Somatosensory system Flashcards
Describe meissner corpsucles
- In the dermal papillae of the skin
- Respond to light touch and temperature
- Vibrations of 30-50Hz
Describe hair follicles
- Located in the epithelia
- Respond to many types of modalities which move the hair follicles
What are primary afferent neurons classified by?
Conduction velocity which reflects diameter
What do the 3rd order neurons in the somatosensory system do?
Reverse topology so lower body neurons synapse on the more medial cortex
What is the occupied size of each body part in the cortex proportional to to?
- Importance
- Density of input (number of neurons)
- More of cortex dedicated to areas with small receptive fields
What sensory receptors have both rapidly and slowly adapting subtypes?
Hair follicles
Describe tactile discs
- Located in the epidermis
- Respond to light touch, texture and temperature
What are the dorsal column nuclei?
- In the brainstem
- Cuneate and gracile nucleus
Describe pacinian corpsucles
- In the dermis, joint capsules, viscera
- Respond to deep pressure and tickle
- Vibrations of 250-350Hz
What is special about the cortical map?
- It has plasticity
- If you stimulate an area more it will take over more of the cortical map in expense to neighboring areas
- Vice versa
Describe ruffini corpsucles
- Located in the dermis, joint capsules and subcutaneous tissue
- Respond to heavy touch, skin stretch and joint movement
What are sensory modalities?
- What we perceive after a stimulus
Eg. hot, cold, rough, smooth, pain, pressure, tickle
As well as the brain, where else in the body is the body mapped?
- Thalamus
- Dorsal column nucleus
How are the somatosensory projections organised?
- Topographically, spatial relations are maintained throughout space
- This reflects the topographic projection in the brain, producing a map of the body in the cortex
What neurons are classed as ‘lower body’?
Below T6
What pathway do the lower body 1st order neurons follow and where do they synapse?
The medial pathway and synapse in the gracile nucleus
What are 3 slowly adapting sensory receptors?
1) Ruffini corpsucles
2) Tactile discs
3) Free nerve endings
What is the spinothalamic tract?
Carries pain and temperature signals to the thalamus
Where are different sensory modalities localised?
Along the sagital axis of the cortex
Where are the areas of the body mapped in the cortex?
In the primary somatosensory cortex (post central gyrus)
What are the 2 rapidly adapting sensory receptors?
1) Meissner corpsucle (tactile)
2) Pacinian corpsucle (lamellated)
What is the relationship between dermatomes, dorsal root ganglions and spinal segments?
Why?
- One dermatomes is innovated by one dorsal root ganglion
- One to one correspondance with spinal segments
- Dermatomes deriving from somites
- In the embryo one somite per dorsal root ganglion
What do the 2nd order neurons do?
- Relay signals from the spinal cord to the thalamus
- Cross the midline (commissural)
What pathway do the upper body 1st order neurons follow and where do they synapse?
The lateral pathway and synapse in the cuneate nucleus
What is the medial lemniscal tract?
Carries mechanoreceptive and proprioceptive signals to the thalamus
What does the size of a receptive field depend upon?
Positon in the body
What is the somatosensory system?
The part of the peripheral nervous system which innovates the skin, joints and skeletal muscle
Describe free nerve endings
- Widespread, located in epithelia and connective tissue
- Respond to pain and temperature
What are primary afferent neurons from the skin designated by?
Letters
A = fastest/fattest
(broken down into greek letters)
C = slowest
What do the 3rd order neurons do?
Carry signals from the thalamus to the cortex
What do the 2nd order neurons in the somatosensory system do?
- Cross the midline and and ascend in the medial lemniscus pathway
- Topology reversed
What are primary afferent neurons from the muscle designated by?
Roman numerals
I = Largest
IV = Slowest
How is sensory information organised in the body?
- Cell bodies of sensory neurons are grouped in the dorsal root ganglion
- Projections are organised into layers in the dorsal horn
- Each layer is a specific receptor and a specific sensory modality
- Spatially organised and remain this way as it is carried into the brain
What do the 1st order neurons do?
Detect stimuli and transmit it to the spinal cord
What are the two proprioceptors?
- Muscle spindles
- Golgi tendon organs
