CNS Lecture 3 Flashcards
The ability to locate a stimulus depends on what two factors?
The Size and Density of receptive fields of the receptors
What is the difference between sensation and perception?
Sensation: conscious awareness of a stimulus
Perception: Sensation combined with an understanding of its meaning
Both result from processing in the cerebral cortex
What process focuses ascending sensory signals to enhance spatial acuity?
Lateral Inhibition focuses ascending sensory signals, enhancing spatial acuity
Acuity: ability to distinguish between things that are near eachother
Ascending sensory axons synapse on neurons in the ______ that then Cross-Over conveying sensory information from the right side of the body to the left side of the brain and vice versa
Ascending sensory axons synapse on neurons in the brainstem that then Cross-Over conveying sensory information from the right side of the body to the left side of the brain and vice versa
What is two point discrimination?
Clinical test used to evaluate sensory discrimination
What is the relationship between spatial acuity and receptive field size?
Smaller receptive fields will provide greater spatial acuity
ie: in areas where receptors are tightly packed, receptive fields will be smaller and sensory acuity (two point discrimination) is high
What would you assume the sensory acuity would be in areas where the receptors are NOT tightly packed?
When receptors are loosely packed, the receptive fields are larger = decrease in sensory acuity (low two-point discrimination)
What kind of information is provided by overlapping stimulation between neighbouring receptive fields?
General information about the location of the stimulus
What is divergence?
Each sensory afferent sends branches to many neurons in the CNS
What is convergence?
A given neuron in the CNS receives inputs from many sensory afferents
What is Lateral Inhibition?
Method of refining sensory information in afferent neurons and ascending pathways whereby fibres inhibit each other; the most active causing the greatest inhibition of adjacent fibres
ie: Sharpens contrast by focusing activation of CNS neurons
What type of neuron functions in lateral inhibition?
Interneurons within the CNS
As shown in the image, lateral inhibition focuses __________ firing to the centre of the stimulus location
Thus increasing:
As shown in the image, lateral inhibition focuses second-order sensory afferent firing to the centre of the stimulus location
Thus increasing: sensory perception
Where is Two-Point discrimination the best and the worst?
Best on hands and face
Worst on abdomen and proximal parts of limbs
Why is Two point discrimination best on hands and face and worst on abdomen and proximal parts of the limbs?
- Density of receptors = highest in skin areas with best 2-pt discrimination
- Surface are of sensory cortex = largest in regions to which receptors from skin areas with the best 2-pt discrimination project
- more surface are of the somatosensory cortex devoted to sensory inputs from the fingers, thumb, forehead, cheek, nose and upper lip
The projection area (topographic maps) is related to _______
The projection area (topographic maps) is related to Functional importance
What is descending inhibition?
The processes through which activity descending from higher centers of the brain and brainstem can “screen out” certain types of sensory information via inhibiting neurons in the afferent pathway
What are the two types of descending inhibition?
Presynaptic inhibition
Postsynaptic inhibition
What is presynaptic inhibition?
Where does it occur?
- results from inhibition of the sensory axon close to the synaptic terminal;
- occurs close to the synapse between the sensory afferents and the neurons that convey information to the brain
How does presynaptic inhibition act?
- Reduces transmitter release at the synapse btwn 1st order and 2nd-order neurons
- inhibits specific sensations (eg pain)
- lasts several milliseconds
What is Postsynaptic inhibition?
How long does it last?
Hyperpolarization of the membrane of the second-order sensory neurons to move their membrane potential further away from the AP threshold
- Non-selective: reduces effect of all synaptic inputs
- lasts less than 1 millisecond
Describe the pathway of pain transmission
- Painful stimulus causes tissue damage
- Damaged tissue releases prostaglandins and histamine which activate the pain receptors
- pain receptors fire AP’s which travel to the dorsal part of the spinal cord
- in the spinal cord, activity in the pain fibres causes release of Substance P
- Substance P activates interneurons called projection neurons
- Signal pain and excite neurons in the thalamus and the cortex
- Results in sensation of pain
How does referred pain occur?
Convergence of visceral and somatic afferents on ascending projection neurons in the spinal cord
- ie sensory afferents from the viscera and internal organs enter the spinal cord and synapse onto the same interneurons as sensory afferents from the skin
- Brain interprets pain from the viscera as pain in those areas of the skin that converge onto the same interneurons
Where do temperature and pain afferents synapse in the anterolateral system?
Pain and temperature afferents synapse onto second-order interneurons in the spinal cord
- these interneurons send axons that cross to the opposite side of the spinal cord (within two segments of the spinal cord generally)
- After crossing, the axons ascend to the brain in the spinothalamic or anterolateral tracts
Describe the Dorsal Column system
- Touch, pressure and stretch afferents enter the spinal cord and form the dorsal columns (DO NOT immediately contact interneurons)
- The dorsal columns ascend to the brainstem where the afferents then synapse onto second order neurons in the brainstem
- the second order neurons send axons that CROSS OVER to the contralateral side of the NS
When does cross-over occur in the anterolateral system vs the Dorsal Column system?
- Anterolateral:
- pathway crosses over within one or two vertebral segments upon entering the spinal cord
- Dorsal Column:
- In the brainstem, second order neurons send axons that cross over
