Lecture 10-12 - Somatosensory System Flashcards
What are muscle spindles?
Specialized sensory receptors found in skeletal muscle that provide feedback on muscle stretching to the CNS
What is another name for skeletal muscle fibres?
Extrafusal muscle fibres
How are intrafusal and extrafusal muscle fibres organized?
they run in parallel
What are intrafusal fibres?
non-force producing muscle fibres that lengthen and stretch with changes in muscle length
What are afferent axons?
innervate sensory endings of muscle spindle and send feedback to spinal cord
What are efferent axons?
gamma MNs innervate polar ends (contractile) of intrafusal fibers and send excitatory inputs from the spinal cord
What do sensory endings do in a muscle spindle do?
sense length of intrafusal muscle fibres
Does the middle (central) part of a muscle spindle contract?
No
What is the capsule of a muscle spindle?
connective tissue surrounding intrafusal fibers and receptors that compose the spindle
What are extrafusal fibres?
fibres that cause muscle contraction
What do gamma motor neuron endings do in a muscle spindle?
stimulate intrafusal muscle fibres
What afferents wrap around all types of intrafusal endings?
primary afferents ( group Ia)
What is a primary afferent and the intrafusal fibres it wraps around called?
muscle spindle primary ending
What afferents make ‘flower spray endings’?
Secondary (group II)
What is the secondary afferent and the intrafusal fibres it contacts called?
muscle spindle secondary ending
What are the two major categories of intrafusal muscle fibres?
Bag and chain
How are bag intrafusal fibres arranged?
nuclei arranged In a central ‘bag’ region
How are chain intrafusal fibres arranged?
nuclei arranged along a straight ‘chain’
How do mechanically gated ion channels in muscle spindles work?
Cytoskeletal strands bind ion channels together so as muscles stretches the strands becomes stiff and pull on the ion channels so they open allowing more Na+ to enter the cell. Larger stretches lead to larger receptor potentials
What does the firing rate of muscle spindles correspond to?
Changes in muscle length
What provides the CNS with a sensory input proportional to the length of the muscle?
Difference in the firing rate between the initial and final length of the muscle
What provides the CNS with a sensory input proportional to the velocity of muscle stretch?
Difference in firing rate of muscle spindles during the dynamic phase (when the muscle is being stretched) of muscle stretch
What do chain type intrafusal muscle fibres do?
Measure muscle length
What do bag type muscle fibres do?
Measure the dynamic phase of muscle stretch “velocity of stretch”
What nerve endings are more sensitive to the dynamic phase of movement (velocity of stretch)?
Primary endings
What does the firing rate of primary endings during the dynamic phase closely correspond to?
the velocity of muscle stretching
What do higher velocities result in?
Larger dynamic responses
What are secondary endings sensitive to?
Muscle length
What type of nerve endings are useful for proprioception?
secondary endings
what type of nerve endings are useful for kinesthesia?
primary endings
What would happen without Gamma MNs?
if the muscle were to contract to a shorter length, the intrafusal fibres would become slack and the spindle would stop providing feedback about where the limbs are
Why do we have gamma MNs?
To make sure that the length of the muscle spindle is the same as the length of the muscle and allows the muscle spindle to keep responding during contractions
What is alpha-gamma co-activation?
Alpha MNs and Gamma MNs are activated together, they receive the same input so muscles stay the same length. Thought to occur because the same motor commands coming from the cerebral cortex connect both the alpha and gamma MN in the spinal cord.
What do muscle spindles play an integral role in?
Proprioception and kinesthesia
What is prorioception?
Our bodies understanding of where it is in space
What is kinesthesia
our brains understanding of movement
What is mechanotransduction?
Mechanical energy gets converted into a voltage change in sensory afferent neurons
What is another name for EPSP?
receptor potential
What is a cutaneous receptive feild?
region of the skin that is capable of evoking action potentials in a given cutaneous afferent (all cells and part of skin that are innervated by one axon)
What is non hairy skin?
“glabrous” skin
How many types of receptors are located in glabrous skin?
4
Where are type 1 cutaneous receptors found?
