What can you tell from the size of a receptive field?
-tells you how the info starts to get analysed, tells you about the resoultion of that system
What are the four types of receptors in the skin?
-Various encapsulated nerve endings -Merkel, Meissner, Pacinian, Ruffini
What does the somatosensory system do?
-mediates sensations from the whole body surface, including skin and deeper tissues
What is the overall structure of the skin?
-most of the body covered by hairy skin. -Palmar surface of the hands and the soles of the feet are covered by glabrous skin, with skin ridges a prominent feature
What type of receptors are the receptors in the skin?
-mechanoreceptors= respond to mechanical stress/stimuli
What 2 mechanoreceptors are close to the surface?
Meissner corpuscles and Merkel complex
What 2 mechanoreceptors are deeper in the skin?
Ruffini organs and Pacinian corpuscles
What are the receptors innervated by and where are their cell bodies?
-innervated by large myelinated axons with cell bodies in dorsal root ganglia
What is hepatic manipulation?
-can work out what an object is just by touch
What is special about the Merkel complex?
-the Merkel cells are not neural
-it is a complex and the nerve endings end around these Merkel cells and transmit information to the CNS
What does the Meissner corpuscle look like and why?
-like a corn cob sort of
-the shape of the end= connective tissue, important as it determines how the nerve will be stimulated depending on its shape
What does the Pacinian corpuscle look like?
-like a marine worm
-flat= because it reacts to vibrations (again the shape confers its properties)
What does the Ruffini organ look like?
-like a Neptune's necklace (sea geep)
-long= thus reacts to stretch of the skin
In which layer are the Merkel and Meissner?
In which layer is the Ruffini organ?
-dermis (below epidermis)
In which layer is the Pacinian corpuscle?
-lower bit of dermis and subcutaneous layer (below dermis) hypodermis
How does transduction work in mechanoreceptors (afferent fibre)?
-afferent fibre is encapsulated by Schwann cells
-then the receptor at the end
-in the membrane of the receptor there are mechanically gated channels (when you touch sth or move sth across= the membrane stretches) it opens a Sodium channel and that results in DEPOLARISATION of the cell
-if the stimuli is strong enough, reach the threshold and fire an action potential (all or nothing system)
-all the skin receptors do it but respond differently to ongoing stimuli
What are the two types of receptors in terms of how they adapt to ongoing stimuli and which one is more common?
1. Slowly adapting 2.Rapidly adapting=by far the most common in the nervous system -most mechanoreceptors are rapidly adapting -burst in activity when the stimuli begins, we are interested in change
Which are the slowly adapting mechanoreceptors and what do they respond to?
1. Merkel complexes=found at the bottom of the epidermal ridges where they respond to indentation 2.Ruffini endings=found in the upper dermis, have sustained response to skin movement
Which are the rapidly adapting mechanoreceptors and what do they respond to?
1.Meissner receptors= found near the skin surface and have a transient response to skin movement 2. Pacinian receptors= located deep in the dermis and hypodermis and have a transient response to vibration
How much of the total do the Merkel complexes make up and how big are their receptive fields?
25% of total -smallest receptive fields -capable of fine discrimination
How much of the total do the Meissner receptors make up and how big are their receptive fields?
40% of the total -small receptive fields -more sensitive
How much of the total do the Pacinian receptors make up and how big are their receptive fields?
-huge receptive fields
Do muscles have mechanoreceptors?
-yes -all muscles have muscle spindles= mechanoreceptors -golgi tendon organ= also mechanoreceptors - used to tell stretch of muscle etc.
How sensitive a field is depends on?
-the size of the receptive field
--green = looks like one point of contact -
red= also can't tell if two points
-blue= can tell it is two
-this is the idea of resolution
Are there big differences in the sensory discrimination in various parts of the body?
-depends on the entire system, peripheral and central mechanisms
-differs a lot
-on your back can't tell if two point even 4cm apart
What fibres are the ones linking the mechanoreceptors of touch with the CNS?
-A beta fibres -second fastest transmission(after muscles) (35-75m/s) - axon diameter= 6-12micrometers
What fibres are the ones linking the mechanoreceptors of pain with the CNS?
-thinner, completely different fibres -the slowest fibres (together with the temp ones) -pain= sustained effect, can still feel it after you remove your hand from the fire or so, so you can learn!
What is a dermatome?
-region of skin that is supplied by nerves of one spinal segment
-(31 spinal segments)
Why are the dermatomes a funny shape?
-because of the shape the embryo has
How is the AP transported from the touch mechanoreceptor to the CNS?
-starts at the receptor ending, then goes down the mechanosensory afferent fiber and then to the dorsal ganglion cells (that is where the cell body is)
-then the axon goes further into the spinal cord and has some local connections there but goes up into the brain into the medulla (dorsal column nucleus)and that is where it synapses with the second neuron
How does the pain circuit work?
-very differently to the mechanosensory circuit,
-goes from the receptor to the dorsal root ganglion cells and then to spinal cord, there it crosses over and goes to the ventral side and up to the brain
What is the main mechanosensory pathway?
1. Mechanosensory receptors from the body to the spinal cord (lumbar= if lower body, cervical if upper)
2. goes up to the Gracile nucleus(lower body)/Cuneate nucleus(upper)
3. There the first neuron synapses with the second and crosses over
4. Second neuron now goes through the brain to the thalamus (the ventral posterior lateral nucleus)
5.Third neuron is from thalamus to the primary somatic sensory cortex
Where is the primary somatic sensory portion of the thalamus and the cortical target?
-target= postcentral gyrus (cenetral gyrus= functional barrier of the cortex)
How many areas is the primary somatosensory cortex divided into?
-4 Area 1, 2, 3a,3b
What does somatotopic mean?
-the map of what is for what -areas of the skin that are next to eah other are innervated ny neurons next to each other= in the somatosensory cortex -almost a continous map
What is the somatotopic order in the human somatic sensory cortex?
-how much is devoted to one depends on how sensitive
-so hands a lot!
What is this?
-scaling of importance
-how much of the cortex devoted to each part
What are the relationships between the different areas of the primary somatic sensory cortex?
each of those regions has to have its own map
- 3a= size and shape
-most input to 3b -
some info interchange
What happens with the information from individual receptors in the primary somatic sensory cortex?
-still separated even here the ruffini will be elswhere than pacini
What happens to the cortical map of a digit if it is amputated?
-the space devoted to it will be devoted to the others
What is the story with the lactating rats?
rats before and after they have babies -how much space devoted to the feeling the nipples - when lactating then much more space devoted to it -plastic, the mapping
Can using one digit a lot, influence the mapping for it?
-if you stimulate a finger more you can expand the space devoted to it
How much of the total do the Ruffini receptors make up and how big are their receptive fields?
20% -stretch sensitive -larger