the somatosensory system -sensory neurones (lecture) Flashcards
role of dendrites?
-receive inputs from other neurons and convey graded electrical signals and passively to the soma
describe the role of the cell body (soma) and what it contains?
-it is the synthetic and metabolic centre of the neuron
-it contains the nucleus, ribosomes, mitochondria and endoplasmic reticulum
-it integrates electrical signals that are conducted passively to the axon hillock
describe the roll of the axon hillock and initial segment?
-site of initiation
-‘all or none’ action potential
role of the axon?
-conducts output signals as action potentials to the presynaptic terminal
-it mediated the transport of materials between the soma and presynaptic terminal (anterograde direction) and vice versa (retrograde direction)
Anterograde= when signals are going towards the axon tip
Retrograde= when signals are going towards the soma
role of the synapse?
-the point of chemical communication between neurons (or other cells)
clinical importance of retrograde transport (transport toward the cell body/soma)?
-several viruses (e.g. herpes, polio, rabies) exploit retrograde transport to infect neurons
what type of neuron is this?
unipolar (sensory neurons in PNS)
what type of neuron is this?
pseudounipolar (dorsal root ganglion and PNS are part of this)
what type of neuron is this?
bipolar (retinal bipolar neurons)
what type of neuron is this?
multipolar (lower motor neuron and motor neurons in PNS are part of this)
what are the 4 functional regions of a neuron?
input- where stimulus is received
integrative- the soma or the axon hillock where all of the signals are integrated
conductile- the length of the axon where the signal is conducted
output- where the neurotransmitter release is
what sensory modalities does the somatosensory system mediate?
-fine discrimatory touch (light touch, pressure, vibration, flutter and stretch)
-joint and muscle position sense (proprioception)
-temperature (thermosensation)
-pain (nociception)
-itch (pruriception)
where is the cell body of the PNS located?
-in the dorsal root ganglia (innervation of limbs, trunk and posterior head)
OR
-cranial ganglia (innervation of anterior head)
what does the somatosensory pathway most often comprise of?
three neurons in series
1st order neuron= PNS
2nd/3rd order neurons= CNS
somatosensory cortex= CNS
describe the physiology behind how neuron terminals (first order neurones) in the somatosensory system transduce a stimulus into electrical activity
-sensory neurons terminals transduce a stimulus into electrical activity
-stimulus (mechanical, thermal or chemical) opens cation selective ion channels in peripheral terminal of primary sensory afferent eliciting a depolarizing receptor potential
-amplitude of receptor potential is graded and proportional to stimulus intensity
-a supra threshold receptor potential triggers ‘all or none’ action potentials, conducted by the axon at a frequency proportional to the amplitude
-action potentials arriving at the central terminal cause the graded release of neurotransmitter on to second order neurons
how is touch, pressure vibration picked up by receptors?
touch, pressure vibration causes mechanical forces acting on the skin which is picked up by skin mechanoreceptors
how is proprioception picked up by receptors?
proprioception creates mechanical forces acting on joints and muscles which is picked up by joint and muscle mechanoreceptors
how is temperature picked up by receptors?
temperature -> heat (thermal energy) -> cold and warm thermoreceptors
how is pain picked up by receptors?
pain -> strong mechanical force on skin/ viscera, heat on skinm mucous membranes and viscera -> mechanical, thermal and polymodal nociceptors
how is itch picked up by receptros?
itch -> irritant on skin or mucous membranes -> itch receptors
what intensity of the stimulus is required for excitation of the sensory receptor?
low threshold mechanoreceptors respond to:
-fine discrimatory touch
high threshold mechanoreceptors respond to:
-high intensity mechanical stimuli
does the sensory unit of a slowly adapting/ tonic response (e.g. stretch receptor) discharge action potentials continuously during the stimulus, or does it respond preferentially to a changing stimulus?
Slowly adapting/ tonic response- continuous information to CNS while terminal deformed e.g. stretch receptors
does the sensory unit for a fast adaptic/phasic response (e.g. some muscle spindle afferents) discharge action potentials continuously during the stimulus, or does it respond preferentially to a changing stimulus?
Fast adaptic/phasic response - detects changes in stimulus strength, number of impulses proportional to rate of change of stimulus e.g. some muscle spindle afferents
does the sensory unit of very fast adapting response (e.g. Pacinian corpuscle) discharge action potentials continuously during the stimulus, or does it respond preferentially to a changing stimulus?
Very fast adapting response - responds only to very fast movement such as rapid vibration e.g. Pacinian corpuscle
how rapidly does the sensory unit conduct action potentials along its axon?
Depends on diameter of axon and whether the axon is myelinated
where is the peripheral terminal of the sensory unit located, do its sensory receptors have a small or large anatomical distribution?
The peripheral termination of cutaneous afferent fibres branch into many fine processes, the tips of which can be free nerve endings or associated with specialised structures
- Receptor field (RF): target territory from which a sensory unit that can be excited
- Sensory acuity correlates inversely with RF size
why is two point discrimination (or spatial acuity) important?
-it is an important measure of somatosensory function
-two point thresholds match the diameter of the corresponding receptor field (target territory from which a sensory unit can be excited)
-regions with highest discriminative capacity have the smallest RFs
how is two point discrimination clinically tested?
-by applying simultaneously two sharp point stimuli, separated by a variable distance, at different sites on the body surface
-subject reports on whether one point, or two, are sensed and a threshold distance between the two is established
wh
what cutaneous receptors measure heavy pressure?
Meissner corpuscle
what cutaneous receptors measure vibration?
Pacinian corpuscle
what cutaneous receptors measure light touch?
Merkel disks
what cutaneous receptors measure skin stretch?
Ruffini endings
what is a dermatome?
-an area of skin innervated by the left and right dorsal roots of a single spinal segment
how is shingles related to dermatomes?
-shingles is due to an infection of the dorsal root ganglion neurones by varicella zoster
-in adulthood the virus may reactivate (typically in a single ganglion), revealing anatomically, as inflamed an blidtered skin, the dermatome innervated by that ganglion