Close to surface of skin (superficial)
What are the 2 type 1 cutaneous receptors?
Meissner and Merkel cell
Are merkel cells slow or fast adapting?
slow
What is the fast adapting type 1 cutaneous receptor?
Meissner’s corpuscle
What type of cutaneous receptors are Ruffinin endings and pacinian corpuscles?
Type 2s (deep)
What is the slowly adapting type 2 cutaneous receptor?
Ruffini endings
Are pacinian corpuscle fast or slow adapting?
fast adapting
What is the ranking of most (lowest threshold) –> least (highest threshold) sensitive to indentation?
- Pacinian
- Meisner
- Merkel
- Rufinni
Which cutaneous receptor has irregular discharge (uneven spacing between AP’s) when stimulated?
Merkel Cells (SA1)
Which cutaneous receptors are highly sensitive to edges and curvature?
Merkel cells
What are ruffini endings very sensitive to?
Skin stretch
Are ruffini endings found in non-human primates?
No
Which cutaneous receptor codes for velocity of skin indentation and motion across skin?
Meissner corpuscles
What is the main cutaneous receptor found in the hand?
Meissner corpuscles
Are meissner corpuscles sensitive to low or high frequency vibration?
Low
Do fast adapting cutaneous receptors constantly fire APs?
No, they only fire when there is a change in stimulus
What do Pacinian corpuscles code for?
acceleration (change in indentation rate)
What type of frequencies do pacinian receptors pick up?
high frequencies
Do deep (type 2) receptors have smaller or larger receptive fields?
Larger
Why do type 1 receptors have smaller receptive fields?
they have more than 1 cell attached to a axon
Where in the hand are Merkel and Meissner recepetors highly dense?
The tip of the finger
What type of cutaneous receptors are more evenly distributed in the hand?
Pacinian and ruffini
What are the functional roles of merkel cells?
Edges, curvature and texture
What are the functional roles of Meissner corpuscles?
slip/motion detection and grip control
What is cutaneous receptor is responsible for detecting skin stretch?
Ruffini endings
What cutaneous receptor is responsible for feeling vibration through objects and perception of fine texture?
Pacinian corpuscles
What is spatial code?
The CNS can determine the location of stimuli based on which afferents specifically are active
Which type or cutaneous receptors code for spatial details (size and shape of objects)?
SA1’s
What happens when you are probed with a small probe?
High discharge rate in few afferents
What happens when you are probed with larger probe?
Individual receptors fire less, but more afferents will be active
What is a nociceptor?
Pain receptor (is a chemoreceptor)
What do nociceptors signal for?
Tissue damage
If you were to put your hand on hot stove when would the nociceptors become active?
Once the heat becomes tissue damaging
How do thermally gated receptors work?
Heat deforms protein channels allowing it to open
T/F: Thermoreceptors are active at lower temperatures than pain receptors
true
Why are spicy food hot?
Capsaicin which is found in spicy food is lipid-based and passes right through the cell membrane. It attaches to VR-1 receptor and opens channel thus artificially creates the sensation of heat
Where do GTO’s attach?
Attache to muscle fibres and tendon
Is there 1 or multiple muscle fibres per GTO?
multiple
What are GTOs innervated by?
type 1b afferent
What are GTOs very sensitive to?
active force production
What is the relationship between GTO firing rate and force output?
linear relationship
What are the 4 types of joint receptors?
- Ruffini endings
- Pacinian corpuscles
- Golgi Endings
- Free nerve endings
What do ruffini endings in the join code for?
joint position and intra-articular pressure
What joint receptor codes for high frequency joint acceleration?
pacinian corpuscles
What do golgi endings code for?
tension in ligaments, mainly at ends of ROM
What joint receptors code for tissue damaging stimuli?
Free nerve endings
Can joint receptors tell the difference between pressure, flexion/extension and intorsion/extorsion?
No
Are joint receptors activated in all directions?
Yes
What are joint receptors most useful for?
Knowing where we are at ends of ROM (joint position)
Do joint angles give us info on joint angle?
No
What amount of joint receptors increase their discharge with on direction of joint rotation?
1/4
Are joint receptors useful for proprioception?
No
