topic 9: somatic sensation Flashcards
What are the 2 main types of skin?
hairy and glabrous (hairless)
What are the layers of skin?
outer layer (epidermis), and an inner layer (dermis)
What is the functions of skin?
-protective function
-prevents evaporation of body fluids into the dry environment
-provide most direct contact with the world –> largest sensory organ
What are mechanoreceptors sensitive to?
-physical distortion such as bending or stretching
Where are mechanoreceptors?
-present throughout the body, monitor skin contact, pressure in heart and blood vessels, stretching of the digestive organs and urinary bladder, and force against the teeth.
describe the mechanoreceptors
-The largest and best-studied receptor is the Pacinian corpuscle , which lies deep in the dermis and can be as long as 2 mm and almost 1 mm in diameter. Each human hand has about 2500 Pacinian corpuscles, with the highest densities in the fingers.
-Ruffini’s endings, found in both hairy and glabrous skin, are slightly smaller than Pacinian corpuscles.
-Meissner’s corpuscles are about one-tenth the size of Pacinian corpuscles and are located in the ridges of glabrous skin (the raised parts of your fingerprints, for example).
-Located within the epidermis, Merkel’s disks each consist of a nerve terminal and a flattened, non-neural epithelial cell (the Merkel cell).
-In Krause end bulbs, which lie in the border regions of dry skin and mucous membrane (around the lips and genitals, for example), the nerve terminals look like knotted balls of string.
Describe the size of the receptive fields of the mechanoreceptors
-Meissner’s corpuscles and Merkel’s disks had small receptive fields, only a few millimetres wide
-Pacinian corpuscles and Ruffini’s endings had large receptive fields that could cover an entire finger or half the palm
What are the rapidly adapting and slow adapting mechanoreceptors?
-rapidly adapting – respond quickly at first but then stop firing even though stimulus continues –> Meissner’s and Pacinian corpuscles
-slow adapting –generate a more sustained response during a long stimulus –> Merkel’s disks and Ruffini’s endings
Where do hairs grow from and explain their innervation?
Hairs grow from follicles embedded in the skin; each follicle is richly innervated by free nerve endings — the terminations of single axons — that either wrap around the follicle or run parallel to it
what do erectile muscles in hair follicles do?
essential for mediating the strange sensation we call goosebumps
what frequencies do mechanoreceptors respond to
-Pacinian corpuscles are most sensitive to vibrations of about 200– 300 Hz,
-Meissner’s corpuscles respond best around 50 Hz
describe the selectivity of Pacinian corpuscle axon
-depends primarily on the structure of its special endings
-has football-shaped capsule with 20-70 concentric layers of connective tissue, arranged like the layers of an onion, with an axon terminal in the middle
What happens when Pacinian corpuscle is compressed?
-Energy transferred to the nerve terminal, membrane deformed, mechanosensitive channels open
-current flowing through the channels generates a receptor potential, which is depolarising (if large enough AP will occur)
-but capsule layers are slick, with viscous fluid between them. If the stimulus pressure is maintained, the layers slip past one another and transfer the stimulus energy in such a way that the axon terminal is no longer deformed, and the receptor potential dissipates. When pressure is released, the events reverse themselves; the terminal depolarizes again and may fire another action potential.
-the layered capsule that makes it sensitive to vibrations and almost unresponsive to steady pressure
describe the function of mechanosensitive ion channels
-convert mechanical force into a change in ionic current
can mechanical stimuli trigger release of second messengers that secondarily regulate ion channels?
Yes
What are Piezo2 channels?
-mechanosensitive in Merkel disks, open in response to pressure and depolarise cell
why is the fingertips better than most parts of body for two-point discrimination?
(1) There is a much higher density of mechanoreceptors in the skin of the fingertip than on other parts of the body
(2) the fingertips are enriched in receptor types that have small receptive fields (e.g., Merkel’s disks)
(3) there is more brain tissue (and thus more raw computing power) devoted to the sensory information of each square millimetre of fingertip than elsewhere
(4) there may be special neural mechanisms devoted to high-resolution discriminations.
what are primary afferent axons?
axons bringing information from the somatic sensory receptors to the spinal cord or brainstem –>enter the spinal cord through the dorsal roots, cell bodies in dorsal root ganglia
in order of decreasing size, what are the axons designations that come from skin?
A-alpha, A-beta, A-delta and C
in order of decreasing size, what are the axons designations that come from muscles?
Groups I, II, III, and IV
which axons designations are myelinated and unmyelinated?
Group C (or IV) are unmyelinated, the rest are myelinated
what is A-alpha (group I) axons used for? and its speed and diameter
proprioceptors of skeletal muscle, 80-120m/sec, 13-20 micrometres diameter
what is A-beta (group II) axons used for? and its speed and diameter
mechanoreceptors of skin, 35-75m/sec, 6-12 micrometres diameter
what is A-delta (Group III) axons used for? and its speed and diameter
Pain, temperature, 5-30m/sec, 1-5 micrometres in diameter
what is C (group IV) axons used for? and its speed and diameter
temperature, pain, itch, 0.5-2m/sec, 0.2-1.5micrometres diameter
how many spinal segments are there? and how many times is the arrangement of the paired dorsal and ventral roots repeated?
