Topic 22: COMMUNICATION BETWEEN CELLS - NEURONS & NERVES (lecture 34)
OBJECTIVES:
1. Be able to describe the relative concentrations of Na +, K+, Cl- and organic anions
between the inside and the outside of neuronal membranes.
2. What is the membrane potential and how is it maintained?
3. Be able to describe an action potential and the role of voltage gated ion channels in
this process.
4. Know how an action potential is propagated in unmyelinated and myelinated axons.
5. Understand how acetyl choline is released and its impact on the post-synaptic cell.
6. Understand the electrical effects of an excitatory vs. an inhibitory neurotransmitter.
Communication by hormone is slow due to the fact that the hormone must be
transported, it must interact with a receptor on the target cell and then the biological
effect takes place. Most complex organisms have nervous systems which transport
information very rapidly.
Neuron- the fundamental cellular component of the nervous system (fig. 48.2); each
neuron consists of dendrite, soma (cell body), axon hillock, long axon, terminal
branches and axon termini. Neurons function by carrying waves of electrical excitation;
this excitation is typically unidirectional; soma  synaptic termini.
(note: neurons are usually bundled together to form nerves)
There are a number of types of neurons/nerves
1. sensory neurons- carry information away from receptors (receptors = structures that
detect changes in the internal/external environment; ca., light, sound, heat, blood
pressure)
2. motor neurons- carry excitation to skeletal muscle
3. interneurons- connect neurons to each other
4. autonomic neurons- are involved in control of the functioning of internal organs (two
kinds- sympathetic & parasympathetic)
Fig. 48.3- gives an example of a neural pathway indicating importance of speed of
communication.
Voltage- electrical term which defines the extent to which there is a difference in charge
between two places; actually referred to as potential difference. In the cases of cells
there is a surplus of negative charges on the inside of the cell relative to the outside.
This potential difference is measured as a very small voltage (see fig. 48.6) and is
known as the membrane potential ( membrane potential = the voltage measured
across the plasma membrane of a cell).
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Virtually every cell has a small membrane potential. However, excitable cells like
neurons and muscle cells (fibers) have a relatively large membrane potential and this
membrane potential changes during excitation.
What makes neurons negatively charged on the inside? Fig. 48.7.
There is an asymmetry of ion distribution between the inside and the outside of the cellmore K+ in than out; more Na+ out than in; more Cl- out than in; inside there are also
negatively charged (anions) organic molecules which are not present on the outside.
1. membrane is more permeable to K+ than Na+
2. K+ flows out down its concentration gradient
3. As it flows out, the inside becomes negatively charged because of anions left
behind
4. The Na+-K+ ATPase (pump) maintains this ion asymmetry by pumping K+ back in
and Na+ out
Neurons are excitable cells because the permeability of their membranes to inorganic
ions can change and these changes may profoundly impact the membrane potential.
Membranes contain protein/protein-complexes known as ion channels that allow
inorganic ions to pass through the membrane. There are three (3) basic kinds:
1. passive channel- always open to ion movement
2. electrically-gated - permeability is controlled by the membrane potential of the cell
3. chemically-gated - permeability is controlled by small molecular weight signaling
molecules known as neurotransmitters
Most ion channels are highly selective for the ion that each transports; thus, there are
potassium, sodium, chloride, calcium etc channels. Also, for each ion there may be
passive, voltage-gated & chemically-gated channels ( ca., voltage-gated K+channel)
Excitation (fig. 48.8)- a neuron receives some kind of stimulus (chemical, electrical,
mechanical) usually in the dendritic region or soma. This causes the membrane
potential to become less negative (called a depolarization). If this depolarization
reaches a certain critical level called threshold, rapid changes take place in the
membrane known as an action potential.
Action potential- a transient reversal of the membrane potential (inside becomes more
positive than outside) that is transferred down the length of the neuron.
Fig. 48.9- resting state; voltage-gated ion channels closed
1. rising (depolarizing) phase- once threshold has been reached, voltage-gated Na+
channels open, Na+ flows in and cell depolarizes.
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2. repolarizing phase- voltage-gated Na+ channels close, voltage-gated K+ channels
open, K+ flows out and cell becomes more negative (membrane potential moves
towards original state)
3. undershoot- voltage-gated K+ channels still open so membrane potential is more
negative than normal (it is hyperpolarized)
4. duration of action potential for neurons is very short (3-10 msec, 3-10 x 10-3 sec!)
Propagated action potential- (fig. 48.10); action potential literally travels from one region
of the membrane to adjacent regions. In vertebrates most axons are covered with lipid
insulation (myelin) with gaps of exposed membrane (fig. 48.11). Action potentials only
take place in the region of exposed membrane. This conduction of action potentials is
known as saltatory conduction. The velocity of conduction of of action potentials varies
with the diameter of the axon and whether it is myelinated or not.
Conduction velocity (m/sec) in selected neurons :
Squid giant axon – 25
Large motor axon to leg muscle in a mammal- 120
Synaptic transmission.
Once the action potential reaches the synaptic terminal, adjacent cells (other neurons,
muscle cells, endocrine cells etc) are impacted by the process of synaptic transmission
(synapse = the junction between a neuron and another cell); two fundamental kinds of
synaptic transmission:
1. electrical – neuron (pre-synaptic cell) is in direct contact with the post-synaptic cell;
depolarization of action potential literally spreads to the post-synaptic cell; not very
common
2. chemical- the neuron releases a neurotransmitter which diffuses across the space
between the two cells (synaptic cleft) and causes some change in the behavior of
the post-synaptic cell.
Fig. 48.12
Table 48.1- neurotransmitters (NT)- acetyl choline, biogenic amines, amino acids and
neuropeptides
Excitatory NT’s produce depolarizations of the post-synaptic membrane (move
membrane potential closer to threshold) while inhibitory NT’s typically produce
hyperpolarizations of the post-synaptic membrane (move membrane potential further
away from threshold).
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Chemical synapses are targets of a variety of drugs, poisons and pharmacological
agents.
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