lecture 8 Flashcards
How is EM radiation sensed by the body?
Receptor: rod and cone photoreceptors
Range: 400 to 600nm wavelength
Sensitivity and dynamic range: single photo to bright sunlight (10^10 fold)
Receptive field: single photoreceptor, concentric ganglion cell
How is distortion of the skin sensed by the body?
Receptor: various encapsulated nerve endings
Range: 10nm to sub-damaging distortion
Sensitivity and dynamic range: mg, 0-1000 Hx
Receptive field: ovaloid from 10mm^2 to entire hand
What does the somatosensory system do?
- mediates sensations from the whole body surface, including skin and deeper tissues
What is the structure of skin?
- most of the body is covered by hairy skin
- the palmar surface of the hands and the soles of the feet are covered by glabrous skin, with skin ridges a prominent feature
Give an overview of the somatosensory system.
- There are four types of mechanoreceptors in glabrous skin
- Meissner corpuscles and Merkel complexes are close to the surface
- Ruffini organs and Pacinian corpuscles are deeper in the skin
- These receptors are innervated by large myelinated axons with cell bodies in the dorsal root ganglia
- Transmission of this information to the brain generates our conscious experience of touch
What are the layers of the skin?
- top dead layer
- epidermis
- dermis
- subcutaneous layer
Where are free nerve endings located?
epidermis
Where are Meissner corpuscles located?
right below the epidermis in the upper part of the dermis
Where are Merkel cell-neutrite complexes located?
In the deep grooves of the epidermis
Where are Ruffini corpuscles located?
dermis
Where are Pacinian corpuscles located?
Dermis/subcutaneous layer
What do Ruffini corpuscles respond to?
Skin being moved or stretched
What do Pacinian corpuscles respond to?
Vibration
How do these mechanoreceptors open sodium channels?
Thought to be literally by force - movement/pressure/whatever that receptor responds to opens the gate and allows the movement of sodium ions across the membrane leading to depolarisation
- however there is greater molecular complexity to it - resistance from ECM and inside of cell
What is the key event for generating an action potential?
depolarisation
What is a crucial difference between the mechanoreceptors?
- how they respond to an ongoing stimulus
How can we talk about stimuli in general?
We can talk about stimuli as causing a transient change or a sustained change. Mechanoreceptors in turn can adapt slowly to a stimulus, or they can adapt rapidly.
Do cells tend adapt rapidly or slowly?
Rapidly - prefer to detect changes. When things stay the same it doesn’t really tell us anything worth knowing e.g. clothes throughout the day
What are the slowly adapting mechanoreceptors?
Merkel:
- complexes are found at the troughs of the epidermal ridges where they respond to indentation
Ruffini:
- endings are found in the upper dermis, have a sustained response to skin movement
What are the rapidly adapting mechanoreceptors?
Meissner:
- receptors are found near the skin surface and have a transient response to skin movement.
Pacinian:
- receptors are located deep in the dermis and hypodermis and have a transient response to vibration
What is the distribution/, receptor field of the different mechanoreceptors in the palm of the hand?
- Merkel: 25% of total, smallest RF
- Meissner: 40% of total, still a small receptor field
- Ruffini: 20%, “proprioceptive”, larger receptor fields
- Pacinian: 15% , most sensitive, vibrations, huge receptor fields
Where else are mechanoreceptors located?
- in the muscle: spindles
- in the tendons: golgi tendon organ
- they are activated by physical deformation forces
- they signal information about muscle stretch or position and tendon force
- not considered part of somatosensation
What are receptive fields?
- it’s really about what the system can do
- resolution
- how sensitive/the smallest difference it can detect
- depending on how centrally in a particular receptive field something is acting, the associated receptor will respond strongly or weakly
- the closest together to points of contact can be while still detecting that it is two points of contact is a way you can define resolution.
- e.g. fingertips = a few mm, thigh = a few cm
What do all four somatosensory mechanoreceptors connect to?
- afferent axon type = A-Beta fibres
- 6 - 12 µm diameter
- 33 - 75 m/s
- large and myelinated
What has the biggest, fastest conducting fibres in the body?
- muscle spindles
Are the conducting fibres connecting to pain receptors fast?
- not particularly: slowest and thinnest of the somatic sensory afferents that link receptors to the central nervous system
- counterintuitive
- however sustains well beyond the stimulus - most slow to adapt
What are dermatomes?
- a dermatome is a region of skin that is supplied by the sensory nerves of a single spinal segment
- this means that location of sensory loss can be indicative of which part of the CNS is damaged - especially if it is evidently a dermatomal region
- relates to the form the embryo was in when the dermatomes/connections were made
What nerve supplies the skin of the face?
- trigeminal nerve
Where is the cell body for the mechanosensory afferent fibres/mechanoreceptor endings?
- dorsal root ganglion
How does the axon travel?
From receptor endings, all the way through the body, past its cell body (offshoot), and then into the spinal cord.
From here it does two things: goes to the brain, make local synaptic connections in the spinal cord too.
The pain and temperature afferent fibre obligatorily makes synaptic connections in the spinal cord - second cell axon crosses over and then travels up to the brain on the other side
important to note that the information for touch and the information for pain travel in completely different paths when they get to the CNS
What happens when we get to the brain?
Gets the medulla - first bit of the brain stem reached - it meets a nucleus called the dorsal column nucleus.
Second neuron in the chain sits here - this neuron decussates (crosses over to the other side)
Project all the way up through the brainstem to the thalamus
In the ventral, posterior part of the thalamus they meet the third neuron in the pathway.
Information goes from here to the cortex e.g. primary somatic sensory cortex
Where is the primary somatic sensory cortex located?
Behind the central sulcus, in the post central gyrus.
How is the primary somatic sensory cortex divided?
- into four areas based on subtle differences in the cellular architecture
- 1, 2, 3b, 3a
How does the information get mapped out in the primary somatic sensory cortex?
- somatopic map
- areas of the skin that are near each other activate neurons that are near each other in that strip of cortex
- so what you get is a sort of map of the body from the medial to lateral part of the cortex
- not a continuous map
- hands have a large amount of the cerebral cortex
- lips and tongue large
- humunculus - reflection of scaling of somatosensory importance - not really one - 4 or 8, 16
- most of the input goes to area 3b - also interpret texture (1), size/shape (2)
- not isolated bits of cortex, they exchange information
Where does the information go?
- a lot to secondary somatosensory cortex and then to amygdala and hippocampus (important for salience, what things mean)
- parietal areas 5, 7 (used for motor planning, how to use to touch to guide movements/actions)
- interplay of motor and sensory information
- separate at tease information out at the level of the primary somatic sensory cortex –> slowly adapting and fast adapting seem to have different areas within their particular regions
Is this regional mapping fixed?
No - very plastic. E.g. if you amputate a finger, the cerebral space that was devoted to that finger starts respond to the neighbouring fingers or if you increase stimulation of a particular area over time more of the cerebral cortex will be devoted to these areas
