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.
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 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 fasiculus neurons synapse?
They synapse in the Cuneate and Gracile Nuclei in the medulla
Describe what happens after these fasiculus 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 the trigeminal sensory 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
List 4 major somatosensory modalities
Pain
Temperature
Touch
Proprioception
3 sensory neurones
A beta
A delta
C fibers
Define receptor
Transducers that convert environmental energy to neuronal action potentials
What is somatosensory function?
Ability to interpret the bodily sensations Mechanical Thermal Proprioceptive Nociceptive
Somatosensory system consists of
Receptors
Nerve cell tracts in body and spine
Parts of brain dealing with processing sensory information
Somatosensory modalities
Touch- mechanical stimuli
Thermal
Nociception
Proprioception
What is modified to determine what is being sensed
Nerve terminal
What do free nerve endings transduce
Nociceptors and thermoreceptors
What do enclosed nerve terminals transduce
Mechanoreceptors
Role of AB fibres
Innocuous mechanical stimulation
The fastest
Role of A delta fibres
Noxious mechanical and thermal stimulation
Role of c fibres
Noxious mechanical, thermal and itch stimulation
Define receptors
Sensory receptors are transducers that convert environmental energy to neuronal action potentials
Thermoreceptor fibres
A delta and C
How do nerves determine temperature changes
Presence of Transient Receptor potential ion channels
4 heat activated and 2 cold activated ion channels which are activated by different temperatures
5 different mechanoreceptors
Hair follicle receptors Meissners corpuscle Merkel cells Pacinian corpuscle Ruffini endings
Role of meissners corpuscles
Transduce low frequency vibration and fine discriminative touch
Role of Merkel cells
Transduce light touch and superficial pressure
Role of pacinian corpuscles
Transduce deep pressure
High frequency vibration
Tickling
Role of ruffini endings
Transduce continuous pressure
Stretch
Role of hair follicle receptors
Light touch
Phasic receptors
Detect change in stimulus strength by transmitting impusle at start and end of a stimulus so when a change has occurred
Example of phasic receptor
Pacinian receptor
Sudden pressure when excited then transmits signal when pressure released
Feature of phasic receptor
Fast adapting
Receptive field
Region of skin which causes activation of one signal sensory neurone
Differences in receptor fields across body
On back is large fields so cells detect over a larger area for less precise perception
Whereas on fingers are densely packed mechanoreceptors with small receptor fields giving very precise perception
2 point discrimination
Minimum distance at which 2 points are perceived as separate
What is 2 point discrimination related to
Size of receptive field
Where are cell bodies for receptors in body
Dorsal root ganglia at that spinal level
Where are cell bodies for receptors found on face
Trigeminal ganglia
Pathway for sensory information from upper limbs
Received at cervical level-> head up cuneate fasiculas tract (known as internal arculate fibres) then cross over at medulla-> form medial lemniscus tract-> thalamus
Types of dorsal horn neurones
Axons that project to brain
Axons remaining in spinal chord (interneurones)
Difference in distribution sensory neurones synapsing in dorsal horn
A delta and c more superficial
A beta deeper
What is lateral inhibition
Can get overlapping of adjacent receptive fields making it hard to distinguish 2 stimulus locations. Lateral inhibition prevents this
How does lateral inhibition work
Mediated by inhibitory interneuones within dorsal horn
What pathway is touch and proprioception carried in
Dorsal column
Path of first order neurones touch and proprioception
Cell body in dorsal root ganglion, travels up either cuneate or gracile tract to terminate in either cuneate or gracile nucleus in medulla
Path of second order neurones
Decussate in caudal medulla then form contralateral medial leminiscus tract and synapse in the ventral posterior lateral nucleus
Differences in termination of second order neurones depending on where they’ve come from
Lower parts of body are more lateral
Somatosensory homunculus
Size of somatotropic area in brain is proportional to density of sensory receptors in that area
Differences in localisation between noxious and inoxious
Inoxious much more precise localisation
Location of somatosensory cortex
Found in posterior part of parietal lobe behind central gyrus
Differences in ascension between pain, temperature and crude touch
Crude touch in anterior spinothalamic
Pain and temp in lateral spinothalamic
Pathway of spinothalamic first order neurones
Terminate immediately in dorsal horn
Pathway second order spinothalamic pathway neurones
Decussate at spinal level and travel in lateral or anterior spinothalamic tract before terminating in VPL in thalamus
Topographic representation of body in VPL spinothalamic pathway
Lower extremities more lateral
Quantitive sensory testing purpose
To test effectiveness of spinal tracts
How quantitive sensory testing works
An quantified stimulus is applied either in form of heat and temperature for spinothalamic tract or 2 point stimulus for dorsal column and integrity is checked
Cause of anterior spinal lesion
Blocked anterior spinal artery
Effect on both tracts of anterior spinal artery blockage
Bilateral loss pain and temperate sensation below that spinal level
Normal touch and 2 point discrimination below that level
Define pain
An unpleasant sensory or emotional experience associated with actual or potential tissue damage
2 types of nociceptors
A delta and C
Pain from a delta nociceptors
Sharp and intense first pain
Type 1- noxious mechanical
Type 2- noxious heat
Pain from c fibre nociceptors
Dull and aching pain from all stimuli
Spinal chord nociceptive processing NT
Glutamate is major pain signalling NT
It is released from sensory afferents in response to acute or persistent noxious stimuli
First synapse in pain pathway
Gulatmergic synapse in superficial dorsal horn
Parts of brain involved in pain reception
Amygdala
Prefrontal cortex
Cerebellum
Gate control theory
There is inhibition of primary afferent inputs to brain via a beta neurone which excites and inhibitory neurone
Descending control pathways
Use of strong emotions to inhibit pain
How does descending control pathways work
Monoamines from descending pathways can inhibit nociceptive processing in the dorsal horn
Examples of monoamines acting in descending control pathways
Serotonin and noradrenaline
Chronic pain
Pain experienced for over 3 months
In the uk believed 28 million people
Difference between nociceptive and neuropathic pain
Nociceptive to do with noxious stimulation of a somatic or visceral nociceptor but neuropathic a disease of somatosensory system
Peripheral sensitisation
With tissue damage a lot of inflammatory molecules are released, these have been proven to increase the sensitivity of nociceptors in this area therefore reducing their threshold for stimuli
Central sensitisation
Increased sensitisation of c fibres from periphery brings about a plasticity in the projection neurone in dorsal horn which affects adjacent cells such as the a delta fibre in dorsal horn
Allodynia
Pain due to a stimulus that does not normally provoke pain
Hyperalgaesia
Increased pain from a stimulus that does normally provoke pain
Remembering 5 mechanoreceptors
PHRMM
