Week 11 - Somatic Senses Flashcards
4 somatic senses
touch, temperature, proprioception, nociception
proprioception
awareness of the position of body bards relative to each other
nociception
detects tissue damage or the threat of it, and it is perceived as pain or itch
Where do receptors for somatic sensation below the chin have their cell bodies
dorsal root ganglia
Where do receptors for somatic sensation in the head have their cell bodies
brain
Where are the parts of neurons that transduce touch, pressure, etc
nerve endings i.e. in the tips of their fibers, in the skin and viscera
what do free nerve endings detect
they detect mechanical stimuli, temperature, and chemicals
Merkel receptors
They are mechanoreceptor nerve endings in contact with specialized epithelial cells called Merkel cells
encapsulated receptors
Meissner and Pacinian corpuscles are mechanoreceptors sheathed in connective tissue
what’s there at the bottom of the epidermis
saucer-shaped Merkle disks
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
they signal contact
are most mechanoreceptors tonic or phasic
phasic
What is there at the top of the dermis
egg shaped Meissner corpuscles
they are mainly in the tongue and hairless skin- erogenous zones, palms, fingertips, etc
inside each corpuscle, there are many looping endings.
what do looping endings in Meissner corpuscles detect
they detect sideways shearing. they are phasic, so they sense changes in shear.
whats there deep in the dermis
onion shaped pacinian corpuscles
te nerve endings are heated in many layer. they can sense tiny displacements if the motion is quick. they are phasic. so they respond strongly to vibration and other fast-changing stimuli.
where are receptors most densely packed
pals, fingertips and lips
test of acuity
2 point discriminatin
thermoreceptors
free nerve endings
cold receptors respond maximally at 30 degrees
warm receptors respond most at 45 degrees.
They are both phasic tonic
Above 45 degrees pain receptors are activated
paradoxical cold
Cold fibers also
respond briefly to temperatures > 45°C, causing paradoxical cold: a
hot object, touched briefly, may feel cold.
nocireceptors
they are free nerve endings that respond to no noxious stimuli
Some respond to damaging mechanical stimuli, others to damaging heat or chemicals
some respond to chemicals released from damaged cells (K+, histamine, prostaglandins) or to serotonin releases by platelets in response to injury
small somatosensory afferents
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. Different small fibers respond to different adequate stimuli, such
as mechanical stimuli, chemicals, or temperature.
large somatosensory afferents
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
how do large fibers project?
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.
how do small fibers project?
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.
large fibers functions
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.
functions of small fibers
Small fibers evoke simple responses to specific stimuli: with-
drawing 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.
how do signals travel thalamus to cortex
Signals from the spinal cord travel via the ventroposterolateral (VPL) nucleus of the thalamus. Signals from the head travel via the ventroposteromedial nucleus (VPM).
Both pass to the primary somatosensory cortex, S1.
Signals travel via thalamus to cortex
what is the primary somatosensory cortex
somatotopic
Neighboring areas of skin project to neighboring cells in cortex, so S1 is a map of the contralateral body surface.
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.
which lobe is the primary somatosensory cortex in
partietal lobe
what enhances spatial differences
lateral inhibition
what family do the ion channels of nociceptors belong to
transient receptor potential channels
vanilloid receptors
TRPV1 channels
they respond to damaging heat and to chemicals (including capsaicin)
TRPM8 channels response to cold and menthol
congenital analgesia
dont feel pain
fast pain
carried by A-delta fibers
slow pain
carried by C fibres
2 responses evoked by pain
quick withdrawal (to get away from the painful thing) and
prolonged immobilization (to promote healing)
responses evoked by nociceptive signals
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.
Nociceptive signals also reach the limbic system and hypothalamus, causing emotional distress, nausea, vomiting, and sweating.
Descending pathways through the thalamus can block nociceptive cells in the spinal cord, e.g. in emergencies where survival depends on ignoring pain.
referred pain
pain in an internal organ that is felt on the body surface.
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.
As pain is more common in skin than in internal organs, the brain
assumes the problem is on the body surface.
liver/gallbladder and heart pain
liver/gallbladder - shoulder
heart - left arm
how can pain be gated by A beta activity
In the dorsal horn, C fibers contact secondary neurons. Those
secondaries are inhibited by Aβ fibers via interneurons.
So Aβ’s can block or dampen pain signals, e.g. if you rub a sore
shoulder, it feels better.
Acetylsalicylic acid
inhibits prostaglandins and inflam-
mation, and slows transmission of pain signals.
opioids
decrease transmitter release from primary sensory neurons and postsynaptically inhibit
secondary sensory neurons.
natural painkillers released by the body
endorphins, enkephalins, and dynorphins.
