|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,
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).
causes the action potential to propagate down the axon
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,
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).
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.
|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).
zone of one neuron at the synaptic junction with the input zone
of the second neuron.
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,
molecules diffusing across the membrane
protein receiving the neurotransmitter molecule which allows sodium
to cross the membrane