Somatosensory System Flashcards
State the three components of the somatosensory system
- exteroceptive
- proprioceptive
- enteroceptive
Describe the exeroceptive component
Cutaneous senses registers information from the surface of the body
Describe the proprioceptive component
Monitors posture and movement (muscles, tendons etc)
Describe the enteroceptive component
Reports upon the internal state of the body and closely related autonomic function
How many neurones are usually in the sensory pathway?
Three
What does a stimulus do?
Opens cation selective channels in peripheral terminal of primary sensory afferent leading to a depolarising receptor potential
State five properties of a sensory neuron
Modality Threshold Adaptation rate Conduction Velocity Site/extent of peripheral termination
For touch/pressure/vibration what is the stimulus and unit?
Mechanical forces act on skin mechanorecptors
For proprioception what is the stimulus and unit?
Mechanical forces act on muscle and joint mechanoreceptors
For heat what is the stimulus and unit?
Heat acts on thermoreceptors
For pain what is the stimulus and unit?
Strong force/heat act on mechanical, thermal and polymodal nociceptors
For itch what is the stimulus and unit?
Irritant acts on itch receptors
What does threshold relate to?
The intensity of stimulus required to excite a sensory unit
Describe low threshold
Responds to low intensity (non-damaging stimuli)
Give two examples of low threshold receptors
LT mechanoreceptors - fine discrimatory touch
LT thermoreceptors - mediate cold to hot
Describe high threshold
respond to high intensity (damaging stimuli)
What are polymodal nociceptors?
At least 2 modalities
Give examples of high threshold receptors
HT mechanoreceptors - mechanical stimuli
HT thermal nociceptors - >45C and <10-15C
HT Chemical nociceptors - substances in tissue
What is adaptation?
Determines whether they change their firing rate in response to a stimulus of changing intensity or continuously throughout
Name three types of adaptation
- slowing adapting/tonic
- fast adapting/dynamic
- very fast adapting/dynamic
Describe slowly adapting/tonic
Continuous information to CNS, provides information about position, degree of stretch, or force e.g stretch receptors
Describe fast adapting/dynamic
Detects changes in stimulus strength, number of impulses proportional to rate of change of stimulus e.g hair follicles
Describe very fast adapting
Only responds to very fast movement such as rapid vibration e.g pacinian corpuscle
How many groups of different neurons are there?
4
What receptors have group one fibres?
Prioprioceptors of skeletal muscle - thick myelination and quick velocity
Which receptors have group two fibres?
Mechanoreceptors of skin - moderate myelination and moderate velocity
Which receptors have group three fibres?
Pain and temperature - think myelination and slower velocity
State the type of receptor that has group four fibres
Temperature, pain, itch - no myelination and very slow velocity
What can the peripheral branch tips be?
Free nerve endings
Associated with specialised structures
Define receptive filed
Target territory from which a sensory unit can be excited
What does a small RF mean?
High acuity and high density - also has high discrimination
What does large RF mean?
Low acuity and low density - low discrimination
Describe the two point discrimination
Simultaneously apply two stimuli at different sites on the body surface - subject reports on whether one point or two are sensed
Name the sensory receptors in skin
- free nerve endings
- meissner’s corpuscles
- merkels discs
- hair end organs
- Krause end bulbs (dry skin - mucous membrane)
- Ruffini endings (dermis and joints)
- Pacinian corpuscles (dermis and fascia_
How can low threshold mechanoreceptors be classified?
Rates of adaptation - Fast and slow
Size of receptive field - small (1), wide (2)
Histologically what do low threshold mechanoreceptors comprise of?
Receptor and parent fibre
What does threshold for detecting vibration correspond to?
The tuning threshold of mechanoreceptor
What is the grey matter of the spinal cord divided into?
Ten distinct laminae of rexed (pain - touch-proprioception from posterior to anterior)
Name the two key fasciculus of the dorsal column
- gracilis
- cuneatus
What order in the dorsal column do the map of sensory inputs appear?
Lateral to medial
Cervical - thoracic - lumbar - sacral
What is the function of the dorsal column tract?
Conveys proprioceptive information to the brain
Describe the pathway of the dorsal column
- Dorsal horn
- Fasciulus (gracilis T6 and below, cuneatus above T6)
- Nucleus in medulla where they cross over and synapse
- Ventral posterior lateral nucleus of thalamus
- Posterior internal capsule
- Primary somatosensory cortex
What are the functions of the dorsal column tract?
- stereognosis (ability to recognise an object by feeling it)
- vibration detection
- fine touch
- conscious proprioception
- weight discrimination
What is contrast enhancement?
As information is conveyed from one neuron to the next in a sensory pathway differences in the activity of adjacent neurons are amplified
What is lateral inhibition?
When one neuron is active it inhibits the activity of its neighbours via inhibitory interneurons
What is the purpose of lateral inhibition?
Sharpens the stimulus
How many trigeminothalamic tracts are there?
Two
Where are the soma of trigeminal sensory neurons?
Trigeminal ganglion
Name the trigeminal sensory nuclei
- mesencephalic
- pontine trigeminal
- spinal nucleus
What happens to trigeminal nerves after the reach the nucleus?
Dessucate and project via the trigeminal lemniscus to the ventroposteriomedial nucleus of the thalamus and then via the thalamocortical neurons to the cortex
Where is the somatosensory cortex?
Post central gyrus of the parietal cortex
What is the name given to the different areas of the somatosensory cortex?
Brodmann Areas
State the four brodmanns areas
1 - cutaneous (texture discrimination)
2 - joint afferents, deep tissues (pressure/joint position)
3a - proprioceptors (body position)
3b - cutaneous, FA1 (touch)
What two things does the homunculus portray?
Area innervated
Density of sensory input
How many layers make up the somatosensory cortex?
Six cell layers
Where do the thalamic inputs terminate?
Layer IV and then project to cells towards the surface and deeper layers
What happens to the sensory area of an amputated finer?
It will respond to stimulation of the adjacent digits
Can sensory input increase?
Yes, the size of the cortex can increase e.g violin players
What happens at the posterior parietal cortex?
Receives and integrates information from inputs and other cortical areas - visual, auditory and the thalamus
What can happen if you damage your posterior parietal cortex?
Bizzare neurological disorders with simple sensory skills in tact
