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Neuroscience -part I > Somatosensory systen > Flashcards

Somatosensory systen Flashcards

1
Q

What is the function of our sensory system?

A

To provide the CNS with a representation of the external
(or internal) world
Accomplishing this requires a close interaction between
ascending (stimulus driven mechanisms) and descending (goal-directed) mechanisms.
TOGETHER, these two mechanisms give rise to
perception and activate stored memories to form the basis of the conscious experience.

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2
Q

What are the properties of sensory receptors?

A
  • SR = organs which receive info from outside or within the body and send it to the CNS for processing
  • Sensory receptors : form an initial neural representation of the internal or external world
  • Activated by various forms of energy , light, sound,thermal, chemical, mechanical etc,
  • Separate populations of receptors are selective for a particular form of energy
  • This is a direct function of the differences in receptor structure and chemistry
  • Transducers : convert one form of energy into another
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3
Q

What are the central pathways of olfaction?

A
  • Olfactory receptory cell TO Olfactory bulb TO Olfactory tract TO Olfactory tubercle TO Thalamus TO medial dorsal nucleus TO Orbitofrontal cortex
  • Olfactorry receptor cell to olfactory cortex and related temporal lobe structures
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4
Q

What are the central pathways of gustation?

A
  • Taste cells (tongue,epiglottis, pharynx) to Primary gustatory axon to Brainstem gustatory nuclei to Thalamus to Gustatory cortex
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5
Q

What are the central pathways for hearing?

A
  • Dorsal and ventral cochlear nuclei contain cell bodies of the 2ary neurones
  • Project to both sides of the brianstem and auditory cortex from the venral cochlear:
  • Processes pass to the superior olive on both sides of the brain
  • Processes then pass to the inferior colliculus (midbrain)
  • Then to the medial geniculate nucleus (thalamus)
  • Then tp the auditory cortex (temporal lobe)
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6
Q

What are the central pathwats for balance?

A
  • Coordinate and integrate information about head and body movement to control motor output of headm eye and body positions
  • CRANIAL NERVE VIII to cerebelleum via lateral vestibicular nucleys
  • Lateral vestibicular nucleus (ispilateral) to cerebellum
  • Cranial nerve VIII to limb motor neurons
  • Cranial nerve VIII to VP nucleus of thalamus
  • Cranial nerve 8 to neck motor neurons
  • Cranial nerve 8 to VP nucleus of thalamus, extraocular motor neurons (III, IV, VI) (Medial longitudinal fasciculus)
  • Cranial nerve VIII ro neck motot neurons (medullary-pontine junction)
  • Numerous connections but the vestibular nerve system maintains equilibrium by adjusting body position when loss of balance is threatened
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7
Q

What are the three types of general sensory endings?

A
  • Exteroreceptors : lie superficially in the skin and respond to different sensory modalities (noiceptive (painful) stimuli, temperature and touch
  • Interoreceptors : occur in the viscera and respond prinicpally to mechanical and chemical stimuli
  • Proprioreceptors : occur in muscles, joints and tendons and provide awareness of posture and movement (kinaesthesia)
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8
Q

What is modality?

A

Discrimination of the sensory experience e.g. what we ‘perceive’
following a stimulus e.g.
Mechanical e.g. touch
Light touch, vibration, pressure
(Mechanoreceptors)
* Temperature e.g. hot or cold
warm, tepid
(Thermoreceptors)
* Painful e.g injurious forces
(Nociceptors)
* Body position e.g. body position, body movement
(Proprioceptors)
N.B The somatic sensory system includes all sensations except those of the special
senses i.e. vision, hearing, smell, taste & balance.

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9
Q

What are the properties of cutaneous receptors?

A
  • Free nerve ending X2 - Function : 1 X temp, threshold varies, adaptation = rapid. 1x pain, Threshold High, Adaptation = slow.
  • Meissner corpuscle : Function = Touch (dynamic deformation, Threshold = low, Adaptation = rapid.
  • Merkel cell disks : Function = touch (indentation), threshold low, adaptation = s;pw
  • Ruffini corpuscle : Function = touch (Stretch), threshold = low, Adaptation = slow
  • Pacinian corpuscle = function : Touch (vibration), Threshold = low, adaptation = very rapid
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10
Q

What is the receptor/ generator potential?

A
  • Depolarisation occuring at the sensory terminals
  • Graded change in membrane potential - when large enough leads to an action potential
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11
Q

Describe the intensity of the stimulus.

