Week 3 - Somatosensory System Flashcards
1
Q
Explain the functions of the somatosensory system:
A
- Proprioception (sense of oneself)
a) Information from the musculoskeletal system
b) E.g. if you close your eyes and move your limbs you still know where your limbs are
c) Important for balance and coordination - Exteroception (sense of external world)
a) Information from the skin is superficial (e.g. touch, pain, and temperature) or cutaneous - Interoception (organ system sense)
a) Information from internal organs - e.g. stretch of bladder or stomach
2
Q
Describe the sensory receptors and their transduction:
Mechanoreceptors
A
- Mediate touch and proprioception by the mechanical deformation of the receptor by touch, pressure, stretch, or vibration
3
Q
Describe the sensory receptors and their transduction:
Cutaneous and subcutaneous
A
- Meissner corpuscle - detection of texture and slow vibration
- Merkel discs - sustained touch
- Pacinian corpuscle - deeper pressure and faster vibrations
- Ruffini organ - detection of pressure and skin stretch
- Free nerve endings - pain and temperature
4
Q
Describe the sensory receptors and their transduction:
Muscle and skeletal mechanoreceptors
A
- Muscle spindles
a) The sensory organs within muscle
b) Intrafusal muscle fibres - detect change in muscle length and relay information to sensory system to prevent over stretching and muscle damage
c) Mediated by Ia and II fibres - Golgi Tendon:
a) Organs that detect the force generated by the muscle contraction by the stretch of the tendon
b) Mediated by Ib fibres
c) Provides proprioception - position of joint and stretch - Joint receptors:
a) Respond to mechanical deformation of joint capsules and ligaments
b) Contain free nerve endings, raffini ending, ligament proprioceptors, and paciniform corpuscles
5
Q
Describe the sensory receptors and their transduction:
Nocioreceptors
A
- Mediate pain
6
Q
Describe the sensory receptors and their transduction:
Special sensory receptors
A
- Rods and cones - vision
- Hair cells in cochlea and vestibular - hearing and balance
- Chemoreceptors - taste (tastebuds) and olfaction
7
Q
Describe the different somatosensory fibres carrying the information from sensory receptors and define where their cell bodies will be located:
A
- Large diameter fibres:
a) Coarse touch, pressure, vibration, fine/light touch (tactile discrimination), proprioception
b) Fast transmission - Small diameter fibres:
a) Pain and temperature
b) Slow transmission
8
Q
What is a receptive field?
A
- The receptive field of a sensory neuron is the area in which stimulation is detected leading to activity in neuron
- This helps us locate with precision where a stimulus was felt
9
Q
Describe the different cortical regions process involved in somatosensory process:
Primary and Associative cortical regions
A
- Primary Sensory neurons
a) Cell bodies located in dorsal root ganglion
b) Primary Axon enters dorsal aspect of spinal cord via dorsal horn to begin relaying information and ascend it to brain - Primary somatosensory cortex
a) Located in the post central gyrus
b) Receives somatotopic input from the thalamus
10
Q
Describe the different cortical regions process involved in somatosensory process:
Primary and Associative cortical regions
A
- Primary Sensory neurons
a) Cell bodies located in dorsal root ganglion
b) Primary Axon enters dorsal aspect of spinal cord via dorsal horn to begin relaying information and ascend it to brain - Primary somatosensory cortex
a) Located in the post central gyrus (Diagram 11)
b) Receives somatotopic input from the thalamus via ascending tract
c) Area 3 - receives information about proprioception
d) Area 1 + 2 - receives information about texture, size, and shape discrimination - Association cortex - area 5 and 7
a) Directly posterior to sensory cortex in the superior parietal lobes
b) Receives synthesised connections from the primary and secondary sensory cortices
c) Neurons respond to several inputs and are involved in complex associations
d) Perception of depth, 3D perception of objects - Secondary somatosensory cortex
a) Area 40 which is in the lower parietal lobe
b) This receives connections from primary sensory cortex
c) Repsonds to sensory stimuli bilaterally with much less precision than the primary cortex
11
Q
What are the three types of ascending tracts?
A
- Conscious relay
- Divergent
- Nonconscious relay
12
Q
What is the conscious relay tract?
A
- Tract that transmits information about the location and type of stimulus with high level of accuracy
a) Allows discrimination of information - Discrimination is achieved because axons in the tract are arranged anatomically and somatotopically
- 3 neuron relay to the cortex:
a) 1st order neuron: cell body in a dorsal root ganglion; one axon connected to a sense organ (receptor) and the other travels into the spinal cord/or brainstem
b) 2nd order neuron: cell body in the spinal cord/or brainstem; axon decussates and goes to the contralateral thalamus
c) 3rd order neuron: cell body in thalamus; axon travels up to primary sensory cortex (via the internal capsule) - Somatic conscious sensations ascend in spinal cord via two routes:
a) Dorsal columns-medial lemniscus tracts
b) Spinothalamic/Anterolateral tracts
13
Q
Describe the dorsal columns/medial lemniscus tract: Diagram 21
A
DCML carries messages of proprioception and touch
- Primary neuron:
a) Axon enters and ascends in dorsal column of spinal cord
b) Leg = Fasciculus gracilis/medial lemnscus pathway; arm = Fasciculus cuneatus/medial lemniscus pathway
b) Axons terminate on the dorsal medulla and synapse with secondary neuron - Secondary neuron:
a) Cell bodies located in the nucleus gracilis for leg or cuneatus for upper limb (in dorsal medulla)
b) Axons decussate at lower medulla and ascend to thalamus - Third-order neuron:
a) Cell bodies located in the thalamus send their axons to the cortex via the internal capsule
b) Synapse to cells in the primary somatosensory cortex - from body area they originated from
14
Q
Describe the spinothalamic/anterolateral tracts: Diagram 22
A
- Transmits information about fast pain and temperature
- Three neuron system:
a) Primary neuron: bring information into the posterior horn (grey matter) of the spinal cord
b) Secondary neurons: have cell body in posterior grey area of the spinal cord; axons decussate immediately and ascend from spinal cord to thalamus anteriolaterally - Tertiary neuron: have cell bodies in the thalamus; axons project from the thalamus to the cerebral cortex (via internal capsule)
15
Q
Describe the lesions of DCML tract:
A
- A lesion of the DCML tract causes a loss of proprioception and fine touch
- Hemi-section of spinal cord = Ipsilateral loss below the level of the lesion
- Hemi-section of brainstem = contralateral sensory loss
- Somatosensory cortex lesion = contralateral sensory loss
