Lecture 8: Touch Flashcards
What are some examples (6) of the diverse range of sensations from periphery?
- Touch
- Pressure
- Limb position
- Vibration
- Heat/cold
- Pain
What are the 4 different types of peripheral ‘receptors’? What do they do?
- Transduce information
- Mechanoreceptors
- Thermoreceptors
- Nociceptors
- Chemoceptors
What do somatosensory afferents do? What type of neuron are they?
- Convey information from skin to CNS
- Pseudounipolar (receptor endings in periphery, peripheral/central components are continuous, attached to cell body in ganglia by single process)
What is sensory transduction?
- Convert stimulation into electrical signal that body/brain can process
- Alters cation channel permeability (depolarizing event)
- Receptor/generator potential (inward current)
Where do receptors end in somatic sensory system?
- Dermis
How are mechanoreceptors activated?
- Physical forces on cell membrane (pulls open receptor/ion channel)
- Direct activation through structural protein (forces act on linking protein which opens channel)
- Indirect activation through force sensor (activates 2nd messengers, inducing ion channel opening)
How is an action potential generated in mechanoreceptors?
- More membrane stretch = more open ion channels
- Sufficient accumulation of receptor potentials leads to AP (influx of +ve ions)
What are tactile senses? What 4 sensory afferents perceive tactile senses?
- Touch
- Merkel Cell
- Meissner Corpuscles
- Ruffini Corpuscles
- Pacinian Corpuscles
What are Merkel Cells?
- 1/4 of all sensory afferents in hand/fingertip
- High spatial resolution
- Useful in discriminating form/texture
- Likely manage reading of Braille
- As little as 0.5mm 2 point discrimination
What are Meissner Corpuscles?
- 40% of mechanosensory nerves in hand
- Contain 2-6 afferent nerve fibres
- 4X more sensitive to skin deformation than Merkel
- Larger receptive fields (less resolution)
- Efficient lo-frequency vibration detection (detecting slippage)
What are Ruffini Corpuscles?
- Least well understood
- 1/5 of hand mechanoreceptors
- Tracking movement/position of hand
What are Pacinian Corpuscles?
- Likely most well understood
- 10-15% of hand mechanoreceptors
- Detect pressure/vibrations (more sensitive than Meissner; skin deformations in nm range)
- Useful in skilled tool usage
- Onion-like appearance
What is the receptive field?
- Area of skin surface over which stimulation results in significant change in AP rate
- Body regions vary with density of afferent fibres
What is 2-point discrimination?
- Minimum interstimulus distance required to perceive 2 simultaneously applied stimuli as distinct
What are rapidly-adapting afferents/what do they do?
- Fire rapidly when stimulus first presented; fall silent with continued stimulation
- Effective in conveying info about changes in ongoing stimulation
What are slowly-adapting afferents/what do they do?
- Spatial attributes of a stimulus
- More constant/ongoing info
- More needed to change (e.g. hands)
What is the sensory function, spatial acuity, and response to sustained indentation in Merkel Cells?
- Shape and texture perception
- 0.5 mm
- Sustained (slow adaptation)
What is the sensory function, spatial acuity, and response to sustained indentation in Meissner Corpuscles?
- Motion detection; grip control
- 3 mm
- None (rapid adaptation)
What is the sensory function, spatial acuity and response to sustained indentation in Pacinian Corpuscles?
- Perception of distant events through transmitted vibrations; tool use
- 10+ mm
- None (rapid adaptation)
What is the sensory function, spatial acuity and response to sustained indentation in Ruffini Corpuscles?
- Tangential force; hand shape; motion direction
- 7+ mm
- Sustained (slow adaptation)
What is proprioception?
- Brain needs to know where body is in space
What is muscle spindle?
- Found in nearly all striated/skeletal muscles
- Detects changes in muscle length
- Info about position of body part in space
- 4-8 intrafusal muscle fibres surrounded by a tissue capsule (interspersed within and parallel to extrafusal fibres)
What are group Ia afferents?
- Rapidly adapting (velocity and direction)
What are group II afferents?
- Sustained response (static position)
What detects stretch?
Type Ia and II afferent axons
What is sensitivity modulated by?
- Gamma-motor neurons from ventral horn of spinal cord
Is the density of spindle fibres consistent or variable?
- Variable
- Relatively few in large muscles with course movements (not a lot of fine control)
- Far more in small muscles with fine movements (hand, neck, surrounding eye)
What is the Golgi tendon organ? Where is it located?
- Located at junction of muscle fibres and tendon (transition)
- Capsule of braided Ib axonal branches interwoven between collagen fibres
- When muscle is stretched, collagen fibres are pulled together which compress Ib afferent
- Efficient measure of tension (force) in muscle
- Ex. knee-jerk reflex (pulls together collagen and compresses 1b afferent)
What is dermatome?