A
  • The number of action potentials is generated by a pressure-sensitive sensory afferent neuron that is directly proportional to the stimulus intensity
  • Stimulus intesnsity is linearly encoded
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12
Q

What is rapid adaptation?

A
  • a mechanism to prevent sensory overload
  • Different receptors vary in their speed of adaptation to stimuli
  • ## Action potentials stop quickly
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13
Q

What is slow adaptation ?

A
  • The generator potential set up by a constant stimulus will wane slowly overtime
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14
Q

What happens when there is no adaptation?

A
  • The generator potential / action potential frequency will remain constant if the stimulus remains constant.

generator potential

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15
Q

What is receptive arrangement?

A
  • Receptors arranged in an orderly fashion across the skin surface
  • The density of the cutaneous receptors differs across the skin surface
  • Greatest density of receptors foudn at fingertips and the mouth
  • High density is linked to spatial acuity
  • Receptive field : area in ehich a stimulus will elicit neural responses when stimulated
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16
Q

What is the 2 point discrimination test?

A
  • Tests to see how far apart the two ends of an unwound paper clip have to be before one can discriminate the two points
  • Subject closes their eyes and paper clip is bent into a U shape.
  • Start at the ends of the paper clip about 1 inch apart. Test the minimum distance at which the subject can distinguish the two ends of the paper clip as two distinct
  • Tested on the subject’s palm, middle finger tip and forearm
17
Q

Why is the activity in a particular population of somatosensory neurones always intepreted by the CNS as a painful stimulus?

A
  • Neurones exhibit modality specificity
  • Neurones are not only selective in what drives them, but also in the post-synaptic targets which they communicate
  • Information passes from receptor to ganglion to central neurones in a highly organised fashion
18
Q

What are the different order neurones and where are they located?

A
  1. First order neurones: have cell bodues in dorsal cell ganglia
  2. Second-order neurones : have cell bodies in the grey matter of the spinal cord or brainstem and give rise to axons that cross the midline and ascend to the thalamus
  3. Third- order neurones : located in the ventral posterior nucleus of the thalamus and project to the somatosensory cortex - arranged in a precise point-to-point fashion. Fibres innervating adjacent parts of the body surface remain side-by-side along the full length of the pathway to the brain = somatotopy
19
Q

What is the somatosensory cortex?

A
  • Cortical homunculus (little man)
  • Located in the post-central gyrus
  • A high degree of spatial segregation of neurones represnting different part of the body’s somatotopic organisation
20
Q

What are the somatosensory information pathways?

A
  • DORSAL COLUMN- MEDIAL LEMIMISCAL PATHWAY:
  • decussation happens at different parts for different sensory modalities
  • Mainly Aa and AB. Fast, high fidelity
  • precise spatial localisation and quality of stimulus
  • rexed’s laminae
  • Histologically visible in the grey matter of the spinal cord. Dorsal horn includes laminae I-VI
  • Branches contribute to local spinal reflexes and to ascending pathways

ANTEROLATERO-SPINOTHALAMIC PATHWAY :
- mainly A delta and C = slower less precise

21
Q

What are rexed’s laminae?

A
  • Layer like organisation of the spinal cord
  • Dorsal hron includes laminae I-VI
  • Primary afferents branch in entering spinal cortd
22
Q

What are spinocerebellar tracts?

A
  • For unconscious proprieception
  • Anterior and posterior
  • Both tracts carry non-conscious proprioceptive information to the ispilateral cerebellum
  • Posterior tract does not cross
  • Anterior crosses twice
  • Both provide input to the ipsilateral cerebellar hemisphere
  • Spinocerebellar tract primarily remains ipsilateral. In general, one side of the cerebellum influences the same side of the body. Cerebellar lesions tend to produce ipsilateral malfunction. Typically presenting akwardness of movement
23
Q

What is the cuneocerebellar tract?

A
  • Neurons located in this nucleus then give rise to the cuneocerebellar tract, functionally related to the upper limb
24
Q

What is the laterality of signs for spinal cord lesions?

A
  • If a lesion occurs above the level at which a pathway has decussated, the signs will be contralateral to the lesion
  • If a lesion occurs below the level at which a pathway has decussated, the signs will be ipsilateral to the lesion
  • True for sensory and motor lesions
25
Q

What is a brown-sequard lesion?

A
  • A rare condition that results from an injury to one side of the spinal cord

Symptons :
Numbness and weakness in limbs.
Loss of proprioception and fine touch in one side of the body
- Loss of pain sensation on the right side below the naval
- His left leg is very weak and he can hardly stand

Neuroscience -part I (21 decks)

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