The nerve impulse

Action potentials are the means whereby information is passed from one neuron to an adjacent neuron. The balance between the excitatory and inhibitory impulses determines how many action potentials will reach the axonal terminal and, by releasing a specific type of neurotransmitter from the terminal, influence the adjacent neuron. Thus, in summary, chemical information in the form of small neurotransmitter molecules released from axonal terminals is responsible for changing the membrane potential at the synaptic junctions which may occur on the dendrites or directly on the cell body. The action potential then passes down the axon to initiate the release

Figure 2.1. A single neuron, represented by a dentate granule cell, receives numerous synaptic contacts which are either of excitatory or inhibitory type.

of the neurotransmitter from the axonal terminal and thereby pass information on to any adjacent neurons.

A summary of the neurotransmitters and neuromodulators that have been identified in the mammalian brain is given in Table 2.1. The term neuromodulator is applied to those substances that may be released with a transmitter but which do not produce a direct effect on a receptor; a neuromodulator seems to work by modifying the responsiveness of the receptor to the action of the transmitter.

The metabolic unity of the neuron requires that the same transmitter is released at all its synapses. This is known as Dale's Law (or principle) which Sir Henry Dale proposed in 1935. Dale's Law only applies to the presynaptic portion of the neuron, not the postsynaptic effects which the transmitter may have on other target neurons. For example, acetylcholine released at motor neuron terminals has an excitatory action at the motor neuron junction, whereas the same transmitter released at vagal nerve terminals has an inhibitory action on the heart.

In addition to the diversity of action of a single transmitter released from a neuron, it has become well established that among invertebrates up to

Table 2.1. Some of the neurotransmitters and neuromodulators that have been identified in the mammalian brain

Transmitter

Distribution in brain

Physiological

Involvement in CNS disease

Noradrenaline

Dopamine

5-Hydroxytryptamine

Acetylcholine

Adrenaline GABA

Glycine

Glutamate and aspartate

Most regions: long axons project from pons and brainstem

Most regions: short, medium and long axonal projections

Most regions: project from pons and brainstem

Most regions: long and short axonal projections from basal forebrain

Midbrain and brainstem

Supraspinal interneurons

Spinal interneurons; modulates NMDA amino acid receptors in brain

Long neurons oîj receptors - inhibitory

/?i receptors - inhibitory

P2 receptors - excitatory?

Dj/D5 receptors - stimulatory

D2 receptors - inhibitory

5-HT1A receptors - inhibitory

Mj receptors - excitatory

M2 receptors - inhibitory

N receptors - excitatory

Possibly same as for noradrenaline

A receptors - hyperpolarize membranes

(inhibitory) B receptors - inhibitory Hyperpolarize membranes Strych-sensitive receptors - inhibitory Strych-insensitive receptors - excitatory

Quisqualate - depolarizes membranes NMDA - depolarizes membranes Kainate - depolarizes membranes

Depression Mania

Schizophrenia ?Mania

Depression

?Schizophrenia

Anxiety

Dementias

?Mania

?Depression Anxiety

Seizures, epilepsy

?Seizures

Learning and memory

?Seizures

Seizures

?Schizophrenia

Substances with a neuromodulatory effect on brain neurotransmitters by direct actions of specific receptors that modify the actions of the transmitters listed include: prostaglandins, adenosine, enkephalins, substance P, cholecystokinin, endorphins, endogenous benzodiazepine receptor ligands, and possibly histamine. CNS, central nervous system. NMDA, N-methyl-D-aspartate. Strych, strychnine.

r1 tn

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