The Somatic Sensory System Flashcards
What are mechanoreceptors?
Sensory receptors that respond to mechanical pressure or distortion
Relay extracellular stimulus to intracellular signal transduction through mechanically gated ion channels
What are the two types of mechanoreceptor?
Slowly adapting receptors - always fire when the stimulus is present (best for providing information about the spatial attributes of the stimulus such as size and shape
Rapidly adapting receptors - only fire at the very start of a stimulation (best for providing information about changes in ongoing stimulation such as those produced by stimulus movement)
Name the rapidly adapting receptors for tactile stimulation
Meissner corpuscle
Pacinian corpuscle
What are the Meissner corpuscles?
- Rapidly adapting
- Most common in smooth hairless skin
- 40% innervation of hand
- For textured objects moving across the skin
- Detection of slippage between hand and object - important for grip
What is the Pacinian corpuscle?
- Rapidly adapting
- More sensitive than Meissner
- Fine textured surfaces
- 15% of hand innervation
- Produce sensation of vibrations or tickle
- Important for the skilled use of tools
Name the rapidly adapting receptors for tactile stimulation
Meissner corpuscle
Pacinian corpuscle
Name the slowly adapting receptors for tactile stimulation
Ruffini’s corpuscles
Merkel’s disks
What are Ruffini’s corpuscles?
- Slowly adapting
- 20% receptors in hand
- Unsure of function
- Something to do with proprioception
- Conformation of the hand in space
What are the Merkel’s disks?
- Slowly adapting
- 25% receptors in hand
- Dense in finger tips
- Stimulation produces sensation of light pressure
- Role in detection of shapes, edges and rough textures
- For Braille reading
What detects pain and temperature in the skin?
Free nerve endings
What determines mechanosensory discrimination across the body surface?
- Receptor density (Regional variations in the average size of afferent receptive fields which reflect the density of afferent fibres suppling the area)
- Receptive field size (the area of the skin surface over which stimulation results in a significant change in the rate of action potentials)
What are proprioceptors?
Provide information about mechanical forces arising from the body itself
Give continuous information about position of the limbs and other body parts in space
What are the main types of proprioceptors?
Muscle spindles
Golgi tendon organs
What are muscle spindles?
Stretch detectors - they sense how much and how fast a muscle is lengthened or shortened
What are Golgi tendon organs?
Proprioceptors which receive information from the tendon and senses tension
Does not respond to passive stretch of muscle only to active contraction
What are the somatic sensory pathways?
The Dorsal Column-Medial Lemniscus System - upper and lower body - fine touch, 2-point discrimination, conscious proprioception, vibration
The Trigeminal System - face
Spinothalamic - pain and temperature
Describe the dorsal column-medial lemniscus system
Mechanosensory receptors in upper body travel through the cuneate tract to the cuneate nucleus in the medulla, decussates and then ascends up to the thalamus and then the primary somatic sensory cortex
Mechanosensory receptors in lower body travel through the gracile tract to the gracile nucleus in the medulla, decussates and then ascends up to the thalamus and then the primary somatic sensory cortex
What are dermatomes?
Area of skin that is mainly innervated by afferent nerve fibres from the dorsal root of any given spinal nerve
substantial overlap between segments especially for touch, vibration and pressure
Describe the trigeminal system
Mechanosensory receptors from face travel through the trigeminal ganglion at the level of the pons
Synapse in the principal nucleus of trigeminal complex and then decussate across the pons
Ascend in the trigeminothalamic tract to the ventral posterior medial nucleus (VPM) of the thalamus
Synapse and then project to the primary somatic sensory cortex
What are the areas of the somatic sensory cortex?
3a = proprioceptive
3b & 1 = cutaneous stimuli
2 = proprioception and tactile stimuli
What are the receptive fields for 3b, 1 & 2?
3b = single finger
1 & 2 = multiple fingers
1 = directions of movement
2 = complex stimuli such as specific shape
What effects do lesions in areas 3b, 1 and 2 have?
3b = severe deficit on both texture and shape
1 = texture discrimination
2 = finger coordination and in size and shape discrimination
What is sensory transduction?
The process of converting energy of a stimulus into an electrical signal
Why do measures of two-point discrimination vary dramatically across the skin surface?
Regional differences in receptive field size and innervation density are the major factors that limit the spatial accuracy with which tactile stimuli can be sensed
On the fingertips, stimuli are perceived as distinct if they are separated by roughly 2mm but the same stimuli applied to the upper arm are not perceived as distinct until they are at least 40mm apart
What sensory afferents are known to detect painful sensations?
Nociceptors which terminate in the skin as free nerve endings
Describe the spinothalamic pathway?
Lateral spinothalamic tract
Information from tactile receptors travel to first order neurons in the dorsal root ganglion
Decussate across the spinal cord, ascend to the ventral posteriolateral nucleus (VPL) of the thalamus, then ascend to the somatosensory cortex
Anterior spinothalamic tract
First order neurons synapse in the nucleus proprius and the second order neurons decussate via the anterior white commissure
Ascend synapsing in the VPL of the thalamus
What is the function of the spinothalamic pathway?
Nociceptive, temperature, crude touch and pressure
