Lecture 2 - Somatosensation Flashcards
What is the somatosensory system responsible for?
Touch
Proprioception
Temperature
Pain
Where do somatosensory afferents synapse?
Spinal cord or brain (in the cranial nerves)
What does it mean if a receptor is fast-adapting (phasic)?
The firing stops quickly as the stimulus continues, but reacts strongly when a stimulus is changing
What is another name for slow-acting receptors?
Tonic receptors
If a receptor has a small receptive field, what is easier to distinguish?
Greater acuity in distinguishing between two points of stimuli
What is lateral inhibition?
Signals from neurons at the center of the receptive field inhibit neurons on the periphery
If the intensity of a stimulus is increased, how will the firing of the action potentials be affected?
There will be an increased number of action potentials
What are 1st order neurons?
Afferent (sensory) neurons
What are 2nd order neurons?
Neurons in the spinal cord or brain stem
What are 3rd order neurons?
Neurons in the thalamus
What are 4th order neurons?
Neurons in the primary area of the cortex
What are A(alpha) fibers?
Largest and fastest sensory neuron; most myelinated. Proprioceptors of skeletal muscle.
What are A(beta) fibers?
The second fastest and second most myelinated axons. Mechanoreceptors of skin
What are A(delta) fibers?
Smaller axons but still have myelination. Pain and temperature
What are C fibers?
The smallest axon, no myelination. Temperature, pain, itch
What are the 4 types of mechanoreceptors?
- Merkel’s Disk
- Meissner’s Corpuscle
- Ruffini’s Corpuscle
- Pacinian Corpuscle
Can a slow-adapting receptor ever adapt to a stimulus?
Yes, over time the continuous pressure being sensed will become adapted to. (ex wearing a watch)
What type of touch do Merkel’s disks sense?
Light touch/pressure
What are the characteristics of Merkel’s disks?
-Located in epidermis (superficial)
-Small receptive field
-Most receptive/sensitive
-Slow adapting*
-A(beta) fiber
-Releases neurotransmitters
What type of touch do Meissner’s Corpuscles sense?
Light touch and movement
What are the characteristics of Meissner’s Corpuscles?
-Located in epidermis (superficial)
-Small receptive field
-Fast adapting*
-A(beta) fiber
-Uses Na+ to trigger action potentials instead of neurotransmitters
What type of touch do Ruffini’s Corpuscles sense?
Stretch and skin tension. Position and movement
What are the characteristics of Ruffini’s Corpuscles?
-Parallel to the skin in the dermis
-Large receptive field
-Slow adapting
-Uses Na+ to trigger action potential
What type of touch do Pacinian Corpuscles sense?
Pressure and deep touch
What are the characteristics of Pacinian Corpuscles?
-Located in subcutaneous tissue
-Large receptive field
-Fast adapting
What is the most abundant type of receptor in the skin?
Free nerve endings (pain and temperature)
How do Pacinian Corpuscles trigger an action potential?
Mechanical pressure distorts the outer layers, causing internal stretch receptors to open Na+ channels. This leads to a graded potential which if large enough, will trigger an action potential.
Which type of receptor will recognize fine details of Braille?
Merkel’s Disks
What do proprioceptors do?
Provide details on position of limbs in space
What are the two types of proprioceptors?
- Muscle spindles
- Golgi tendon organs
Where are muscle spindles located and what do they do?
-Located in skeletal muscle
-Signal changes in muscle length (stretch receptors)
Where are golgi tendon organs located and what do they do?
-Located within tendons
-Detects amount of tension that muscle exerts on tendons. Can also activate to decrease muscle contraction and force during overload
What type of fiber is responsible for proprioception?
A(alpha) fiber
What type of fiber is responsible for mechanoreception?
A(beta) fiber
What is the Dorsal Column-Medial Lemniscus Pathway (DCML)?
A pathway from the spinal cord to the cerebral cortex. Carries fine touch, pressure and proprioceptive sensations
Info from the upper limb ascends within the___________?
Fasciculus cuneatus
Info from the lower limb ascends within the___________?
Fasciculus gracilis
What is processed by the somatosensory cortex?
Localization of touch and intensity
If an area has a greater sensory receptor innervation, what will the corresponding area of the somatosensory cortex be like?
It will be a larger area of the cortex
What is thought to be an adaption to nocturnal life in mice and rats?
The barrel cortex, which is a topographic map of each whisker
What are the characteristics of thermoreceptors?
-Free nerve endings
-A(delta) and C fibers
-Fast adapting to constant temp
-Warm and cold thermoreceptors
What is the range of temperature where pain receptors would be activated if exceeded?
Temperatures below 15C or above 43C
What are TRP channels?
Transient receptor potential channels
–> triggered at a certain temp will cause an influx of Na+ and Ca2+
What receptor do avians have a mutation in that allows them to be resistant to capsaicin (spicy)?
TRPV1 receptor
Are Merkel’s discs slow or fast adapting?
Slow
How big is the receptive field of Merkel’s discs?
Small
Which mechanoreceptors are responsible for tactile acquity?
Merkel’s discs
Are Meissner’s corpuscles slow or fast adapting?
Fast
How big is the receptive field of Meissner’s corpuscles?
Small
Which mechanoreceptor detects vibration?
Meissner’s corpuscles
What percentage of the sensory innervation in the hand is due to Meissner’s corpuscles?
40%
How big is the receptive field of Ruffini’s corpuscles?
Large
Are Ruffini’s corpuscles slow or fast adapting?
Slow
Are Pacinian corpuscles slow or fast adapting?
Fast
How big is the receptive field of Pacinian corpuscles?
Large
