Sensory Receptors and Pathways

Question 1

What parts of the CNS recieve sensory information?

Answer 1

Cerebrum, cerebellum, brainstem and spinal cord

Question 2

What are the two types of sensory systems?

Answer 2

Somatosensory system - responds to the external environment

Viscerosensory system - responds to the internal environment

Question 3

How can we differentiate between perception and reflex?

Answer 3

Perception - slower response

Reflex - immediate response

Question 4

What are the types of peripheral innervation?

Answer 4

Afferent - axons from somatic receptors to the CNS (primary afferents)
Afferents feed into the spinal cord via a spinal nerve and the dorsal root

Efferent - axons from the CNS to peripheral structures (motor efferent)
Efferents leave the spinal cord via a ventral root and the spinal nerve

Question 5

What does the somatosensory system detect?

Answer 5

The system informs body about the external environment

Examples:
Touch - mechanoreceptors in the skin
Temperature - thermoreceptors in the skin
Pain - nociceptors in the skin

Question 6

What is the structure of a sensory neurone?

Answer 6

Receptor endings in the periphery
Axon to the cell body (dorsal root ganglia)
Terminals in the spinal cord

Pseudounipolar (meaning not really unipolar)

Question 7

What can receptors be?

Answer 7

Unimodal or polymodal

Respond to only one or multiple stimulus

Question 8

What are some types of mechanoreceptors?

Answer 8

Merkle
Ruffini
Meissner
Pacinian
Free nerve endings

Question 9

Describe Meissner and Merkle mechanoreceptors?

Answer 9

Meissner - they sit between the dermal papillae (just below the epidermis)
Comprised of looping axonal terminals and intertwined supporting cells. Stimulus = low frequency vibrations
Most common mechanoreceptor of smooth hairless skin (fingertips) - 40%

Merkle - They are aligned with the papillae (just below the epidermis)
Dome structure comprised of axon terminals and Merkel cells
Account for 25% of mechanoreceptors in the skin - mainly abundant in the fingertips, lips and genitalia

Question 10

Describe Ruffini, Pacinian and free nerve ending mechanoreceptors?

Answer 10

Ruffini - located deep in dermis, long axis of the corpuscle is orientated parallel to the skin - 20 % of receptors in skin
Nerve terminals intertwined with collagen fibrils (20%)
Stimulus = pressure

Pacinian - They are large encapsulated endings located in the subcutaneous tissue
sensory axon surrounded by fluid filled capsule gives it an onion-shaped appearance
Stimulus = high frequency vibrations

Free nerve endings - They penetrate into epithelial cells, no apparent morphological specialisation. Noxious (harmful)

Question 11

Describe the map of receptive field in a human hand?

Answer 11

Receptive field - area of skin where stimuli evoke sensory receptor activation
Variable in size
Punctate (discrete) zones characteristic of Meissners and Merkels
Broad regions more characteristic of Pacinians and Ruffini

Question 12

Describe sensory transduction?

Answer 12

A stimulus changes the nerve ending
It alters the membrane permeability of the receptor membrane
This produces a generator potential which triggers an action potential, to travel along the axon to the CNS

Sensory receptor - spinal nerve - dorsal root ganglion - dorsal root - grey matter

Question 13

What is the 3 neurone chain order?

Answer 13

1st order neurone – primary afferent (into dorsal horn, spinal cord)
2nd order neurone central neurone (to thalamus)
3rd order neurone – central neurone (to cortex)

Question 14

What is the dorsal column-medial lemniscal pathway used for?

Answer 14

A sensory pathway that conveys senses of: touch, vibration, two-point discrimination and proprioception

Question 15

Describe the dorsal column-medial lemniscal pathway?

Answer 15

Cerebral cortex
Thalamus
Medial lemniscus
Dorsal column nuclei
Dorsal column
Dorsal root axon

Question 16

How does diameter effect sensory afferents?

Answer 16

Large diameter - rapidly conducting afferents (I/II) associated with low threshold mechanoreceptors.
Small diameter - slow conducting afferents (III/IV) associated with nociceptors and thermoreceptors

Question 17

What do we name axons in different locations?

Answer 17

Axons from the skin are called A alpha, beta, etc…

Axons from the muscles are called Group I, II, III, etc…

Question 18

What is pain felt by?

Answer 18

Nociception
This is the sensory process that provides the signal that triggers pain
Pain is a feeling or perception (irritation, soreness, aching etc…)
Pain is detected by nociceptors, not the over stimulation of receptors
Nociceptors have cell bodies in the dorsal root ganglia (like any other sensory receptor)

Question 19

What are the types of nociceptors?

Answer 19

One modality:
Mechanical: respond to strong pressure
Thermal: respond to burning heat / extreme cold
Chemical: respond to histamine or other chemicals

Most are polymodal and will respond more than one stimulus

Question 20

What is the anatomy of nociceptors?

Answer 20

Nociceptors are found in the periphery as simple free nerve endings
Peripheral nerve fibre branches & innervates as naked, unmyelinated endings the dermis
They are the afferents with the smaller diameter (slow conducting)

Question 21

How do nociceptors respond to stimulus?

Answer 21

Pain afferents send signals to the CNS but also release signalling molecules such as Substance P locally to increase the inflammatory response (mast cell/neutrophil)

A nociceptor needs a strong enough stimulus to have any response (threshold)
They use several neurotransmitters: glutamate (excitatory) and vesicle containing neuropeptides such as substance P

Question 22

What is the spinothalamic pathway?

Answer 22

A sensory pathway that conveys a sense of: Pain, temperature and touch

It is a contralateral pathway i.e. sensory inputs cross at the level of spinal cord and ascend on opposite side
Information is relayed to the thalamus and then onto the somatosensory cortex

Question 23

Describe the spinothalamic pathway?

Answer 23

Cerebral cortex
Thalamus
Medulla
Lateral spinothalamic tract
Dorsal root axon

Question 24

What is the anterolateral system?

Answer 24

The overall pathway for conveying pain / temperature

The pain and temperature fibre crosses in the spinal cord
Other mechanosensory afferent fibres will cross later on in the medulla not in the spinal cord