Biology 101 - Nervous System Part 3

It is a region where one nerve cell adjoins another. Here, signals can pass between the two, generally from the axon of the presynaptic cell to the dendrite of the postsynaptic one. Synapses can be electrical or chemical.

Electrical synapses involve the direct transmission of a signal between cells connected by gap junctions. This type of synapse is prevalent in the heart. Chemical synapses are small gaps between cells. The presynaptic cell releases neurotransmitters in membrane-bound vesicles; these molecules cross the gap and bind to receptors on the postsynaptic cell.

Ca2+ Since this synapse is described as cholinergic, it must be a chemical synapse; more specifically, it must use acetylcholine as its neurotransmitter. All chemical synapses require Ca2+ for proper fusion of neurotransmitter-containing vesicles to the membrane of the axon terminal.

Electrical and chemical gradients combine to form the electrochemical gradient. The electrical potential is produced by the difference in charge across the membrane, while the chemical gradients are formed by differences in ion concentration between the two sides. Like any gradients, both of these are under pressure to dissipate.

-70 mV The resting potential is negative because more cations are present outside the cell than inside. Note that one mV, or millivolt, is one thousandth of a volt.

Outside the cell The large amounts of Na+ outside the membrane contribute to the relative negativity of the inside of the neuron.

Inside the cell Though K+ concentrations are much greater inside the neuron than outside, the distribution of other ions, especially Na+, causes the interior of the cell to be relatively negative.

The Na+/K+ ATPase, sometimes called the sodium-potassium pump, transfers three sodium ions out of the cell and two potassium ions in. Though the pump is present in other cell types, it is especially relevant in neurons, where it contributes to the high Na+ concentration outside the cell and the high K+ concentration inside.

active transport Active transport requires energy, generally in the form of ATP, and transports molecules or ions against their concentration gradients.

At values greater than (or in other words, less negative than) -65 mV. In the nervous system, depolarization is one step of an action potential. During this step, the membrane potential moves from -70 mV to values as high as +40 mV.

A neuron must reach a certain depolarization threshold to trigger an action potential. This threshold is around -55 mV in a normal cell. Neurons are commonly referred to as "all-or-none." In other words, a full action potential will occur when threshold is surpassed, but nothing will happen if that value is not reached.

repolarization Like its name implies, repolarization references the act of becoming "more polar." Specifically, after depolarization, the cell must repolarize to become negative again.

At values more negative than -65 mV. In the nervous system, hyperpolarization occurs as an "undershoot" after the repolarization phase of an action potential. During this step, the membrane potential briefly moves below its resting value of -70 mV.

It is an electrical event that allows a signal to propagate down the axon of a neuron. Action potentials involve a brief variation in membrane potential from its resting value of -70 mV. This process is facilitated by voltage-gated sodium and potassium channels.

The neuron begins at its resting potential with all channels closed. A stimulus, often neurotransmitter binding, triggers the opening of some Na+ channels. If this Na+ influx depolarizes the cell past its threshold, Na+ channels continue to open, further depolarizing the cell to a value of around +40 mV. K+ voltage-gated channels open; Na+ channels begin to close. The efflux (outflow) of K+ causes the cell to repolarize. This repolarization generally produces a hyperpolarized "undershoot" where the membrane potential dips below -70 mV. Leak channels and the Na+/K+ ATPase contribute to the neuron's return to resting potential.

Summation If the summed value is sufficient to overcome the postsynaptic cell's threshold, an action potential will be produced. Note that inputs can be either excitatory or inhibitory. Often, both types of signals are present at the same time.

spatial summation temporal summation Spatial summation occurs when multiple presynaptic cells synapse onto the same postsynaptic neuron. When these distinct cells fire in close succession, their signals can sum to produce an action potential. Temporal summation occurs when a single presynaptic cell synapses onto a postsynaptic neuron. When this cell fires multiple times in a row, its signals can sum to produce an action potential.

An EPSP, or excitatory postsynaptic potential, occurs when inputs to a neuron stimulate it to depolarize. An EPSP increases the likelihood of an action potential occurring. An IPSP, or inhibitory postsynaptic potential, occurs when inputs to a neuron cause it to hyperpolarize. An IPSP decreases the likelihood of an action potential occurring.

refractory period This happens during and immediately after an action potential, when another action potential is difficult or impossible to produce. Refractory periods can be either absolute, in which sodium channels are inactivated and no stimulus can trigger an action potential, or relative, in which a stronger stimulus than usual is necessary.

glutamate

GABA | (gamma-aminobutyric acid)