Lecture 20 Flashcards
What are the two major input components of the somatic sensory system?
Mechanical stimuli (light touch, vibration, pressure and cutaneous tension) Painful stimuli and temperature
What does input and interpretation of the two major components enable us to do?
Identify the shape and textures of objects
Monitor the internal and external forces acting on the body
Detect potentially harmful circumstances
Have a sense of ourselves within our environment and so plan our actions accordingly
Where are hair follicles located?
Widespread in epithelia
What is the modality of hair follicles?
Varied according to type
- Both rapid and slowly adapting subtypes
Where are Meissner’s corpuscles (Tactile) located?
Dermal papillae of skin, especially palms, eyelids, lips and tongue etc
What is the modality of Meissner’s corpuscles?
Light touch, texture (movement)
- Sensitive to 30-50 Hz
- Rapidly adapting
Where are Pacinian corpuscles (Lamellated) located?
Dermis, joint capsules, viscera
What is the modality of Pacinian corpuscles?
Deep pressure, stretch, tickle, vibration
- Sensitive to 250-350 Hz
- Rapidly adapting
Where are Ruffini corpuscles located?
Dermis, subcutaneous tissue, joint capsules
What is the modality of Ruffini corpuscles?
Heavy touch, pressure, skin stretch, joint movements (kind of proprioceptor…?
- slowly adapting
Where are Merkel (or Tactile) Discs located?
Superficial skin (epidermis)
What is the modality of Merkel discs?
Light touch, texture, edges, shapes
- slowly adapting
What are examples of encapsulated nerve endings?
Pacinian corpuscles, Ruffini Corpuscles
What are examples of unencapsulated nerve endings?
Merkel discs
Where are free nerve endings located?
Widespread in epithelia and connective tissues
What are modalities for free nerve endings?
Pain, heat , cold
What is the difference between cellular receptors and the molecular receptors?
Cellular receptor is “device” made of cells that detects changes in the body or the environment
Molecular receptor is a molecule, usually located on the surface of a cell with a transmembrane linkage to the cytoplasm, that detects changes in the molecular environment (e.g. growth factors, hormones etc)
What are the different classes of mechanoreceptor response?
Rapidly adapting (phasic receptors) - Give information about changes in the stimulus Slowly adapting (tonic receptors) - Continue to respond as long as stimulus is present (gives information about persistence of stimulus)
How are primary afferent axons classified?
According to conduction velocity which broadly reflects diameter (faster = larger diameter)
What is the difference between the labeling of axons coming from the skin and from the muscles?
Skin: Letters (A,B,C; A = largest/fastest, C = slowest/smallest)
Muscles: Roman numerals (I, II, III and IV; largest to smallest)
Are pain fibers fast?
No
How is the sensory information is organized?
Into layers in the spinal cord dorsal horn
- The cell bodies of sensory neurons are grouped in the DRG and their projections are organized to different layers of the dorsal horn:
- e.g. information from different classes of hair follicle is represented in different layers
- Applies to different sensory modalities e.g. pain vs touch
- Sensory information remains spatially organized as it is carried into the brain by different pathways…
What are the two somatosensory projection pathways?
1, The medial lemniscal tracts
- carry mechanoreceptive and proprioceptive signals to the thalamus
2. The Spinothalamic tract - carries pain and temperature signals to the thalamus
What are the three neurons which reach higher centers?
- First-order neurons
- detect the stimulus and treatment to spinal cord - Second-order neurons
- relay the signal to the thalamus, the “gateway” to the cortex - Third-order neurons
- carry the signal from the thalamus to the cortex
(second order neurons cross the midline: commisural)
How are the axons of the medial lemniscal pathway organized?
Topographically
- First-order axons from the upper body follow the lateral pathway and synapse on second-order neurons in the cuneate nucleus
- First-order axons from the lower body (below vertebra T6) follow the more medial pathway and synapse on neurons in the gracile nucleus
- Together these are known as the dorsal column nuclei
After the second order axons cross the midline in the medial lemniscal pathway, where do they ascend?
Medial lemniscus
- then topology is reversed, relative to the midline, so that lower body axons are more lateral on reaching the thalamus
Where do third-order axons synapse?
Lower body axons synapse on more medial cortical neurons
What does the topographic projection imply?
Map of the body in the cortex
- This indicates that the map is very fine, not simply lower vs upper body
- This reflects the fact that each DPG innervates a specific domain of the body called a dermatome
What does each sensory ganglion innervate?
Specific region of skin called a dermatome
Why do dermatomes arise?
Because the dermis of each region is derived from a specific embryonic structure called the somite
What are somites?
Iterated stuctures that give rise to the underlying musculature and skeleton
In the embryo, what is each sensory ganglion (DRG) associated with?
A specific somite and subseqquently innervates the tissues arising from that somite
What is notable about the somatosensory homunculus?
Representations not proportional
- number of different receptors (more specifically axons) in each different regions on the body
What are receptive fields?
Each sensory neurons has a receptive field
The size of the receptive field for any particular neurons will vary depending on where it is in the body
The size of a receptive field can be measured by assessing the ability to discriminate two sharp points set apart at different distances
When does two point discrimination occur?
If the subject feels two pin points then the distance between the points is larger than the receptive field
Where receptive fields are large, discrimination is…?
Low (legs and arms)
Where receptive fields are small, discrimination is…?
High (fingers)
More cortex is dedicated to areas where receptive fields are …?
small
Why is the amount of derived information is also higher?
The number of receptors (nerve endings) may be similar, the number of endings from different neurons per unit area is higher
The number of sensory neurons innervating a particular area is related to…
the behavioral significance of that area
Cortical representation is determined by behavioral significance. This implies…
While the representation of the hand is large in human cortex, in rodents it is the fields corresponding to whiskers that dominate
How is sensory modality also represented in the cortex?
While the somatotopic map is preserved in the coronal plane throughout the postcentral gyrus, different sensory modalities are localized along the sagittal axis
- 3b: texture and shape discrimination
- 2: finger coordination, shape and size discrimination
- 3a: proprioception sensory input from muscles etc. - sense of body position
What is cortical map plasticity?
Amount of cortex dedicated to specific area between species
e. g. monkeys
- digits were removed; other parts of the body take over the are
e. g. Taxi drivers
- area of cortex dedicated to mapping of streets are increased
