your lips and fingertips you can distinguish points 2–4 mm
   apart, but on your calves you need 40 mm
2. Thermoreceptors are free nerve endings
   a) Cold receptors respond maximally at ~30°C (which is well below body temperature) .; warm receptors at ~45°C Both are phasic-tonic, which is why we get used to a hot bath or a cold lake.
   b) Above 45°C, pain receptors are activated. Cold fibers also respond briefly to temperatures > 45°C, causing paradoxical cold: a hot object, touched briefly, may feel cold.
   c) We have more cold receptors than warm, and few thermoreceptors in total — as few as 1000 fibers may carry temperature
   information up the spinal cord to the brain (precise localization isn’t crucial for temperature).
3. Nociceptors are free nerve endings that respond to noxious stimuli
   a) Some respond to damaging mechanical stimuli, others to damaging heat or chemicals.
   b) Some respond to chemicals released from damaged cells (K , histamine, prostaglandins) or to serotonin released by platelets in response to injury

Question 5

1. Somatosensory afferents fall into 2 groups: X
   a) The small fibers, called x and x, come mainly from x. C fibers are x, and conduct x at speeds up to x. Aδ’s are x than C’s, x, and conduct at up to x.
   b) Different small fibers respond to different x, such as x, x, or x.
   c) Large fibers, called x, come from x or x such as x or x corpuscles. They are x, and conduct at x
2. Large and small fibers project differently
   a) Large fibers turn x on reaching the x, and run x up to the x in tracts called the x. In the medulla they x on cells whose x
   b) Small fibers synapse x or via x on x (for x) or on x neurons whose axons cross the x and run in the x , in the x of the cord, between the dorsal and ventral horns.
3. This anatomy reflects the fibers’ different functions
   a) Large fibers x, especially to x, as it manipulates x. Their information has to x
   b) Small fibers x: . Many of these tasks can be handled in x without immediate x

Answer 5

1. Somatosensory afferents fall into 2 groups: small and large
   a) The small fibers, called C and Aδ (A-delta), come mainly from free nerve endings. C fibers are unmyelinated, and conduct spikes at speeds up to 2 m/s. Aδ’s are thicker than C’s, myelinated, and conduct at up to 30 m/s.
   b) Different small fibers respond to different adequate stimuli, such as mechanical stimuli, chemicals, or temperature.
   c) Large fibers, called Aβ (A-beta), come from Merkel disks or encapsulated mechanoreceptors such as Meissner or Pacinian corpuscles. They are myelinated, and conduct at .70 m/s
2. Large and small fibers project differently
   a) Large fibers turn upward on reaching the spinal cord, and run ipsilaterally up to the medulla in tracts called the dorsal columns. In the medulla they synapse on cells whose axons cross the midline
   b) Small fibers synapse directly or via interneurons on motoneurons (for reflex responses) or on dorsal-horn neurons whose axons cross the midline and run in the spinothalamic tracts, in the lateral part of the cord, between the dorsal and ventral horns.
3. This anatomy reflects the fibers’ different functions
   a) Large fibers provide feedback to the brain, especially to motor cortex, as it manipulates objects. Their information has to travel a long way (up to the brain) quickly.
   b) Small fibers evoke simple responses to specific stimuli: withdrawing from pain, brushing away a bug, thermoregulatory and sexual responses. Many of these tasks can be handled in the spinal cord, without immediate input from the brain

Question 6

1. Primary somatosensory cortex is x
   a) Neighboring areas of skin project to x, so S1 is a x
   b) The map is x, as areas of high sensitivity and acuity (such as hands and lips) get a lot of x, just as the x do in the visual system
2. Primary somatosensory cortex (S1) is in
   x
3. There is x among x fibers
   a) As in the visual system, lateral inhibition x, i.e. x.
   b) If you step into a very hot bath, you feel the most discomfort x, because x
   c) This is a somatosensory version of the x

Answer 6

1. Primary somatosensory cortex is somatotopic
   a) Neighboring areas of skin project to neighboring cells in cortex, so S1 is a map of the contralateral body surface.
   b) The map is distorted, as areas of high sensitivity and acuity (such as hands and lips) get a lot of cortical space, just as the foveas do in the visual system
2. Primary somatosensory cortex (S1) is in
   the parietal lobe
3. There is lateral inhibition among somatosensory fibers
   a) As in the visual system, lateral inhibition enhances spatial differences, i.e. edges.
   b) If you step into a very hot bath, you feel the most discomfort not in your foot but at the line formed by the water surface around your leg, because that is the temperature edge.
   c) This is a somatosensory version of the Chevreul illusion.

