Somatosensory System Flashcards
What is the role of the somatosensory system?
Tells us what is happening within the body and environment through our senses and pathways
What is true about the distribution of the somatosensory system?
It is not confined to the head, but is spread throughout the body
What determines our sensitivity to stimulation?
The density of somatosensory receptors
What is true about the density of somatosensory receptors?
Areas with many receptors = More sensitive to stimulation
Areas with fewer receptors = Less sensitive to stimulation
What are two skin types found in humans?
Hairy skin and glabrous skin
What are features of hairy skin?
Hair follicle presence, less sensitive to touch
What are features of glabrous skin?
No hair follicles, larger number of sensory receptors –> mainly used to explore our environments
How can we test the sensitivity of body parts?
Two point sensitivity test
What is the two-point sensitivity test?
2 sharp points simultaneously vs. perceived as a 1
Glabrous skin = detect 2 points up until about 3mm apart
Hairy skin = only detect 2 points up until about 2 – 5 cm
What is the two-point sensitivity test?
2 sharp points simultaneously vs. perceived as a 1
Glabrous skin = detect 2 points up until about 3mm apart
Hairy skin = only detect 2 points up until about 2 – 5 cm
What are the 3 functional groups of somatosensory receptors?
Nociceptive
Haptic
Proprioceptive
What are nociceptive receptors?
Receptors of irritation, perception of pain, temperature and itch –> free nerve endings that secrete chemicals
What are haptic receptors?
Receptors of pressure and fine touch –> help us identify what we touch and grasp –> found in superficial layers and attached to hairs
What are proprioceptive receptors?
Receptors for body awareness (perception of spatial location) –> found in muscles, tendons and joints
What is the role of rapidly adapting receptor?
Tells us when a stimulus occurs –> activates only when stimulation begins and stimulation ends.
What is the role of a slowly adapting receptor?
Tells us whether a stimulus is still occurring –> continuously responds so long as the stimulus is still present
What neurons carry afferent information INTO the CNS?
Sensory neurons
Where is the dendrite located outside of the spinal cord?
In the posterior root ganglion neurons
What is true about spinal cord segments and posterior root ganglion neurons?
Each spinal cord segment has one posterior root ganglion on each side (left/right) that contains many posterior root ganglion neurons.
What differences exist in the diameter and the degree of myelination of the axons based on what kind of sensory information the neuron is conveying?
Proprioceptive and haptic neurons = large and well myelinated –> carry information faster
Nociceptive neurons = small with little myelination and carry info slower
True or false: Our somatosensory system is closely linked to movement; evident with damage to the PRG.
True
What is deafferentation?
Loss of incoming sensory input due to damage to sensory axons/neurons
- Will cause motor symptoms as well, despite no damage to motor neurons
What can happen with selective adamage to neurons that carry proprioceptive information?
Movement abnormalities –> eg. loss of proprioception from vitamin B6 overdose
What are the two somatosensory pathways to the brain?
The posterior spinothalamic tract
The anterior spinothalamic tract
What somatosensory information is carried by the posterior spinothalamic tract?
Haptic and proprioceptive information
How is haptic and proprioceptive information carried by the posterior spinothalamic tract?
Axons from posterior root ganglion neurons enter the spinal cord and go up on the same side (ipsilaterally)
Then the axons enter the brainstem and cross over to the opposite side (contralaterally) and project up to the medial lemniscus
After getting to the medial lemniscus, axons go to the ventrolateral nucleus of the thalamus then projecting to the somatosensory cortex
What somatosensory information is carried by the anterior spinothalamic tract?
Nociceptive information
How is nociceptive information carried by the anterior spinothalamic tract?
Axons from the posterior root ganglion neurons enter the spinal cord and immediately cross over.
These axons then climb to the brainstem where they join the other axons forming the medial lemniscus.
These axons then travel again to the ventrolateral nucleus of the thalamus then project to the somatosensory cortex.
What are two important differences between the two spinothalamic tracts?
i) the sensory information they carry, and ii) where the information crosses over.
Posterior = touch/pressure/location = brainstem
Anterior = pain/temperature/irritation = spinal cord
Why are there 4 somatosensory tracts going up into the brain?
Left = 1 posterior tract (solid red) and 1 anterior tract (dashed red)
Right = 1 posterior tract (solid blue) and 1 anterior tract (dashed blue)
What is the primary somatosensory cortex and its role?
S1
- Consists mainly of the postcentral gyrus in the parietal lobes; lies adjacent to the primary motor cortex on the other side of the central sulcus
- Receives projections from the thalamus (as we just saw)
- Where initial somatosensory processing occurs
What is the secondary somatosensory cortex and its role?
S2
- Located behind S1; receives information from S1
- Begins to combine somatosensory information with our other senses (vision, etc.)
What did Penfield say about the somatosensory humonculus of S1
Identified a somatosensory homunculus present in S1 that is almost identical to the one present in M1
Helps explain why we can distinguish between different types of sensory stimuli at the same time on the same body part
What is Astereognosis?
The inability to recognize objects by touch alone
Damage to the postcentral gyrus also results in marked changes in what 3 things?
- Sensory thresholds become abnormally high
- Position sense of physical contact
- Weakened sterogenesis leading to deficits in tactile and detail perception
What is the main pain pathway to the brain?
The anterior spinothalamic tract
What are some other connections to the anterior spinothalamic tract that carry pain information from the spinal cord to the brain?
Reticular formation —> arousal presence of pain
Amygdala –> emotional responses to pain
Hypothalamus –> coping with pain
What is phantom limb pain?
Pain, phantom movement, muscle cramps
What is the gate theory of pain
Idea that activity in different sensory pathways can compete against each other to diminish the activity of other pain and temperature pathways –> eg. rubbing a toe after stubbing it
What is the periaqueductal grey matter (PAG)?
Nucleus that surrounds the cerebral aqueduct
How does the PAG suppress pain?
Opioid receptors
Inhibiting neurons that form the ascending pain pathways
What is the only localized part of the somatosensory system?
Vestimular organ with semicircular canals and otolith organs
What are the functions of the vestibular organs?
Detect changes in rotation and acceleration of head movements and sense the position of the body in relation to gravity
What are the semicircular canals of the vestibular system?
Oriented canals filled with endolymph fluid that provide our sense of head rotation.
What is the cochleovestibular nerve (cranial nerve 8)?
Cranial nerve that is connected to cilia and semicircular canals
What occurs in the otolith organs in response to acceleration?
Hair cells embedded in otoconia shift according to movement
What is a common disorder of the vestibular system?
Vertigo
What happens when someone has vertigo?
Incongruous information is provided to the brain –> The vestibular system detects movement, but the visual system does not.
What is Ménière’s disease?
Reocurring vertigo, tinnitus, hearing loss and fullness in inner ear due to an increased volume of endolymph fluid in the inner ear.
