Somatosensation Flashcards
what does the somatosensory system do
- Tells us what the body is up to and what’s going on in the environment by providing bodily sensations such as:
- Touch, temperature, pain, position in space, and movement of the joints
- Allows us to distinguish between what the world does to us and what we do to it
- It has a closer relationship with movement than the other senses do
where and what are somatosensory receptors
- receptors are all over: skin, muscles, tendons, joints
- more receptors = increased sensitivity to stimulation than area with relatively fewer receptors eg hands, lips
- sensitivity to different somatosensory stimuli is a function of the kinds of receptors
what are the two types of skin humans have
- hairy skin
- glaborous skin
what are the properties of hairy skin
Relatively low sensitivity
what are the properties of glabrous skin
- Skin that does not have hair follicles but contains larger numbersof sensory receptors than do other skin areas (e.g. palm ofhands/feet, lips)
- Sensitive to wide range of stimuli
what is the two-point discrimination test
reveals differences in skin sensitivity across the body
what does the two-point discrimination test
- fingertips most sensitive
- higher density of mechanoreceptors
- receptors with small receptive fields
- brain devotes a lot of resources
- back, calf → very low sensitivity
what are the 3 main types of somatosensory perception
- Nocioception
- perception of pain and temperature
- Hapsis
- perception of fine touch and pressure [e.g grasp, manipulate ]
- Proprioception
- perception of the location and movement of the body
nocioceptors
Nocioceptors: free nerve endings
- sharp/dull pain and heat/cold
- Damage to dendrite or surrounding cells release chemicals (e.g. prostaglandins) that stimulate dendrite and produces an action potential
haptic receptors
Haptic receptors: dendrite attached to hair, connective tissue, or dendrite encased in capsule of tissue (see diagram)
- Distinguish touch, pull, vibration, indentation
- Mechanical stimulation produces action potential e.g.:
- hair receptor: bending of hair
- Ruffini’s corpuscle: stretch
specifci types of haptic receptors
- light touch = steady skin indention
- light touch = changes
- pain and temperature
- stroking
- steak skin indention, skin stretch, perception of grasp objects
- vibration, rapid skin indention, deeper pressure
proprioceptors
Movements stretch the receptors to mechanically stimulate dendrites and produces an action potential
what do somatosensory receptors tell us about a sensory event
when it occurs, and whether it is still occurring
what are two types of somatosensory receptors
- rapidly adapting receptor
- slowly adapting receptor
what does the rapidly adapting receptor do
Body sensory receptor that responds briefly to the beginning and end of a stimulus on the body
- motions of object on skin
- gives good responses to repeated stimuli
- eg vibration, tickle
what does a slowly adapting receptor do
Body sensory receptor that responds as long as a sensory stimulus is on the body
- signals the presence of a long sustained stimulus
- eg pain, long pinch
3 types of somatosensation and adaption types
- Nocioception [pain and hot/cold] → slow
- Hapsis [fine touch and pressure] → slow and rapid
- Proprioception [body awareness] → slow and rapid
the dorsal-root ganglion neuron
- The dendrite and axon are continuous and carry sensory information from the skin to the CNS via the spinal cord
- The tip of the dendrite is responsive to sensory stimulation
- Each spinal cord segment has one dorsal-root ganglion on each side that contains many dorsal-root ganglion neurons
- In the spinal cord, the axons of these neurons may synapse onto other neurons or continue up to the brain
Proprioceptive and Haptic Neurons
- Carry information about locationand movement (proprioception)and touch and pressure(hapsis)
- Large, well-myelinated axons(fast)
Nocioceptive Neurons
- Pain and temperature information
- Small axons with little or nomyelination (slow)
Deafferentiation
Loss of incoming sensory inputusually due to damage to sensory fibers; also loss of anyafferent input to a structure
dorsal-root ganglion neurons consequence of deafferentiatiation
[patient]
- Patient G.O. lost sensation due to peripheral neuropathy after influenza
- Did not lose motor control, butsimple actions for prolonged periods (right) would require visual feedback
- Could not perform many daily tasks
- Afferent feedback is required forfine movements
the dorsal spinothalamic tract
- Carries haptic and proprioceptiveinformation
- Axons from the dorsal-root ganglion neurons enter the spinal cord and ascend ipsilaterally until they synapse in the dorsal column nuclei (base of brain)
- Axons from the dorsal columnnuclei cross over to the oppositeside of the brain and project upthrough the brainstem as part ofa pathway called the mediallemniscus
- Axons synapse with neurons located in the (ventrolateral nucleus of the) thalamus, which projects to the somatosensory cortex and motor cortex
ventral [or anterior] spinothalamic tract
- Carries nocioceptive information
- Axons from the dorsal-root ganglion neurons enter the spinal cord and synapse onto other neurons. These ‘2nd order’ neurons, in turn send their axons to the contralateral side.
- Axons from contralateral spinal cord then ascend to the brain where they join with other axons forming the medial lemniscus, eventually synapsing with neurons located in the (ventrolateral nucleus of the) thalamus
- Neurons from the thalamus then project to the somatosensory cortex
somatosensory pathways to the brain
- two separate pathways convey somatosensory info
- haptic-proprioceptive
- nociceptive
- because of this arrangement, unilateral spinal-cord damage results in distinctive sensory losses to both side of the body below the site of injury
what are the two main somatosesnory areas in the cortex
- primary somatosensory cortex
- secondary somatosensory cortex
primary somatosensory cortex
- receives projections from the thalamus
- begins the process of constructing perceptions from somatosensory info
secondary somatosensory cortex
- located behind the primary somatosensory cortex
- continues the construction of perceptions, projects to the frontal cortex
the somatosensory homunculus -> history
- Wilder Penfield (1930s) stimulated the cortical surface with electrodes and recorded patient responses.
- Wanted to locate source of seizures
- Penfield’s original studies suggested that there was a single ‘somatosensory homunculus’→ representation of human body in cortex
- Distinct parts of the somatosensory cortex senses different somatosensation
effects of damage to the somatosensory cortex
- damage to the primary somatosensory cortex results in impairment in:
- pressure sensitivity, proprioception,
- hapsis [ ability to identify objects by touch], and simple movements [eg reaching and grasping]