30 for both
where does each spinal nerve, consisting of dorsal root and ventral root axons pass through?
a notch between the vertebrae (“back bones”) of spinal cord –> there are as many spinal nerves are there are notches between vertebraes
What are the 30 spinal segments divided into?
-cervical (C) 1-8
-thoracic (T) 1-12
-lumbar (L) 1-5
-sacral (S) 1-5
What is the dermatome?
the area of skin innervated by the right and left dorsal roots of single spinal segment –> one-to-one correspondence between dermatomes and spinal segments
where does the spinal cord end and what continues on from there?
ends at the level of the third lumbar vertebrae, the bundle of spinal nerves streaming down within the lumbar and sacral vertebrae column are called the cauda equina
Describe the cauda equina
-Courses down the spinal column within a sack of dura filled with cerebrospinal fluid (CSF).
what information does the dorsal column-medial lemniscal pathway carry?
-touch or vibration of skin
where does the ascending branch of the large sensory axons (A-beta) enter?
enters ipsilateral dorsal column of the spinal cord, the white matter tract medial to the dorsal horn
where do the axons of the dorsal column terminate?
dorsal column nuclei, which lie at the junction of spinal cord and medulla
where does the dorsal column decussate?
at the level of the medulla, ventromedially
where do the axons ascend to after decussating in the medulla in dorsal column?
ascend within white matter tract called the medial lemnicus
where does the medial lemniscus white matter tract travel?
rises though the medulla, pons and midbrain and its axons synapse upon neurons of the ventral posterior (VP) nucleus of the thalamus
where does the dorsal column go after synapsing in the thalamus?
thalamic neurons of the VP nucleus then project to specific regions of primary somatosensory cortex (S1)
what is the somatic sensation of the face supplied by mostly?
the large trigeminal nerves (cranial nerve V/5)
Where does the trigeminal nerve enter the brain?
at the pons
what does the trigeminal nerve break up into?
3 peripheral nerves that innervate the face, mouth areas, the outer thirds of the tongue and the dura mater covering the brain
describe the sensory connections of the trigeminal nerve
Analogous to those of the dorsal roots. The large-diameter sensory axons of the trigeminal nerve carry tactile information from skin mechanoreceptors. They synapse onto second-order neurons in the ipsilateral trigeminal nucleus, which is analogous to a dorsal column nucleus. The axons from the trigeminal nucleus decussate and project into the medial part of the VP nucleus of the thalamus. From here, information is relayed to the somatosensory cortex.
where is the somatosensory cortex located?
in the parietal lobe
What is area 3b in the brain and why?
-Primary somatic sensory cortex because:
(1) it receives dense inputs from the VP nucleus of the thalamus
(2) its neurons are very responsive to somatosensory stimuli (but not to other sensory stimuli)
(3) lesions here impair somatic sensation
(4) when electrically stimulated, it evokes somatic sensory experiences.
What is area 3a concerned with?
receives input from thalamus, this region concerned with the sense of body position rather than touch
describe areas 1 and 2 in the brain
-receive input from area 3b
-projections from 3b to area 1 sends mainly texture information
-projection to area 2 emphasizes size and shape
what layer do thalamic inputs to S1 terminate?
layer IV (4) –> these neurons project to other layers
what is somatotopy?
the mapping of the body’s surface sensations onto a structure in the brain
what does the somatotopic map resemble and what is it sometimes called?
roughly resembles a body with its legs and feet at the top of the postcentral gyrus and its head at the opposite, lower end of the gyrus –> sometimes called homunculus
what happens to somatosensory area is that area is removed in body? and what happens if that part of body is used more?
that area is used for other parts of the body, e.g., if a finger is amputated that area of brain responsible for sensing that finger response to stimulation in adjacent digits –> i.e., rearrangement of circuitry occurs
-opposite also occurs, if part of body used more, that part will be expanded more
what is agnosia and how does it occur?
-the inability to recognise objects even though simple sensory skills seem normal
-damage to posterior parietal area
What is astereognosia? and how does it occur?
-cannot recognise common objects by feeling them, although sense of touch is otherwise normal
-damage to parietal lobe
What are nociceptors?
free, branching, unmyelinated nerve endings that signal the body tissue is being damaged or is a risk or being damage
what is the difference between pain and nociception?
-Pain is the feeling
-nociception is the sensory process that provides signals that trigger pain
what happens when nociceptors membrane is activated?
-Activates mechanically gated ion channels that cause the cell to depolarize and generate action potentials.
-In addition, damaged cells at the site of injury can release a number of substances that cause ion channels on nociceptor membranes to open (e.g., proteases (enzymes that digest proteins), ATP, K+))
What does protases do in the nociception response?
Can break down an abundant extracellular peptide called kininogen to form another peptide called bradykinin
What does bradykinin do?
