Neurology - Somatosensory Flashcards
(46 cards)
What is a sensory modality
A modality is a type of stimulus that has specialised receptors which transmit information through specific anatomical pathways to the brain
What are the different sensory modalities and what receptors detect them
Mechanoreceptors
- Touch
- Pressure
- Vibration
- Proprioception (joint position, muscle length, muscle tension)
Thermoreceptor
- Temperature
Nociceptor
- NociceptionWhat are the three types of sensory fibres
AB - thickest, myelinated, fastest transmission - mechanoreceptors of the skin
Ad - thick, myelinated, fast transmission - pain temperature
C - thin, slow, unmyelinated - temperature, pain and itch
A peripheral nerve will have all of these different fibres in it
What are the different types of nerve endings
C fibres have free nerve endings which are very close to the skin and are responsive to heat
Mechanoreceptor ending are encapuslated
What is the absolute threshold
The level of stimulus that produces a positive result 50% of the time
This will create a generator potential which will depolarise the nerve
The stronger the stimulus the more neurotransmitter is released
Explain the functionality of thermoreceptors
- Free nerve ending with high thermal sensitivity
- Change in temperature activates a family of transient receptor potential ion channels
- There are 4 heat activated TRP channels (TRPV1-4) ranging from low heat to high heat
- There are 2 cold activated TRP channels (TRPM8, TRPA1)
What are the different types of mechanoreceptors and what do they sense for
- Meissner’s corpuscle - fine discriminative touch
- Merkel cells - light touch and superficial pressure
- Pacinian corpuscle - detects deep pressure, vibration and tickling
- Ruffini endings - continuous pressure or touch and stretch
What are the two types of adaptation receptors
Tonic receptors
- Detect continous signal strength
- Continue to transmit impulses as long as the stimulus is present
- keeps the brain informed of the status of the body
- e.g. merkel cells - slowly adapt allowing for fine touch to be perceived
Phasic receptors
- Detect changes in stimulus strength
- Transmit an impulse at the start and at the end (when a chane is taking place)
- Also called movement receptors or rate receptors
- e.g. Pacinian corpuscle - sudden pressure excites receptor, transmits a signal again when pressure is released
What are somatosensory dermatomes
- Each spinal nerve has a specific dermatome on the skin
- Each spinal nerve innervates a certain level in the spinal cord
What are receptive fields
- The receptive field is the region of skin which causes activation of a single sensory neuron when activated
- There are different size receptive fields on the body e.g. smaller on the fingers to allow fine touch but much larger on the back
Define two point discrimination
The minimum distance at which two points are perceived as separate (relative to the size of the receptive field)
What are the different fibre types used for in nociception
Ad fibres mediate sharp, intense or first pain
- Myelinated
- Type 1 : ad-mechanoheat receptors (noxious mechanical and thermal stimuli)
- Type 2 : mechanoreceptors (noxious mechanical stimuli)
C fibres mediate dull, persistent or second pain
- Unmyelinated
- Respond to thermal, mechanical and chemical stimuli (polymodal)
Where are sensory cell bodies found within the body and face
In the body they are in the dorsal root ganglia
In the face they are in the trigeminal ganglia
How are the sensory pathways organised within the dorsal horn
Organised into layers - the rexed laminae (1-7)
What terminates into each of the lamina
Pain and temperature (Ad and C fibres) terminate in laminae 1-2 (superficial)
Innocuous mechanical stimuli (AB fibres (and Aa)) termiante in laminae 3-6 (deep)
What connects between the different laminae and adjacent peripheral inputs
Inter neurons
What is lateral inhibition and why does it occur
- Receptive fields can overlap and make it difficult to establish between two stimuli locations
- Lateral inhibition prevents overlap and facilitates pinpoint localisation of the stimulus
- It is mediated by interneurons within the dorsal horn of the spinal cord
What are the central sensory structures and where are they found
Primary somatosensory cortex
- In the postcentral gyrus
Secondary somatosensory cortex
- In the parietal operculum
Posterior parietal complex
- Spatial awareness of the bodyWhat is the function of the dorsal column system
- Transmits innocuous mechanical stimuli (fine discriminative touch, vibration)
- AB fibres enter via the dorsal horn and enter the ascending dorsal column pathways
- Information from the lower limbs and body (below T6) travel ipsilaterally along the gracile tract
- Information conveyed from the upper limbs and body (above T6) travel ipsilaterally along the cuneate tract
Where do first order neurons of the dorsal column system terminate
First order neruons terminate in the medulla
- Fibres in the gracile tract have their first synapse in the Gracile nucleus
- Fibres in the Cuneate tract have their first synapse in the Cuneate nucleus
Where do second order neurons of the dorsal column system cross and terminate
Second order neurons cross in the medulla
- Second order axons decussate in the caudal medulla
- Forms the contralateral medial lensicus tract
Second order neurons terminate in the thalamus
- Axons of second order neurons terminate in the ventral posterior lateral nucleus of the thalamus
- There is a topographical representation of the body in this nucleus as the limbs terminate more laterally
Where do third order neurons of the dorsal column system terminate
Third order neurons terminate in the somatosensory cortex
- Third order neurons from the VPL project into the somatosensory cortex
- Size of the somatotopic areas is proprtional to the density of sensory receptors in that body region
What is the function of the spinothalamic (anterolateral) pathway
- Pain and temperature sensations ascend within the lateral spinothalamic tract
- Crude touch ascends within the anterior spinothalamic tract
Where do first order neurons of the spinothalamic pathway terminate
First order neurons of the spinothalamic pathway terminate in the dorsal horn
- Primary afferent axons terminate upon entering the spinal cord
- Second order neurons decussate immediately in the spinal cord and form the spinothalamic tract
