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)
