The Somatosensory System Flashcards
What are the two types of mechanoreceptor and what do they transmit?
Responsible for the transduction of tactile sensory information into electrical signals
Rapidly adapting - sensitive to change in stimulation
Slowly adapting - sensitive to sustained stimulation
What are Meissner corpuscles?
Superficial (upper dermis/epidermis)
Rapidly adapting
Most common in smooth/hairless skin
40% innervation of the hand
Detects texture of objects passing over the skin and grip/slippage
What are Pacinian corpuscles?
The most deep (dermis)
Rapidly adapting
More sensitive than Meissner - fine textured surfaces
15% innervation of the hand
Vibration/tickle sense
Important for skilled use of tools
What are Merkel’s disks?
Superficial
Slowly adapting
25% innervation of the hand - dense in the finger tips
Light pressure - edge/rough texture/shape detection incl. Braille reading
What are Ruffini’s corpuscles?
Found in the upper dermis
Slowly adapting
20% innervation of the hand
Exact function unknown but likely proprioceptive
What do free nerve endings carry?
Pain and temperature (spinothalamic tract)
What is two point discrimination determined by and how does it vary with body surface?
Receptor density - greater density = greater discrimination; receptive field size - smaller receptive field size = greater discrimination
Greatest 2-point discrimination in the fingers and lips/face; least on the calf, back etc
As highlighted, cells are also specialised for detection of specific sensory modalities = better discrimination
What are proprioceptors?
Provide feedback about spatial orientation/tension of body parts
Muscle spindles and golgi tendon organs
What is the structure and function of muscle spindles?
Detect change in muscle length/stretch; even passive stretch - group 1 = dynamic movement; group 2 = static position of limbs
Embedded within skeletal muscles - greater number of spindles = greater control of muscle required ie extraocular muscles
Composed of intrafusal fibres innervated by gamma motor neurons that keep fibres at an optimum length for stretch detection
Relay messages to dorsal root ganglion via 1a afferent fibres
(alpha-gamma coactivation = alpha MN innervating muscle is activate at the same time as the gamma MN innervating receptor so they change together)
(Phase 1 neuro 2 notes have stuff on myotactic reflex - check book for relevance)
What is the structure and function of golgi tendon organs?
Detect change in muscle tension; only in response to active contraction
Located in muscle tendons
Ends of 1b afferent fibres wrap collagen bundles in the tendon - when tension increases (due to muscle contraction), fibre ends distort and activate 1b afferent neurons
(similar stuff on reflex involvement)
How does afferent sensory information travel distally to proximally?
Dorsal column medial lemniscus pathway - upper and lower limbs
Trigeminal - face
Spinothalamic - pain and temperature
What is the path information travels in if carried in the dorsal columns?
Transmit mechano- and proprioception ie light touch, pressure and spatial orientation
Mechanoreceptor-peripheral nerve-dorsal root ganglion (cell body) - dorsal horn of the spinal column (synapse) - spinal nerve (= dorsal column - gracile tract = lower limbs; cuneate = upper limbs) - gracile/cuneate nuclei in the caudal medulla (synapse) - medial lemniscus pathway - ventral posteriolateral nucleus of the thalamus (synapse) - primary somatosensory cortex
What are dermatomes?
Areas of innervation arising from the same dorsal root
Large overlap between segments for touch, vibration, and pressure; less so for pain = better modality for detecting nerve lesions
What is the path information travels in if carried in the trigeminal system?
Mechanoreceptor from face - trigeminal ganglion (cell body) - principle nucleus of trigeminal complex (synapse) - medial lemniscus pathway/trigeminothalamic tract - ventral posterior medial nucleus of the thalamus (synapse) - primary somatosensory cortex
Three branches of the nerve - V1/occipital = tip/bridge of nose and above; V2/maxillary = cheeks and top lip; V3/mandibular = side of head by ears, jaw line, lower lip and chin
How is sensory information represented in the somatosensory cortex?
Arranged like a homunculus - larger areas dedicated to body parts with greater sensory innervation
Face and neck most lateral; arms and hands mediolateral; trunk, legs, feet and genitals more medial
Primary cortex - S1 - areas 1 (texture), 2 (size/shape), 3a; 3b (= processes bulk of VPN thalamus, projects to 1+2)
Secondary cortex - S2 (more lateral), receives from S1 and projects to parietal cortices (then to motor/premotor areas)
