CNS 3 Flashcards
Neural signals of a given modality from a particular region of the body travel along specific pathways to the brain (Called
“labelled” or “private” lines).
The ability to locate a stimulus depends on the
size and density of receptive fields of the receptors.
Within the CNS, lateral inhibition focuses
ascending sensory signals, enhancing spatial acuity.
Ascending sensory axons synapse on neurons in the brainstem that then cross the
midline, conveying sensory information from the right side of the body to the left side of the brain and vice versa.
Two-point Discrimination
• Sensory afferent A has a smaller receptive field than afferent B, which provides greater spatial acuity
Overlapping Receptive Fields
Overlapping stimulation between neighboring receptive fields provides general information about the location of a stimulus
Divergence:
Each sensory afferent sends branches to many neurons in the CNS
Convergence
a given neuron in the CNS receives inputs from many sensory afferents
Lateral inhibition:
– “Sharpens contrast” by focusing activation of CNS neurons
– Stimulus location is perceived more precisely (↑ “acuity”)
Two-point discrimination is best on
hands and face, worst on abdomen and proximal parts of limbs
Two-point discrimination is best on hands and face, worst on abdomen and proximal parts of limbs
WHY?
- Density of receptors is highest in skin areas with the best 2-point discrimination
- Surface area of sensory cortex is largest in regions to which receptors from skin areas with the best 2-point discrimination project
The conscious awareness of a stimulus is called a
sensation
A sensation combined with an understanding of its meaning is called
perception
Are both sensation and perception a result form processing in the cerebral cortex?
YES
Topographic Maps are within the
Sensory cortex
Topographic maps Project area that’s related to
functional importance ie. Lips, tongue,
thumb, fingers in humans and monkeys; snout, periodontal tissue in
rabbits & cats
Topographic Maps change according to use:
“plasticity” ie. Shoulder & arm areas
enlarge after below-elbow amputation.
- constantly change, sometimes within seconds: “dynamic plasticity”.
Descending Inhibition
Activity descending from higher centers in the brain and brainstem can “screen out” certain types of sensory
information by inhibiting neurons in the afferent pathway
Presynaptic inhibition:
Acts by reducing transmitter release at synapse between first-order and second-order sensory neurons
- inhibits specific sensation (pain)
- lasts several seconds
Postsynaptic inhibition:
- acts by hyperpolarizing memb. Of second order sensory neurons
- non-selective: reduces effect of all synaptic inputs
- lasts less then 1 millisecond
Analgesia
(Suppression of Pain)
- aspirin
- opioids
Activity in the analgesic system in brainstem can suppress pain by releasing
“endogenous” opiate neurotransmitters onto pain fibre endings in spinal cord eg. During fright/fight reactions
Projection neurons can also be inhibited by the firing of
large sensory afferents (eg. Those signalling touch, pressure, muscle stretch)
– Explains analgesia elicited during rapid movement and by acupuncture, rubbing the skin and transcutaneous electrical stimulation (TENS)
Re ferred Pain
• Convergence of visceral and somatic
afferents on ascending projection neurons in the spinal cord
• Regions of body surface where referred pain from visceral organs is perceived
Pain & temperature afferents synapse on 2nd-order interneurons in spinal cord that cross within 2 segments and ascend in the
spinothalamic (Anterolateral) tracts
To uch, pressure and stretch afferents from muscle & skin ascend in the dorsal columns. They contact 2 nd-order neurons in brainstem and then cross to
contralateral side
