the somatosensory system -sensory neurones (lecture)

Question 1

role of dendrites?

Answer 1

-receive inputs from other neurons and convey graded electrical signals and passively to the soma

Question 2

describe the role of the cell body (soma) and what it contains?

Answer 2

-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

Question 3

describe the roll of the axon hillock and initial segment?

Answer 3

-site of initiation
-‘all or none’ action potential

Question 4

role of the axon?

Answer 4

-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

Question 5

role of the synapse?

Answer 5

-the point of chemical communication between neurons (or other cells)

Question 6

clinical importance of retrograde transport (transport toward the cell body/soma)?

Answer 6

-several viruses (e.g. herpes, polio, rabies) exploit retrograde transport to infect neurons

Question 7

what type of neuron is this?

Answer 7

unipolar (sensory neurons in PNS)

Question 8

what type of neuron is this?

Answer 8

pseudounipolar (dorsal root ganglion and PNS are part of this)

Question 9

what type of neuron is this?

Answer 9

bipolar (retinal bipolar neurons)

Question 10

what type of neuron is this?

Answer 10

multipolar (lower motor neuron and motor neurons in PNS are part of this)

Question 11

what are the 4 functional regions of a neuron?

Answer 11

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

Question 12

what sensory modalities does the somatosensory system mediate?

Answer 12

-fine discrimatory touch (light touch, pressure, vibration, flutter and stretch)
-joint and muscle position sense (proprioception)
-temperature (thermosensation)
-pain (nociception)
-itch (pruriception)

Question 13

where is the cell body of the PNS located?

Answer 13

-in the dorsal root ganglia (innervation of limbs, trunk and posterior head)
OR
-cranial ganglia (innervation of anterior head)

Question 14

what does the somatosensory pathway most often comprise of?

Answer 14

three neurons in series

1st order neuron= PNS
2nd/3rd order neurons= CNS
somatosensory cortex= CNS

Question 15

describe the physiology behind how neuron terminals (first order neurones) in the somatosensory system transduce a stimulus into electrical activity

Answer 15

-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

Question 16

how is touch, pressure vibration picked up by receptors?

Answer 16

touch, pressure vibration causes mechanical forces acting on the skin which is picked up by skin mechanoreceptors

Question 17

how is proprioception picked up by receptors?

Answer 17

proprioception creates mechanical forces acting on joints and muscles which is picked up by joint and muscle mechanoreceptors

Question 18

how is temperature picked up by receptors?

Answer 18

temperature -> heat (thermal energy) -> cold and warm thermoreceptors

Question 19

how is pain picked up by receptors?

Answer 19

pain -> strong mechanical force on skin/ viscera, heat on skinm mucous membranes and viscera -> mechanical, thermal and polymodal nociceptors

Question 20

how is itch picked up by receptros?

Answer 20

itch -> irritant on skin or mucous membranes -> itch receptors

Question 21

what intensity of the stimulus is required for excitation of the sensory receptor?

Answer 21

low threshold mechanoreceptors respond to:
-fine discrimatory touch

high threshold mechanoreceptors respond to:
-high intensity mechanical stimuli

Question 22

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?

Answer 22

Slowly adapting/ tonic response- continuous information to CNS while terminal deformed e.g. stretch receptors

Question 23

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?

Answer 23

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

Question 24

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?

Answer 24

Very fast adapting response - responds only to very fast movement such as rapid vibration e.g. Pacinian corpuscle

Question 25

how rapidly does the sensory unit conduct action potentials along its axon?

Answer 25

Depends on diameter of axon and whether the axon is myelinated

Question 26

where is the peripheral terminal of the sensory unit located, do its sensory receptors have a small or large anatomical distribution?

Answer 26

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

Question 27

why is two point discrimination (or spatial acuity) important?

Answer 27

-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

Question 28

how is two point discrimination clinically tested?

Answer 28

-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

Question 29

wh

Question 30

what cutaneous receptors measure heavy pressure?

Answer 30

Meissner corpuscle

Question 31

what cutaneous receptors measure vibration?

Answer 31

Pacinian corpuscle

Question 32

what cutaneous receptors measure light touch?

Answer 32

Merkel disks

Question 33

what cutaneous receptors measure skin stretch?

Answer 33

Ruffini endings

Question 34

what is a dermatome?

Answer 34

-an area of skin innervated by the left and right dorsal roots of a single spinal segment

Question 35

how is shingles related to dermatomes?

Answer 35

-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