Sensory Receptors and Pathways Flashcards
What parts of the CNS recieve sensory information?
Cerebrum, cerebellum, brainstem and spinal cord
What are the two types of sensory systems?
Somatosensory system - responds to the external environment
Viscerosensory system - responds to the internal environment
How can we differentiate between perception and reflex?
Perception - slower response
Reflex - immediate response
What are the types of peripheral innervation?
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
What does the somatosensory system detect?
The system informs body about the external environment
Examples:
Touch - mechanoreceptors in the skin
Temperature - thermoreceptors in the skin
Pain - nociceptors in the skin
What is the structure of a sensory neurone?
Receptor endings in the periphery
Axon to the cell body (dorsal root ganglia)
Terminals in the spinal cord
Pseudounipolar (meaning not really unipolar)
What can receptors be?
Unimodal or polymodal
Respond to only one or multiple stimulus
What are some types of mechanoreceptors?
Merkle Ruffini Meissner Pacinian Free nerve endings
Describe Meissner and Merkle mechanoreceptors?
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
Describe Ruffini, Pacinian and free nerve ending mechanoreceptors?
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)
Describe the map of receptive field in a human hand?
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
Describe sensory transduction?
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
What is the 3 neurone chain order?
1st order neurone – primary afferent (into dorsal horn, spinal cord)
2nd order neurone central neurone (to thalamus)
3rd order neurone – central neurone (to cortex)
What is the dorsal column-medial lemniscal pathway used for?
A sensory pathway that conveys senses of: touch, vibration, two-point discrimination and proprioception
Describe the dorsal column-medial lemniscal pathway?
Cerebral cortex Thalamus Medial lemniscus Dorsal column nuclei Dorsal column Dorsal root axon
How does diameter effect sensory afferents?
Large diameter - rapidly conducting afferents (I/II) associated with low threshold mechanoreceptors.
Small diameter - slow conducting afferents (III/IV) associated with nociceptors and thermoreceptors
What do we name axons in different locations?
Axons from the skin are called A alpha, beta, etc…
Axons from the muscles are called Group I, II, III, etc…
What is pain felt by?
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)
What are the types of nociceptors?
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
What is the anatomy of nociceptors?
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)
How do nociceptors respond to stimulus?
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
What is the spinothalamic pathway?
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
Describe the spinothalamic pathway?
Cerebral cortex Thalamus Medulla Lateral spinothalamic tract Dorsal root axon
What is the anterolateral system?
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