Question 7

1. Many nociceptors have ion channels of the x
   a) Many nociceptors (and also thermoreceptors) have ion channels belonging to a family called x
   b) e.g. TRPV1 channels, called x receptors, respond to x and to x, including the x in chili peppers; x channels respond to cold and to menthol.
2. Nociceptive signals report x or x, and evoke x or x
   a) People with x usually die before they are x, because of x and x.
   b) We have 2 types of pain: x and x, e.g. when you stub your toe, x. Fast pain is carried by x , slow by x
   c) The reason for the 2 types is likely that pain evokes 2 distinct responses: x
3. Nociceptive signals evoke responses from
   the x
   a) Nociceptive signals trigger x, e.g. pulling your hand back from a hot stove. This is a x , and so it doesn’t need x
   b) Nociceptive signals also reach the x and x, causing 4x
   c) x can block nociceptive cells in the x , e.g. x

Answer 7

1. Many nociceptors have ion channels of the TRP type
   a) Many nociceptors (and also thermoreceptors) have ion channels belonging to a family called transient receptor potential (TRP) channels.
   b) e.g. TRPV1 channels, called vanilloid receptors, respond to damaging heat and to chemicals, including the capsaicin in chili peppers; TRPM8 channels respond to cold and to menthol.
2. Nociceptive signals report damage or danger, and evoke pain or itch
   a) People with congenital analgesia usually die before they are 20, because of injury and infection.
   b) We have 2 types of pain: fast and slow, e.g. when you stub your toe, you feel an immediate sharp pain, followed ~1 s later by a duller sensation. Fast pain is carried by Aδ fibers, slow by C fibers.
   c) The reason for the 2 types is likely that pain evokes 2 distinct responses: quick withdrawal (to get away from the painful thing) and prolonged immobilization (to promote healing)
3. Nociceptive signals evoke responses from
   the CNS
   a) Nociceptive signals trigger withdrawal, e.g. pulling your hand back from a hot stove. This is a spinal reflex, and so it doesn’t need
   immediate input from the brain.
   b) Nociceptive signals also reach the limbic system and hypothalamus, causing emotional distress, nausea, vomiting, and sweating.
   c) Descending pathways through the thalamus can block nociceptive cells in the spinal cord, e.g. in emergencies where survival depends on ignoring pain

Question 8

1. Pain in an x is often felt on x— x pain
   a) x from different locations converge on a x. So when that x sends signals to the x, the brain doesn’t know x
   b) As pain is more common in x than in x, the brain assumes x
2. Pains from different organs are x
3. Pain can be gated by x activity
   a) In the dorsal horn, x contact x . Those x are inhibited by x fibers via x.
   b) So x’s can x or x pain signals, e.g. x

Answer 8

1. Pain in an internal organ is often felt on the body surface — referred pain
   a) Nociceptors from different locations converge on a single ascending tract. So when that tract sends signals to the brain, the brain doesn’t know where the stimulus came from.
   b) As pain is more common in skin than in internal organs, the brain assumes the problem is on the body surface.
2. Pains from different organs are referred to different regions on the body surface
3. Pain can be gated by Aβ activity
   a) In the dorsal horn, C fibers contact secondary neurons. Those secondaries are inhibited by Aβ fibers via interneurons.
   b) So Aβ’s can block or dampen pain signals, e.g. if you rub a sore shoulder, it feels better

Question 9

Analgesics work by various mechanisms

1. Acetylsalicylic acid (aspirin) inhibits x, and slows x
2. Opioids (such as morphine and codeine) decrease x and x
3. The body makes natural painkillers such as x

Answer 9

1. Acetylsalicylic acid (aspirin) inhibits prostaglandins and inflammation, and slows transmission of pain signals.
2. Opioids (such as morphine and codeine) decrease transmitter release from primary sensory neurons and postsynaptically inhibit
   secondary sensory neurons.
3. The body makes natural painkillers such as endorphins, enkephalins, and dynorphins