Neurons And The Action Potential

Electrical and chemical

Chemical signals received in dendrites from axons that contact them are transformed into electrical signals, which add to or subtract from electrical signals from all other synapses, thus making a decision about whether to pass on the signal somewhere else. Electrical potentials then travel down axons to synapses on the dendrites of the next neuron and the process repeats

Dendrites, cell body, axons and synaptic terminals

Dendrite received, cell-body integrates, axons transmit through polarisation

The outer membrane of neurons which are made of fatty substances are arranged around a cytoskeleton that is built up of rods of tubular and filamentous proteins that extend out into dendrites and axons

Dendrites change shapes, sprouting new connections and withdrawing others while axons grow new endings

Proteins are mostly manufactured in the cell body and are transported along the cytoskeleton

Dendritic spines

It is where incoming axons make most of their connections

Molecules called growth factors

Growth factors are taken inside the neuron and transported to cell body where they influence the expression of neuronal genes and hence the manufacture of new proteins, these enable neuron to grow longer dendrites or make other dynamic changes to its shape or function

Small gap where dendrites have close contacts with incoming axons of other cells

Dendritic spines

Synaptic transmission

When dendrite receives one of the chemical messengers that has been fired across the gap separating it from sending axon, miniature electrical currents are set up inside the receiving dendritic spines. All these positive and negative waves of current are accumulated in the dendrites and spread down the cell body. If they don’t add up to much activity, currents soon die and nothing happens. If currents add up to a value that crosses a threshold, the neuron will send a message on to other neurons

Currents that come into the cell

Currents that move out of the cell

Axons of neurons transmit electrical pulses called action potentials and travel along nerve fibres. Axonal membrane contains ion channels that open and close to let through electrically charged ions. Some channels let through sodium ions (Na+) while others let through potassium ions (K+). When channels open, the Na+ or K+ ions flow down opposing chemical and electrical gradients, in and out of the cell in response to electrical depolarisation of the membrane

The Na+ channel opens, and Na+ ions moves into a cell, quickly, the transmembrane voltage switches by about 100mV; it flips from inside membrane voltage that is -70mV to one that is positive +30mV. This switch opens K+ channels, triggering a pulse of K+ to flow out of the cell which causes the membrane potential to swing back to its original negative value on the inside

Concentrations of Na+ and K+ within the cytoplasm do not change significantly during an action potential, however in the long run, they are kept in balance by ion pumps which remove excess Na+ ions

Nerve fibres behave like electrical conductors and action potential generated at one point creates another gradient of voltage between the active and resting membranes adjacent to it, the action potential is actively propelled in a wave of depolarisation that spreads from one end of nerve fibre to the other

After a very brief period of silence, the refractory period, the spent membrane recovers its explosive capability, readying the axon membrane for the next action potential

Placing tiny electrodes inside very large neurons and axons in certain sea creatures to measure changing electrical voltage

Patch-clamping