Lecture 8: Somatosensation Flashcards
What is the role of somatosensation
Tells us what the body is up to and what’s happening in the environment by providing bodily sensations such as touch, temperature, pain, position in space, movement of the joints.
It allows us to distinguish between what the world does to use and what we do to it. The skin is all over the body - the biggest sensory organ.
The more receptors you have…
The higher the sensitivity to stimulation than areas with relatively fewer receptors e.g. hands or lips
What two kinds of skin do humans have
1) Hairy skin: relatively low sensitivity
2) Glabrous skin: Skin that has no hair follicles but contains a larger number of sensory receptors than other skin areas + sensitive to wide range of stimuli.
How to determine sensitivity of skin
Using a two point discrimination test which reveals differences in skin sensitivity across the body. It uses two needles which have different cm apart in whatever area you want to test. Can it distinguish the points as two? or as one?
Which areas are most or least sensitive
The fingertips are the most sensitive as they have a higher density of mechanoreceptors which have small receptive fields.
Back,calf - very low sensitivity
What are the three types of somatic sensation
1)Nocioception: perception of pain and temperature.
2)Hapsis: Perception of fine touch and pressure e.g. grasp, manipulate.
3)Proprioceptive: Perception of the location and movement of the body.
How do nociceptors work
Nocioreceptors are free nerve endings which detect sharp/dull pain and heat/cold temperatures. Damage happens to dendrite or surrounding cells which releases chemicals like prostaglandins that stimulate dendrite and produce an action potential/noxious signal. Ibuprofen blocks prostaglandin synthesis. Histamines can also be released from allergies/inflammation which can lead to pain/higher temp.
How do haptic receptors work
They are dendrites attached to hair follicles, connective tissue or dendrites encased in capsule of tissue. They distinguish touch, pull, vibration and indentation. Mechanical stimulation produces action potential e.g. hair receptor: bending of hair or Ruffini’s corpuscle: stretch.
How do proprioceptors work
Movements stretch the receptors to mechanically stimulate dendrites and produces an action potential.
- muscle spindles: muscle stretch
- golgi tendon organs: tendon stretch
- joint receptors: joint movement
What are the two categories of somatosensory receptors in terms of adaptation?
1) Rapidly adapting receptor: 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 e.g. vibration, tickle.
2) Slowly adapting receptor: Body sensory receptor that responds as long as a sensory stimulus is on the body. Signals the presence of a long sustained stimulus e.g. pain or long pinch.
What neuron is responsible for sensing sensory information signals from receptors to the brain, and how?
The dorsal root ganglion neurons:
- The dendrites and axons are continuous and carry sensory information from the skin to the CNS via the spinal cord.
- The tip of dendrite is responsible for 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.
Describe the structure of the 3 somatosensory neuron types
- Proprioceptive & haptic neurons have large,well-myelinated axons (fast)
- Nocioceptive neurons have small axons with little to no myelination (slow)
Define deafferentiation
Loss of incoming sensory input usually due to damage to sensory fibers; also loss of any afferent input to a structure.
Patient example of consequences of deafferentiation
Patient G.O. lost sensation due to peripheral neuropathy after influenza.
- Did not lose motor control, but simple actions for prolonged periods (right) would require visual feedback.
- Could not perform many daily tasks.
- Afferent feedback is required for fine movements.
Sensation is important for movement. e.g. mouth with anesthesia you can’t feel if it’s open or how it is moving.
How does the dorsal spinothalamic tract carry sensory information to the brain
- Carries haptic and proprioceptive
information (movement/pressure/touch) - 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 column
nuclei cross over to the opposite
side of the brain and project up
through the brainstem as part of
a pathway called the medial
lemniscus. - Axons synapse with neurons
located in the (ventrolateral
nucleus of the) thalamus, which
projects to the somatosensory
cortex and motor cortex.
How does the ventral spinothalamic tract carry somatosensory information to the brain?
- Carries nocioceptive information (pain/temperature)
- 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.
Why are there two different pathways that convey somatosensory information
Because of this arrangement, unilateral spinal-cord damage results in distinctive sensory losses to both sides of the body below the site of injury. Like loss of touch and pressure sensation on the side below the damage, and loss of pain and temperature sensation on the opposite side below the cut. So it’s easier to find the source/side of the damage.
What area of the brain processes somatosensory information
1)Primary Somatosensory Cortex
- Receives projections from the
thalamus
- Begins the process of
constructing perceptions from
somatosensory information
.2) Secondary Somatosensory Cortex
- Located behind the primary
somatosensory cortex
- Continues the construction of
perceptions, projects to the
frontal cortex
How was the somatosensory cortex discovered
- Wilder Penfield (1930s) stimulated the cortical surface of brain areas with electrodes and recorded patient responses whilst awake. 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.
- The brain lacks nocireceptors so the patient would not feel pain whilst the brain receives the currents.
Damage to the primary somatosensory cortex results in impairment in…
- Pressure sensitivity: proprioception.
- Ability to identify objects by touch and simple movements like reaching and grasping: Hapsis
