Cellular neuroscience

Some neurons such as photoreceptor cells, for example, do not have myelinated axons that conduct action potentials.

These cells are only recently beginning to receive attention from neurobiologists for being involved not just in nourishment and support of neurons, but also in modulating synapses.

Following several local graded depolarizations of the membrane potential, the threshold of excitation is reached, voltage-gated sodium channels are activated, which leads to an influx of Na+ ions.

As Na+ ions enter the cell, the membrane potential is further depolarized, and more voltage-gated sodium channels are activated.

As the rising phase reaches its peak, voltage-gated Na+ channels are inactivated whereas voltage-gated K+ channels are activated, resulting in a net outward movement of K+ ions, which re-polarizes the membrane potential towards the resting membrane potential.

Repolarization of the membrane potential continues, resulting in an undershoot phase or absolute refractory period.

Nevertheless, as more voltage-gated K+ channels become inactivated, the membrane potential recovers to its normal resting steady state.

Electrical synapses are characterized by the formation of gap junctions that allow ions and other organic compound to instantaneously pass from one cell to another.

[2] Chemical synapses are characterized by the presynaptic release of neurotransmitters that diffuse across a synaptic cleft to bind with postsynaptic receptors.

Once bounded with Ca2+, the vesicles dock and fuse with the presynaptic membrane, and release neurotransmitters into the synaptic cleft by a process known as exocytosis.

Metabotropic receptors on the other hand activate second messenger cascade systems that result in the opening of ion channel located some place else on the same postsynaptic membrane.

Although slower than ionotropic receptors that function as on-and-off switches, metabotropic receptors have the advantage of changing the cell's responsiveness to ions and other metabolites, examples being gamma amino-butyric acid (inhibitory transmitter), glutamic acid (excitatory transmitter), dopamine, norepinephrine, epinephrine, melanin, serotonin, melatonin, endorphins, dynorphins, nociceptin, and substance P. Postsynaptic depolarizations can either transmit excitatory or inhibitory neurotransmitters.

"Current Clamp" is a common technique in electrophysiology. This is a whole cell current clamp recording of a neuron firing a train of action potentials due to it being depolarized by current injection
Illustration of the major elements in a prototypical synapse . Synapses are gaps between nerve cells . These cells convert their electrical impulses into bursts of neurochemical relayers, called neurotransmitters , which travel across the synapses to receptors on the dendrites of adjacent cells, thereby triggering further electrical impulses to travel down the latter cells.