-binds to specific receptors molecules that activates ionic conductance in some nociceptors
-in addition bradykinin stimulates long-lasting intracellular changes that make heat-activated ions channels more sensitive
what does ATP do in nociception response?
causes nociceptors to depolarise by binding directly to ATP-gated ion channels
where does transduction of painful stimuli occur?
in the free nerve endings of unmyelinated C fibres and lightly myelinated A-delta fibres
what are polymodal nociceptors?
nociceptors that respond to mechanical, thermal and chemical stimuli
what are mechcanical, thermal and chemical nociceptors?
showing selective response to strong pressure, burning heat or extreme cold and histamine and other chemicals
What is hyperalgesia? describe primary and secondary hyperalgesia
-Can be the reduction of threshold for pain, an increases intensity of painful stimuli or spontaneous pain (things are more painful than usual)
-primary hyperalgesia occurs within area of damaged tissue
-secondary hyperalgesia surrounding tissues becoming supersensitive
what is inflammation?
a natural response of the body’s tissues as they attempt to eliminate injury and stimulate the healing process
what are prostoglandins?
chemicals generated by the enzymatic breakdown of lipid membrane –> do not elicit pain, but increase greatly the sensitivity of nociceptors to other stimuli
What is substance P?
-Peptide synthesized by the nociceptors, activation of one branch of nociceptors axons can lead to secretion of substance P by other branch’s of that axon in the neighbouring skin
-substance P causes vasodilation (swelling of the blood capillaries) and release of histamine from mast cells
-sensitization of other nociceptors around the site of injury by substance P is one cause of secondary hyperalgesia
what is itch?
defined as a disagreeable sensation that induces a desire or reflex to scratch
What is the 2 distinct perceptions of pain?
-fast, sharp, first –> caused by A-delta fibres
-followed by duller, longer lasting second pain –> activation of C fibres
where do the small-diameter fibres of nociceptor cells have their cell bodies, where do they enter and where do they travel?
-cells bodies in segmental dorsal root ganglia
-enter dorsal horn of spinal cord
-fibres branch immediately and travel short distance up and down the spinal cord in a region called the zone of Lissauer, and then synapse on cells in the outer part of the dorsal horn in a region known as the substantia gelatinosa
what is the neurotransmitter of pain afferents?
glutamate, but neurons also contain peptide substance P
where does the nociceptor axons from viscera enter spinal cord?
Same route as cutaneous nociceptors
What is referred pain?
within the spinal cord, there is substantial mixing of information from the cutaneous and visceral nociceptors, where visceral nociceptor activation is perceived as a cutaneous sensation
describe the spinothalamic pain pathway
-information about pain (as well as temperature) is conveyed using this pathway
-the axons of second-order neurons immediately decussate (unlike dorsal column which decussate at level of medulla)
-ascend though the spinothalamic tract running along the ventral surface of the spinal cord
-traverse through brainstem without synapsing until reaching thalamus
describe the trigeminal pain pathway
-take path to thalamus analogous to the spinal path
-small-diameter fibres in trigeminal nerve synapse first on second-order sensory neurons in the spinal trigeminal nucleus of brainstem
-then axons of these cells cross and ascend to the thalamus in the trigeminal lemniscus
what is the gate theory of pain?
-certain neurons of dorsal horns, which project an axon up the spinothalamic tract are excited by both large-diameter sensory axons and unmyelinated pain axons
-the projection neuron is also inhibited by an interneuron, and the interneuron is both excited by large sensory axons and inhibited by pain axon.
-By this arrangement, activity in the pain axon alone maximally excites the projection neuron, allowing nociceptive signals to rise to the brain. However, if the large mechanoreceptive axon fires concurrently, it activates the interneuron and suppresses nociceptive signals.
What is an important region for suppression of pain? and what can electrical stimulation of this region do? and where does this area project to
-periventricular/periaqueductal gray matter (PAG),
-electrical stimulation can cause profound analgesia
-PAG neurons send descending axons into various midline regions of the medulla, particularly to the raphe nuclei (which use the neurotransmitter serotonin). These medullary neurons in turn project axons down to the dorsal horns of the spinal cord, where they can effectively depress the activity of nociceptive neurons.
What are opiods?
-drugs like opium, morphine, codeine and heroin
-produces profound analgesia when taken systemically
-produce mood changes drowsiness, mental clouding, nausea, vomiting and constipation
what is a endogenous morphine like substance that the brain produces?
endorphins
where are endorphins and their receptors concentrated?
in areas that process or modulate nociceptive information
Which do temperature-sensitive neurons are important for physiological responses that maintain stable body temperature
clusters of neurons in the hypothalamus and spinal cord
which thermoreceptors contribute to out perceptor of temperature?
the ones in the skin
What does the sensitivity of a sensory neuron to a change in temperature depend on?
the type of ion channels the neurons express
e.g.,
-TRPV1 for hot
-TRPM8 for cold
how many TRP channels are there?
6 that confer different temperature sensitivities
how many types of channels do thermoreceptors express?
only 1 single type –> thus explaining how different regions of skin can show distinctly different sensitivities
what is the temperature pain pathway?
-identical to pain pathway already described
-cold receptors with A-delta and C fibres, warm with only C fibres
-small diameter axons synapse within substantia gelatinosa of dorsal horn, second-order neurons immediately decussate and ascend in contralateral spinothalamic tract
