Sensory Pathways: Touch and Proprioception Flashcards
What receptors are responsible for the sensory modalities of touch and proprioception?
Mechanoreceptors
Describe the structure of mechanoreceptors involved in touchand proprioception.
The receptor is NOT a separate entity but is actually the peripheral terminal of the peripheral axon of the primary sensory neuron.
Describe the structure of a Pacinian corpuscle and explain how this structure relates to its function.
There is an axonal ending in the middle and it is wrapped around several concentric circles of epithelial cells – this allows the receptor to be very sensitive to vibration.
What is the difference between slow adapting and fast adapting receptors?
Slow adapting receptors continue firing impulses for as long as the stimulus is present
Fast adapting receptors tend to fire at the start of the stimulus and sometimes when the stimulus switches off but they tend to fade in the middle
What type of receptors are mechanoreceptors?
Mixture of slow and fast adapting receptors
Describe how sensory neurons vary in their properties.
They vary in SIZE and CONDUCTION VELOCITY
What are the two classifications of axons?
Anatomical = based on axon diameter (labelled using LETTERS) eg A-beta, c fibres, A-delta
Physiological = based on conduction velocity (labelled using ROMAN NUMERALS)eg I, II, III
As axon diameter and conduction velocity are related, there is a lot of overlap in the classifications
Describe the general structure of sensory neurons that convey touch and proprioceptive information.
They are LARGE and have a FAST conduction velocity
What is a receptive field?
An area of skin that is innervated by ONE sensory axon and its branches
Describe how the receptive fields in the lips and mouth vary from the receptive fields of the upper arm.
Lips and Mouth – high-density innervation with very small receptive fields
Upper arm – larger receptive fields and thinner innervation
Describe how neurons can code for the intensity of a stimulus.
It is coded by the FREQUENCY of the action potentials going down the sensory fibres
Which part of the spinal cord carries sensory axons for touch and proprioception?
Dorsal columns
What are the bundles of axons within the spinal cord that havecome from above and below the waist called? Describe their spatial arrangement within the spinal cord.
Above the waist – Cuneate Fasciculus Below the waist – Gracile Fasciculus Axons from below the waist are packed more medially in the dorsal column and above the waist are more lateral Lower = Medial Higher = Lateral
Where do these neurons synapse?
They synapse in the Cuneate and Gracile Nuclei in the medulla
Describe what happens after these neurons synapse and the tract that they run in.
The second order neurons then cross the midline (decussation) and continue up the brainstem in the MEDIAL LEMNISCUS
Which thalamic nucleus is responsible for relaying somatosensory information from the neck down?
Ventral Postero-lateral
Describe the passage of the third order sensory neuron.
The third order neurone travels from the ventral postero-lateral nucleus in the thalamus to the primary somatosensory cortex
Where does the trigeminal nerve enter the brainstem and where does it synapse with a second order neuron?
Pons
It synapses at the trigeminal cranial nucleus
Describe the passage of this second order neuron.
The second order neuron crosses the midline (decussation) and joins the medial part of the median lemniscus
Which thalamic nucleus is responsible for relaying sensory information from the face?
Ventral Postero-medial
What is the purpose of lateral inhibition?
Improves the resolution of localising the stimulus
Name the three parts of the somatosensory cortex.
Primary Somatosensory Cortex (SI)
Secondary Somatosensory Cortex (SII)
Posterior Parietal Cortex
What is the posterior parietal cortex mainly involved in?
Spatial relationships
Damage to the touch and proprioception pathway will halt sensory information going up to the primary somatosensory cortex. What effects will this have?
Anaesthesia (complete cessation of sensation)
Parasthesia (sensation is there but it isn’t normal)
What is the most common cause of peripheral neuropathy?
Diabetes mellitus
what are the 4 main sensory modalities
touch
proprioception
temperature
pain
what are the 3 main receptors for these modalities
mechanoreceptor-touch/proprioception
thermoreceptor- temperature
nociceptor- pain
what modalities come under “touch”
light touch
pressure
vibration
what modalities come under proprioception
joint position
muscle length
muscle tension
what is lateral inhibition
activation of one neural unit inhibits the neural units surrounding it
This facilitates pinpoint accuracy in localisation of the stimulus
So each axon will stimulate a second order neuron and inhibit neighboring first order neurons
what is 2 point discrimination
minimum distance between 2 stimuli in order to perceive them as 2 separate stimuli
more densely packed receptive fields- smaller 2 point discrimination
what is neural adaptation
a form of desensitisation-
if a stimulus of constant strength is maintained for a period of time, the frequency of Ads diminishes,
this length of time is different for different receptors:
phasic-Rapidly adapting
tonic-Slowly adapting
what are the three main types of fibres and give their functions
A-beta fibres-innocuous mechanical stimulation
A-delta fibres-noxious mechanical and thermal stimulation
C-fibres-noxious mechanical, thermal and chemical stimulation
give so e characteristics of nociceptor fibres
high threshold for activation
triggered by direct activation of ion channel proteins
which part of the spinal cord does crude touch ascend in
anterior spino-thalamic (A-delta fibres)
which part of the spinal cord does pain/temp ascend in
lateral spino-thalamic tract
where do the anterior and lateral spinothalamic tracts decussate
at the level they enter the spinal cords (first order neurones synapse in dorsal horn and second order decussate immediately)
where do both dorsal and spinothalamic second order neurones synapse with their third order neurones
VPL in thalamus - There is a topographic representation of the body in the VPL (lower extremities are lateral)
