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 touch and proprioception.
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: fire impulses for as long as stimulus is present
Fast: fire at start of stimulus + sometimes when stimulus switches off, but tend to fade in the middle
What type of receptors are mechanoreceptors?
Mixture of slow + fast adapting receptors
Describe how sensory neurons vary in their properties.
SIZE
CONDUCTION VELOCITY
What are the two classifications of axons?
Anatomical = based on axon diameter (LETTERS)
Physiological = based on conduction velocity (ROMAN NUMERALS)
Diameter + conduction velocity are related, there is a lot of overlap in the classifications
What is a receptive field?
A region on the skin which causes activation of a single sensory neurone when activated
What is the difference between small + large receptive fields?
Small: allow for detection of fine detail over a small area= precise perception
Large: allow cell to detect changes over a wider area= less precise perception
Describe how neurons can code for the intensity of a stimulus.
By FREQUENCY of the APs 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 have come from above and below T6 called? Describe their spatial arrangement within the spinal cord.
Above: Cuneate Fasciculus (lateral in cord)
Below: Gracile Fasciculus (medial in cord)
Travel ipsilaterally, desiccate in medulla
Where do the fibres in the Gracile + Cuneate tracts have their 1st synapse?
In the medulla
Cuneate: Cuneate nucleus
Gracile: Gracile Nucleus
Describe what happens after the gracile + cuneate neurones synapse and the tract that they run in.
2nd order neurons decussate in the caudal medulla + continue up the brainstem in the contralateral MEDIAL LEMNISCUS tract
Which thalamic nucleus is responsible for relaying somatosensory information from the neck down?
Ventral Posterior-lateral nucleus (VPL)
Describe the passage of the 3rd order sensory neuron.
Project from VPL 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 (from the trigeminal nerve).
The 2nd order neuron decussates+ 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 branches of primary axons stimulate interneurones that are inhibitory on neighbouring axons
Prevents the overlap of receptive fields, facilitating pinpoint accuracy in localisation of stimulus
Mediated by interneurones in dorsal horn
What is the purpose of lateral inhibition?
Facilitates enhanced sensory perception (discrimination)
Name the 3 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 the 4 sensory modalities
Touch (light mechanical stimuli)
Thermosensation (temperature)
Nociception (noxious + potentially damaging stimuli)
Propriception (mechanical displacement of muscles + joints)
Describe the nerve endings in nociceptors, thermoreceptors + mechanoreceptors
Thermo- + Nociceptors = Free nerve endings
Mechanoreceptors = Enclosed nerve endings
Describe 3 types of sensory neurone (diameter, conduction speed, myelination, type of receptor)
AB: Largest diameter, fastest conduction, highly myelinated = innocuous mechanical stimuli
AD: Medium diameter, medium conduction + myelination = noxious mechanical + thermal stimulation
C fibres: Smallest diameter, slowest conduction, no myelination= noxious mechanical, thermal + chemical stimulation
What are sensory receptors?
Transducers that convert energy from the environment into neuronal action potentials
Which neurones transmit hot and cold stimulation?
Heat= C fibres Cold= AD fibres
What type of ion channels are involved in thermoreception?
Transient receptor potential (TRP)
List the 5 different types of mechanoreceptor + their function
Meissner corpuscles: Discriminative touch
Merkel discs: Light touch, superficial pressure
Hair follicle receptors: Light touch (hairy skin)
Ruffini endings: continuous touch, pressure
Pacinian corpuscles: Vibration
Define absolute threshold
Point of intensity at which the person can just detect the presence of a stimulus 50% of the time
What does increased stimulus strength + duration result in?
Increased NT release
More intense response to a stimulus
Describe tonic receptors and give an example
Detect continuous stimulus strength
Continue to transmit impulses to brain as long as stimulus is present
e.g. Merkel cells (slowly adapting for superficial pressure + fine touch to be perceived)
Describe phasic receptors and give an example
Detect a change in stimulus strength
Transmit an impulse at the start + end of the stimulus
e.g. when a change is taking place
E.g. Pacinian receptor (Sudden pressure excites receptor, transmits a signal again when pressure is released)
What is 2 point discrimination and what is it related to?
Minimum distance at which 2 points are perceived as separate
Related to size of receptive field
Where are cell bodies for the body and face?
Body: Dorsal root ganglia
Face: Trigeminal nerve
What are the 2 main types of dorsal horn neurone?
Projection neurones: axons project to the brain
Interneurones: axons remain in spinal cord
How do AB fibres differ to AD + C fibres in terms of where they relay info to?
AD + C fibres: relay to superficial layers (lamina 1+2)
AB fibres: relay to deeper layers (lamina 3,4,5)
What is size of somatotropin area proportional to?
Density of sensory receptors in that body region
