CNS - Lecture 3 Flashcards
What is localization
determining the position of a stimulus on or within the body
explain the location and transmission of modality-specific signals
travels along sensory axons in specific spinal cord tracts towards the brain
The ability to locate a stimulus depends on:
the size and the density of the receptive fields of the receptors
lateral inhibition
enhances spatial acuity within the CNS by refining sensory information in afferent neurons (like making an image sharper)
Acuity is
the ability to differentiate between closely spaced stimuli
How does spatial acuity work
ascending sensory signals are focused by lateral inhibition within the CNS
process and outcome of ascending sensory axons that cross the midline in the brain
process: ascending sensory axons synapse on brainstem neurons that cross the midline
outcome: conveys sensory information from the right side of the body to the left side of the brain & vice versa
what is two-point discrimination
the ability to distinguish between two closely spaced stimuli
small receptive fields and high two-point discrimination/acuity are a result of
tightly packed receptors
large receptive fields and low two-point discrimination/acuity are a result of
spaced receptors
what is the relationship between receptive field size and acuity
receptive field size determines sensory acuity or two-point discrimination
what are receptive fields
specific areas or regions of the body that, when stimulated, activate a particular sensory neuron or receptor
overlapping stimulation between neighbouring receptive fields provides
general information about the location of a stimulus
what is divergence
each sensory afferent sends branches to many neurons in the CNS
see document for diagram
what is convergence
a given neuron in the CNS receives inputs from many sensory afferents
see document for diagram
how does lateral inhibition allow the location of a stimulus to be perceived more accurately?
focuses the activation of the neurons on the centre of a stimulus
describe firing rate profile in the presence of lateral inhibition
narrower firing rate profile due to lateral inhibition
two-point discrimination is best and worst where?
best: hands and the face
worst: abdomen and parts of the limbs
why is two-point discrimination best on hands and face
- there is a higher density of receptors
- more surface area of sensory cortex is devoted to sensory input from the face and hands
sensation
the conscious awareness of a stimulus
perception
when a sensation is combined with an understanding of it’s meaning
sensation and perception result from processing in the
cerebral cortex
what parts of the body is the somatosensory cortex largely devoted to receiving input from? which parts not as much?
- face and hands
- abdomen, legs and feet
what does a sensory topographic map show
which areas of the skin are connected to specific regions in the brain.
what is projection area
a specific region in the brain that receives and processes information from a particular sensory organ
projection area of any given organ in the sensory cortex is related to
functional importance
plasticity
topographic maps change according to usage
dynamic plasticity means
the maps constantly change, sometimes within seconds
what is descending inhibition
activity descending from higher centers in the brain and brainstem that can block out certain types of sensory information by inhibiting neurons in the afferent pathway
how does presynaptic inhibition act
by reducing transmitter release at synapse between first and second order sensory neurons. inhibits specific sensations
how does postsynaptic inhibition act
acts by hyperpolarizing second order membranes to move their potential further away from threshold
how long does postsynaptic inhibition last?
very short, less than a ms
what mediators are released when tissue is damaged, what do they do?
prostaglandins and histamine - activate the pain receptors
pain receptors fire _______ _________ which travel to the _______ part of the spinal cord
action potentials
dorsal
what releases substance P and what does it do
- activated pain fibres in the spinal cord
- activates interneurons called projection neurons
what do projection neurons do
signal pain (pain sensation)
what is analgesia
suppression of pain transmission
how does aspirin work
blocks production of prostaglandins released by damaged tissue
what does gabapentin do
blocks conduction of nociceptors
how do opioids (morphine) work
block the release of substance P onto projection neurons in spinal cord
What neurons release endogenous opiate neurotransmitters
Neurons in the brainstem and reticular formation
What are the functions of endogenous opiate neurotransmitters
- reduce the release of substance P (reduce activity in the projection neurons that signal pain to the thalamus and the cortex)
- gets released during fright, flight and flight reactions to reduce pain (nociceptive input)
Why is pain from the viscera and internal organs interpreted by the brain as pain on the skin
Sensory afferents from the vicera enter the spinal cord and synapse onto the same interneurons as sensory afferents from the skin
in the somatosensory system, pathways that mediate pain and temperature are different than those that mediate
touch pressure and muscle stretch (mechanoreceptive)
What afferents are involved in the Anterolateral system
Pain and temperature
What afferents are involved in the Dorsal column system
Touch, pressure and stretch
** See document for the processes of neuron transmission in the somatosensory system
