A Channelopathy Is A Medical Condition In Which The Form

Chapter 3 A channelopathy is a medical condition in which the form and function of ion channels is altered as a result of a mutation. Batrachotoxin: a toxin, produced by poison arrow frogs, that selectively interferes with Na+ channel (forcing it to stay open).

Characteristic

Electrical synapse

Chemical synapse

Width of synaptic cleft

2 to 4 nm

20 to 40 nm

Molecules that pass from presynaptic cell to postsynaptic cell

ions

transmitters

Synaptic delay

0 ms

0.5 to 1.0 ms

Electrical synapses are found where conduction must occur very rapidly, such as in circuits involved in escape behavior, and also in regions where a large number of fibers must be activated simultaneously, such as in the oculomotor system. Electrical synapses feature special large ion channels lined up on both sides of the synapses, which allow ions to flow directly from the presynaptic cell into the postsynaptic cell across a very small cleft. Transmission at these synapses resembles the conduction along a cell's axon. Glia can increase the overall strength of the postsynaptic potentials, perhaps by preventing neurotransmitter leakage from synapses. Imagine that a single vesicle discharges its transmitter into the cleft, followed a short time later (after the transmitter has been cleared from the synapse) by the release of transmitter from a second vesicle. Compared to the size of the effect that the first vesicle has on the postsynaptic potential, the size of the second vesicle's effect is identical, because each vesicle contains about the same number of molecules of transmitter. The nicotinic ACh receptor is a(n) ligand-gated ion channel consisting of 5 protein subunits, of which 2 are identical and contain Ach-binding sites; 2 such sites must be bound in order for the channel to open. Life span events, daily and seasonal rhythms, and the use of drugs can all affect the numbers of specific receptors in the brain. Use of such a second messenger may allow for amplification of the effect of the transmitter (e.g., many channels may open, or long-lasting biochemical changes in the neuron may occur). The basic neural chain forming the knee jerk reflex has three neural components: a sensory neuron, a motor neuron, and a single synapse between the two. Thousands of such circuits work in parallel to produce the jerk response to a hammer tap—which has a latency of just 40 ms ERP stands for event-related potential; it reflects the gross changes in electrical potentials provoked by discrete sensory stimuli. Hence the activity it measures is evoked. A particularly important clinical application of ERP techniques is the assessment of a subject's hearing via the “auditoryevoked brainstem potential”; this is especially useful in very young children. A generalized seizure involves loss of consciousness and symmetrical involvement of the body musculature; two common forms are grand mal and petit mal seizures.