Neuronal Signaling

Signals which transmit information among nerve cells consist of electrical potential changes produced by ionic currents flowing across their surface membranes. The currents are carried by ions such as sodium, potassium, calcium, and chloride and occur due to the opening and closing of membrane protein ion channels. The nerve maintains an electrical polarization across its membrane by actively pumping sodium ions out, and potassium ions in. Thus when the ion channels are opened (by voltage change, neurotransmitters, drugs, etc.) sodium and potassium rapidly flow through the channel creating a depolarization. Depending on the spatial location and temporal sequence of channels, activation can result in waves used as signals.

Neurons carry only two obvious types of signals: localized "gated" potentials which are older on the evolutionary scale and analog, and propagating "all or none" action potentials which are newer and digital. Localized gated potentials can spread only one to two millimeters, are attenuated and distorted by local resistivity, and are essential where spatial and temporal summation ("integration") is required. This occurs at sensory nerve endings ("receptor potentials"), neuronal synaptic junctions where both excitatory and inhibitory potentials are integrated ("synaptic potentials"), and as slow waves arising as rhythmic depolarizations in dendrites. Gated potentials in dendrites are also integrated at the cell body to initiate (when appropriate) propagating action potentials along axons. A primary role for localized dendritic potentials in cerebral neuron information processing has been emphasized by several authors including Alwyn Scott (1977) and Ross Adey (1966) who feel dendritic slow wave potentials allow cerebral neurons to "whisper together."

Action potentials ("nerve impulses") propagate as membrane depolarization waves along axons. They occur due to sequential opening of membrane channels which allow passive diffusion of ions. Gaps in myelinization ("nodes of Ranvier") along axons contain abundant ion channels so that impulses propagate rapidly between nodes where they are slowed and susceptible to modulation ("saltatory conduction"). With this exception, action potentials occur on an "all or none" basis (i.e. digital) from integration of dendritic input (i.e. analog) at the cell body region of the neuron. The frequency of firing is related to the stimulus intensity; a sensory nerve responding to muscle stretch fires at a rate proportional to the degree of stretch. Action potential velocity is fixed for given axons dependent on axon diameter, degree of myelinization, and distribution of ion channels. Typical action potential velocities of about 100 meters per second allow effective communication within relatively large nervous systems.

0 0

Post a comment