Neuro Definitions Flashcards
Nerve Cell (neurone)
a highly specialised cell that generates and transmits nerve impulses (action potentials) as part of a neural circuit.
Glial cell
supporting cells of the nervous system, of which there are three types; astrocytes, oligodendrocytes and neurolemmocytes. Microglial cells are immune cells rather than true glia.
Receptor
has multiple meanings, especially in sensory neuroscience:
a) protein complex that binds to and is activated by a signalling molecule
b) nerve cell, often highly specialised, that is adapted to detect and respond to an external (ie external to the receptor) event, converting the energy in that event into a change in membrane potential (the receptor potential / generator potential)
Primary afferent
first cell in the chain that carries a sensory signal from the sensory apparatus to the cerebral cortex. In some systems the receptive apparatus is part of the primary afferent, but in the “special senses” the receptors are separate and highly specialised nerve cells.
Threshold
has multiple meanings, especially in sensory neuroscience:
a) action potential threshold, which is the level of depolarisation necessary to allow voltage-gated sodium channels to open and hence trigger an action potential. Roughly -50mV.
b) minimum sensory input necessary to activate a sensory receptor, and hence potentially generate a sensation. Different types of receptor have varying sensitivity and hence activation threshold (NB action potential threshold does not vary).
c) perceptual threshold, which is the minimum sensory input needed to generate conscious perception of the stimulus. Depends not only on the strength of activation of the receptors, but also the attention of the subject (see modulatory systems lecture).
Sensitivity
the relative responsiveness of a sensory system (or component thereof) to stimulation. Higher sensitivity correlates with lower response thresholds, larger response magnitude to a given stimulus, and a tendency to saturate at lower stimulus strengths.
Specificity
a measure of how finely a system (or component thereof) differentiates between similar stimuli.
Receptor adaptation
in sensory physiology this refers to a common feature of receptor cells, which respond strongly to changes in stimulus strength but reset their membrane potentials back to rest if a stimulus remains constant. Some adapt very rapidly (eg Pacinian corpuscles in the skin, which only respond repeatedly to rapid vibration), others very slowly (eg cone photoreceptors adapt over several seconds). C fibre nociceptors are probably the only ones that don’t obviously adapt. Adaptation damps down neural responses to homogeneous stimuli while allowing small differences in stimulus strength to be signaled strongly. This extends dynamic range and as well as saving energy.
Lateral inhibition
a key mechanism in sensory neurophysiology, typified by inhibition of one part of a sensory pathway by excitation of a neighbouring part. It is seen for example in the somatosensory system, where the cells carrying information from neighbouring patches of skin inhibit on another, and in the visual system where cells carrying information from neighbouring patches of retina inhibit one another. Less obviously, cells carrying information about similar qualities rather than location (eg sound frequencies) may inhibit one another. In each case the inhibition damps down neural responses to homogeneous stimuli while allowing small differences in stimulus strength to be signaled strongly. This inhibition extends dynamic range as well as saving energy.
Dynamic range
the range of stimulus strengths over which a sensory system can respond. A single receptor type without the ability to adapt would either have to have very poor sensitivity (it could only afford to respond weakly, even to big jumps in stimulus strength) or a very limited response range (if it responded strongly to even small changes in stimulus strength, it would rapidly reach its maximum response magnitude and be unable to signal changes in stimulus strength beyond that point). Sensory systems overcome this by 1) having receptors that vary in sensitivity and 2) through adaptation of those receptors to extend their individual dynamic ranges.
Saturation
point at which a sensory system (or component thereof) has reached its maximum response magnitude, beyond which it cannot signal increases in stimulus strength or differentiate between stimuli that differ in strength
Synaptic plasticity
the changes in the strength and effectiveness of synapses in response to patterns of use. The best understood mechanism is long-term potentiation (LTP), which increases the strength of fast excitatory (ionotropic) synapse by increasing transmitter availability, receptor numbers and receptor effectiveness. The trigger for LTP is that the synapse releases glutamate and immediately afterwards the postsynaptic cell strongly depolarises. The strong depolarisation indicates that many other synapses activated simultaneously, suggesting that they are all involved in a useful circuit.
Specific circuit / pathway
neural machinery performing “computations” linked to sensory, motor and cognitive functions.
Modulatory pathway
neural inputs that adjust how a cell responds to its specific inputs, which control the sleep-wake cycle, attention and concentration, mood and some emotional states. Related inputs are essential to the function of some motor and cognitive pathways.
