Impulse Transmission 2

Impluse Transmission

Nerve impulses are transmitted through the body by neurons. When a neuron is at rest (not transmitting an action potential) it maintains a voltage difference across its plasma membrane; the cytoplasmic fluid next to the membrane is negatively charged in comparison to the interstitial fluid outside the membrane. This voltage difference is called the resting membrane potential (4, 18).

Picture adapted from Starr and Taggart

Sodium-Potassium Pump: maintains the resting membrane potential across the neuron plasma membrane. A. Sodium leaks into the neuron by diffusion; B. Sodium is actively pumped out of the cell through channel proteins by active transport; C. Potassim is actively pumped into the neuron through channel proteins; D. Potassium leaks into the neuron by diffusion; E. Potassium leaks out of the neuron by diffusion; F. Carrier protein that serves as the sodium-potassium pump to actively transport sodium and potassium into and out of the cell to maintain the membrane potential; G. Outside the neuron; H. The plasma membrane; I. Inside the neuron (31)

Signals arise in the neuron's trigger zone and when they reach the neuron's input zone, usually the dendrite, the neuron is stimulated by an action potential: a very brief voltage reversal across the plasma membrane. An action potential can arise, however only if the voltage reversal reaches the threshold level. Once the threshold level is reached in one area of the neuron, the action potential triggers the voltage reversal at an adjacent area of membrane, making the action potential self-propagate along the neuron (3, 4, 6, 18).

Picture adapted from Starr and Taggart

A disturbance causes the action potential to propagate down the axon

The disturbance at a. creates another action potential at b. and c. This distrubance causes sodium gates at the nodes to open, and sodium flows in to create another action potential.

Action potentials travel rapidly along the axons of both sensory and motor neurons due to the myelin sheaths that surrounds them (6, 18, 31). The myelin sheath is made by oligodendrocytes and it is important because it insulates the axons and greatly increases the speed of neural transmission (2).

Picture provided by The National Institutes of Health

A. Synapses coming from different nerve cells B. Axon C. Synaptic vessicles that contain neurotransmitter molecules D. Synapse E. Synaptic Cleft F. Neurotransmitter receptors on post-synaptic cell membrane G. synaptic vessicle releasing neurotransmitter H. Node of Ranvier I. Myelin Sheath J. Oligodendrocyte K. nucleus L. nerve cell body M. dendrites

Small gaps called nodes of Ranvier separate the cells, and the action potential jumps from node to node making neural transmission very fast. As the action potential reaches the output zone, gated channels for calcium ions, which extend across the membrane, open. The ions travel down the concentration gradient, which induces synaptic vesicles containing neurotransmitters to fuse with the membrane (4).

Picture provided by The National Institutes of Health

The output zone of one neuron at the synaptic junction with the input zone of the second neuron.

A. mitochondrion B. axon terminal C. synaptic vesicle releasing neurotransmitter molecules D. synaptic vessicles containing neurotransmitter molecules E. synapse F. synaptic cleft G.Neurotransmitter receptors on the post synaptic cell membrane

The neurotransmitter molecules diffuse across the synaptic cleft and bind with receptor proteins on the adjoining post synaptic cell. Binding causes channels to open through the protein allowing ions to cross the plasma membrane. Depending on the type of neurotransmitter and the post-synaptic cell, the neurotransmitter may have excitatory or inhibitory effects (3, 6, 31)

picture adapted from Starr and Taggart

neurotransmitter molecules diffusing across the membrane

receptor protein receiving the neurotransmitter molecule which allows sodium to cross the membrane