3 functionally distinct systems in somatic sensory system ?
1. Cutaneous mechanoreceptors Fine touch Vibration Pressure 2. Specialized receptors associated with muscles, tendons and joints Proprioception 3. Free nerve endings Pain Temperature Course touch
pseudounipolar neurons
Neurons found in peripheral nervous system
• Attach to cell body in ganglia by a single process
• Signal does not pass through cell body
• Cell body maintains cellular machinery that mediates
transmission
Sensory transduction
- process of converting energy of a stimulus into an electrical signal
- Stimulus alters permeability of ion channels in the afferent nerve endings to generate a receptor potential; or action potential if reaches threshold
Types of sensory receptors ?
-Mechanoreceptors
• Afferent fibers can be encapsulate by specialized receptor cells to help tune to particular feature
-Free nerve endings
• Lack encapsulation by specialized receptors
• Sensation of pain
• Higher threshold for action potential
What are the different kind of afferents and what information do they convey in what pace ?
• Ia afferents = largest and fastest, supply sensory receptors to muscles for proprioception
• Aβafferents = smaller, convey touch
• Aδand C afferents = small and slow, pain and temp
*Only C are unmyleinated
Receptive field
• Receptive field = area of the skin surface over
which stimulation results in activation of a neuron
• Highly variable per neuron; function of branching of afferent
• Some overlap
• More innervation by many neurons = smaller receptive fields (finger, lips vs back, forearm)
what is Two-point discrimination, and what stimuli activates what fibers ?
• Two-point discrimination = apply 2 stimuli at same
time and measure distance required to receive stimuli as distinct
- Closely placed stimuli (green dots) = activate mostly the b fiber
- Move farther apart (red and blue) = activate mostly the a and c fibers
Afferents are adapting in two ways. what are they and what are they used for ?
- Slowly adapting afferents
• Generate sustained discharge during ongoing stimulus
• Provide spatial info; size and shape of stimulus
- Rapidly adapting afferents
• Fire rapidly when a stimulus is first presented
• Fall silent with continual stimulation
• Convey changes in movement
- Determined by receptor type
MECHANORECEPTORS SPECIALIZED
FOR TOUCH
Motion detection & grip; _____ adapting
Form & texture; _____ adapting
Cutaneous stretch during movement of fingers to
detect hand shape; _____ adapting
Vibrations (ie skilled usage of tools); _____ adapting
Motion detection & grip; rapidly adapting
Form & texture; slowly adapting
Cutaneous stretch during movement of fingers to
detect hand shape; slowly adapting
Vibrations (ie skilled usage of tools); rapidly adapting
Only ____ adapting Merkel cells provide highly accurate representation of Braille pattern
Only slowly adapting Merkel cells provide highly accurate representation of Braille pattern
MECHANORECEPTORS SPECIALIZED FOR PROPRIOCEPTION, what are they and what do they convey ?
- Muscle spindles = coil around muscle fiber bundles; stretch triggers action potentials
- Golgi tendons = distributed among tendons; inform on changes in muscle tension
- Joint receptors = in and around the joints; relay finger position for range of motion protection
Describe the Dorsal column medial-lemniscus pathway.
(serve touch from body)
- First order neurons
• Info ascends in columns ipsilaterally in spinal cord to the medulla
• Topographic organization
•• Fasiculus gracilis to the gracile nucleus = lower limbs
•• Fasciculus cuneatus to cuneate nucleus = upper limbs, trunk and neck
- Second order neurons
• Internal arcuate fibers cross (decussation) the midline and form medial lemniscus; remains separated
• Synapse at ventral posterior lateral nucleus (VPL) of the thalamus
- Third order neurons
• VPL neurons send axons to synapse in somatosensory cortex
Describe the Trigeminothalamic pathway.
(serve touch from face)
- First order neurons
• Cell bodies in trigeminal ganglia
• Nerve has 3 subdivisions: opthalamic, maxillary, mandibular
• Enter brainstem at pons to synapse on trigeminal brainstem complex; differ nuclei process differ stimuli submodalities
- Second order neurons
• Decussate and ascend as the trigeminothalamic tract
• Synapse at ventral posterior lateral nucleus (VPL) of the thalamus
- Third order neurons
• VPL neurons send axons to synapse in somatosensory cortex
Describe the Spinocerebellar tract.
(serve proprioception)
- First order neurons
• Bifurcate into ascending and descending branches (dorsal and ventral horns)
• Lower limbs innervate Clarke’s nucleus in the dorsal horn (blue), upper limbs (blue)
- Second order neurons
• Travel to medulla and into the cerebellum via the dorsal
spinocerebellar tract
• Send collaterals to synapse in nucleus gracilus and travel with third order neurons in medial lemniscus
- More laterally located ventral posterior lateral nucleus (VPL) receives projections from ______ (and serve _____)
- Medially located ventral posterior medial nucleus (VPM) receives input from _______ (and serve _____)
- More laterally located ventral posterior lateral nucleus (VPL) receives projections from medial lemniscus (and serve body)
- Medially located ventral posterior medial nucleus (VPM) receives input from trigeminal lemniscus (and serve face)
What are the functions of area S1: 3b, 1, 3a, 2
Areas of SI :
• 3b and 1: respond to cutaneous stimuli
• 3a = proprioceptors
• 2 = tactile and proprioception
what is the function of different areas in S1
= 3b: obligatory 1st step in cortical processing
Lesions produce profound deficits
= Area 1
Lesions produce inability to discriminate texture of objects
= Area 2
Lesions produce inability to discriminate shape of objects
Vernon Mountcastle proposal
Vernon Mountcastle proposed that neurons with similar response properties might cluster in functionally-distinct columns
Pain reception is poorly localized (as is temperature) (T/F)
True
Nocireceptors
• Free nerve endings network that respond to intense pressure = mechanical
• Free nerve endings that respond to heat, acids, and capsaicin = thermal, chemical
• Polymodal respond to all
• Adapt slowly, if at all
Innervate:
• Skin
• Sheath around muscles, internal organs
• Cornea of the eye
• Pulp of the teeth
Cutaneous somatosensory axons ______ at a greater rate when same stimuli becomes painful; instead nociceptors begin to ______
Cutaneous somatosensory axons do not discharge at a greater rate when same stimuli becomes painful; instead nociceptors begin to discharge. (which means pain is processed differently)
Difference between fast and slow pain
- Fast pain
- Myelinated (A delta) fibers
- Sharp pricking sensation
- Well localized
- “First pain”
- Slow pain
- Unmyelinated C fibers
- Dull aching
- Poorly localized
- “Second pain”
Generally, cold is processed by _ fibers while warm is processed by _ fibers.
Generally, cold is processed by A fibers while warm is processed by C fibers.
Describe Anterolateral pathway (pain)
- First order: neurons synapse in dorsal horn
- Second order: neurons decussate and ascend to brainstem and thalamus in the spinothalamic tract
Trigeminothalamic tract = face/head/teeth pain
