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 thestimulus 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) Physiological = based on conduction velocity (labelled using ROMAN NUMERALS) 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 conveytouch 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
What is the main sensory nerve of the face?
Trigeminal Nerve (CN V)
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 lateral inhibition?
Lateral inhibition takes place in the cuneate and gracile nuclei Each axon has lateral branches that are inhibitory on neighbouringaxons So each axon will stimulate a second order neuron and inhibit neighboring first order neurons
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 examples of the 2 types of nerve ending?
Free nerve endings - noiciceptors Enclosed nerve endings - mechanoreceptors
Describe the different kinds of sensory neurones
Aα - Biggest and fastest, Propiception Aβ - 2nd biggest and 2nd fastest, mechanoreceptors Aδ - 3rd biggest 3rd fastest, pain and temperature C - smallest and slowest, temperature, pain, itch.
What are the types of thermoreceptor?
Aδ - 2 Cold activated TRPM8 & TRPA1 C - 4 Heat activated TRPV1-4
What are the characteristics of thermoreceptors?
Free nerve endings, Transient receptor potential (TRP) ion channels
What are the types of mechanoreceptor? What are they stimulated by?
- Meissner’s corpuscle - Fine discriminative touch, low frequency vibration
- Merkel cells - light touch and superficial pressure
- Pacinian corpuscle - Deep pressure, high frequency, vibration and tickling
- Ruffini endings - Continuous pressure or tough and stretch
Define Stimulus threshold
‘’A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)’’
Define Stimulus intensity
The amount of neurotransmitter released as a direct result of the strength and duration of the stimulus
Define adaptation
How the sensory organs transmit sensation and how the brain interprets that
Define receptive field
‘’The receptive field is the region on the skin which causes activation of a single sensory neuron when activated’’ Small receptive fields allow for the detection of fine detail over a small area. Precise perception Large receptive fields allow the cell to detect changes over a wider area (less precise perception)
Define 2 point discrimination
Minimum distance at which two points are perceived as separate Related to the size of the receptive field
Draw the different mechanoreceptors
Define lateral inhibition
Interneurons in the dorsal horn inhibit lateral neurones within the dorsal horn of the spinal cord This is because receptive fields can overlap Lateral inhibition helps facilitate pinpoint accuracy in the location of stimulus and discrimination
What are the 2 types of receptor adaptation?
Tonic receptors - slow adapting detect constant stimulus strength transmit impulses to the brain as long as the stimulus is present e.g. merkel cells Phasic receptors - fast adapting detect a change in stimulus strength trasmit impulses at the beginning and end of a stimulus e.g. pacininan corpuscle
What is the dorsal column system?
Innocuous mechanical stimuli Fine discriminative touch Vibration Aβ fibers enter via the dorsal horn and enter the ascending dorsal column pathways Information conveyed from lower limbs and body (below T6) travel ipsilaterally along the gracile tract Information conveyed from upper limbs and body (above T6) travel ipsilaterally along the cuneate tract
What is the pathway of 1st order dosal column neurones?
1st order neurones synpase in the medulla Fibers in the Gracile Tract have their first synapse in the Gracile Nucleus Fibers in the Cuneate Tract have their first synapse in the Cuneate Nucleus
What is the pathway of 2nd order dosal column neurones?
Second order axons decussate (cross the midline) in the caudal medulla Forms the contralateral medial lemniscus tract 2nd order neurons terminate in the thalamus The axons of the second order neurons terminate in the ventral posterior lateral nucleus of the thalamus (VPL).
What is the pathway of 3rd order dosal column neurones?
3rd order neurons from the VPL project to the somatosensory cortex
What is the spinothalamic pathway?
Pain and temperature sensations ascend within the lateral spinothalamic tract Crude touch ascends within the anterior spinothalamic tract
What is the pathway of 1st order spinothalamic pathway neurones?
Primary afferent axons terminate upon entering the spinal cord Second order neurons decussate immediately in the spinal cord and form the spinothalamic tract
What is the pathway of 2nd order spinothalamic neurones?
2nd order neurons terminate in the ventral posterior nucleus of the thalamus
What is the pathway of 3rd order spinothalamic neurones?
3rd order neurons from the VPL project to the somatosensory cortex
How do you assess damage to the sinal columns?
2 point discrimination - assess dorsal root column Temperature and pain tests assess the spinothalamic tract
