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What is somatosensation?
Somatosensation is sensory input from the skin and musculoskeletal systems.
Skin: touch, pain, temperature
Musculoskeletal: proprioception and pain
What determines the speed of sensory information processing?
Axon diameter
Degree of myelination
Number of synapses in the pathway
What is a sensory receptor?
A receptor is a specialized cell or cell part that detects a specific stimulus and produces a receptor potential.
What are the types of sensory receptors and examples of each?
Chemoreceptors: Smell, taste, pH, blood chemistry
Photoreceptors: Light (rods and cones of the retina)
Thermoreceptors: Temperature (free nerve endings)
Mechanoreceptors: Touch, pressure, vibration, stretch, hearing
Nociceptors: Pain (e.g., extreme heat, pressure, inflammatory chemicals)
How is sensory information coded?
Through receptor potentials which may lead to action potentials depending on the receptor’s properties.
What is a receptive field?
The specific area within which a stimulus will activate a given sensory receptor.
What is sensory transduction?
The process of converting external stimulus energy into an electrical signal within the receptor.
What are the characteristics of receptor potentials?
Local and graded
Can be excitatory (depolarizing) or inhibitory (hyperpolarizing)
Trigger action potentials in some receptors
Can open ion channels directly or use G-protein-coupled pathways
What is adaptation?
Reduced sensitivity of a receptor to a constant stimulus over time.
What are slow adapting receptors better for?
Detecting sustained stimuli (e.g., pressure).
What are rapid adapting receptors better for?
Detecting changing stimuli or the start/stop of a stimulus (e.g., vibration).
Why do some receptors produce action potentials while others do not?
Receptors sending signals long distances must generate action potentials.
Others use local graded potentials and influence nearby synapses instead.
Some have a trigger zone within the same neuron (no synapse).
Are there receptors that do not adapt?
Yes, some nociceptors (pain receptors) show minimal or no adaptation.
What is each somatosensory receptor associated with?
A pseudounipolar neuron with:
A central process projecting to the CNS
A peripheral receptive ending
What are the two classifications of somatosensory receptors?
1. Encapsulated:
Meissner’s corpuscle: Fine touch – rapid adapting
Pacinian corpuscle: Vibration – rapid adapting
Ruffini ending: Pressure – slow adapting
2. Nonencapsulated:
Hair follicle receptor: Movement/touch – rapid adapting
Merkel ending: Sustained touch – slow adapting
Free nerve endings: Pain, crude touch, temperature
What is the difference between fast and slow pain?
Fast pain (Aδ fibers): Sharp, localized, myelinated
Slow pain (C fibers): Dull, aching, unmyelinated
What are muscle spindles and what do they sense?
Sensory receptors in muscles that detect muscle length and rate of stretch.
What’s the difference between intrafusal and extrafusal fibers?
Intrafusal: Inside the spindle, detect stretch
Extrafusal: Generate muscle contraction
What neurons innervate extrafusal and intrafusal fibers?
Alpha motor neurons → Extrafusal fibers
Gamma motor neurons → Intrafusal fibers
How are aplaha and gamma neurons activated?
They are coactivated to maintain muscle spindle sensitivity during movement.
What are Golgi tendon organs?
Spindle-shaped receptors at the junction between muscle and tendon.
What do golgi tendon organs sense?
They monitor tension or force from active contraction or passive stretch.
Contribute most to the sense of force exerted.
What types of receptors are in joint capsules and ligaments?
Free nerve endings
Golgi tendon organs
Pacinian corpuscles
(All respond to mechanical deformation)
What are the types of visceral receptors and what do they monitor?
Mechanoreceptors (e.g., in aorta): Monitor pressure/volume
Chemoreceptors (e.g., carotid body): Monitor blood gases & pH
Nociceptors (e.g., organ capsules): Monitor distention(swelling)
What are the three connective tissue coverings?
Endoneurium: Loose CT around individual axons
Perineurium: Coarse CT bundling axons into fascicles
Epineurium: Tough outer sheath enclosing the nerve