- Helps to define location of suspected spinal lesion/injury
- Innervation of spinal cord arising from a single dorsal root ganglion and its spinal nerve
What is the medial lemniscal pathway?
- Lower limb, upper limb, trunk and neck
How do the first-order neurons travel in the medial lemniscal pathway?
- Enter dorsal root and bifurcate into ascending/descending branches
- Project into grey matter (cell bodies) of dorsal horn
- Ascending branches ascend ipsilaterally through dorsal columns to lower medulla
- Synapse onto neurons in dorsal column nuclei
How are the dorsal columns of the spinal cord organized?
- topographically
What is the gracile tract?
- Lower limb fibres are medial
- Track ends at gracile nucleus (in medulla)
What is the cuneate tract?
- Upper limbs, trunk, neck are more lateral
- Track ends at cuneate nucleus (in medulla)
How do the second-order neurons travel in the medial lemniscal pathway?
- Axons exiting dorsal column nuclei projecting to somatosensory thalamus are internal arcuate fibers
- Cross midline and follow a tract called medial lemniscus to ventral posterior lateral (VPL) nucleus of thalamus
How do third-order neurons travel in the medial lemniscal pathway?
- Synapse in thalamus then project to appropriate region of primary somatic sensory cortex (SI)
What is the trigeminothalamic system?
- Face
How do the first-order neurons travel in the trigeminothalamic system?
- Trigeminal nerve mechanoreceptors (face)
- Trigeminal ganglion cells enter brainstem at level of the pons to terminate in the trigeminal brainstem complex
How do the second-order neurons travel in the trigeminothalamic system?
- Axons cross midline and ascend to ventral posterior medial (VPM) nucleus of the thalamus by way of trigeminal lemniscus
How do the third-order neurons travel in the trigeminothalamic system?
- Neurons in the VPM send axons ipsilaterally to cortical areas SI and SII
What are the proprioceptive receptor afferents?
- Muscle spindles
- Golgi tendon organs
How do lower body first-order neurons in the spinocerebellar tract travel?
- Enter dorsal horn of spinal cord
- Bifurcate into ascending and descending branches
- Some synapse within dorsal horn (e.g. knee-jerk)
- Proprioceptive afferents that enter the spinal cord b/n mid-lumbar and thoracic levels synapse on neurons in Clarke’s nucleus in medial aspect of dorsal horn
- Neurons entering below this level ascend through dorsal column then synapse
How do lower body second-order neurons in the spinocerebellar tract travel?
- From Clarke’s nucleus ascend ipsilateral posterior lateral column of spinal cord to dorsal spinocerebellar tract in medulla
- Continue to cerebellum but have collateral connections with neurons just outside nucleus gracilis
How do lower body third-order neurons in the spinocerebellar tract travel?
- Join the medial lemniscus
- Combine with cutaneous mechanoreceptors terminating at the VPL nucleus of the thalamus
What is the function of the cerebellum?
- Proper proprioception
- Maintain order of body in space
How do the upper body first-order neurons in the spinocerebellar tract travel?
- Enter dorsal horn of spinal cord
- Travel via dorsal column cuneate tract to level of medulla
- Synapse on proprioceptive neurons in dorsal column nuclei, including external cuneate nucleus
How do the upper body second-order neurons in the spinocerebellar tract travel?
- Project axons into ipsilateral cerebellum
- Project collateral branches across midline to join medial lemniscus pathway to VPL nucleus of thalamus
What are the proprioceptive projections to thalamus and how are they organized?
- Segregated passage of fibres throughout ascent from mechanoreceptors through organized topographical connectivity with the cortex
Where is the somatotopic map of?
- Postcentral gyrus of parietal lobe
- 4 distinct regions (Brodmann’s areas)
- Based on cytoarchitectural organization of brain regions
- Many areas are functionally correlated
What does the primary somatosensory cortex consist of?
- Brodmann areas 1, 2, 3a and 3b
What do Brodmann areas 3b and 1 do?
- Sensory representation of cutaneous stimuli (skin)
What does Brodmann area 3a do?
- Proprioception
What does Brodmann area 2 do?
- Both tactile and proprioceptive stimuli
What is the homunculus?
- Somatotopic map
- Body represented
- Size of cortical representation is proportional to density of sensory receptors (smaller 2-point discrimination) in corresponding body part
- Each Brodmann area is nearly an entire map
What Brodmann area receives the bulk of input from ventral posterior thalamus? Which area(s) is it then projected to?
- Area 3b
- All inputs are integrated here
- First step in cortical processing of somatosensory info
- Projects to areas 1 and 2
Where do all Brodmann areas project to, and where do they project from there?
- SII
- Believed to play a role in learning/memory and emotional processing
- Project to amygdala and hippocampus
Where does Area 2 project to?
- Parietal areas 5a and 7b
- Input into motor and premotor areas of frontal lobe (motor outputs)
- Primary point where somatosensory inputs inform and execute voluntary movement
