Nerve cells

The biology of nerve cells:

• The neuron doctrine - the neuron is the fundamental building block and elementary signaling unit of the brain.
• The ionic-hypothesis - focuses on the transmission of information within the nerve cell. It describes the mechanisms whereby individual nerve cells generate electrical signals, called action potentials, that can propagate over a considerable distance within a given nerve cell.

The chemical theory of synaptic transmission - focuses on the transmission of information between nerve cells. It describes how one nerve cell communicates with another by releasing a chemical signal called a neurotransmitter. The second cell recognizes the signal and responds by means of a specific molecule in its surface membrane called a receptor.

Despite their complex shape, nerve cells are single, coherent entities. The fine processes surrounding them are not independent but emanate directly from the cell body. The entire nerve cell, including the processes, is fully enclosed by a surface membrane, consistent with the cell theory.

Cajal went on to distinguish the following parts of the neuron:

• a nucleus
• a single axon - emerges at one end of the cell body and can extend up to several feet. Often splits into one or more branches along its length, with many tiny axon terminals at the end of each branch. Send information to other cells
• dendrites - emerge on the opposite side of the cell body. They branch extensively (up to 40 branches in the human brain), forming a treelike structure that grows out from the cell body and spreads over a large area. Receives signals from other nerve cells.
• synapses - sites of connection between an axon and a dendrite. Communication across them has three components
  • The presynaptic terminal of the axon that sends a signal
  • The synaptic cleft or gap
  • The post-synaptic site on the dendrite that receives the signal

Typically, a single neuron makes contact through its many presynaptic terminals with the dendrites of many target cells and can disseminate the information it receives widely to different target neurons, sometimes located in different regions of the brain.

Conversely, the dendrites of a target nerve cell can receive information from the presynaptic terminals of a number of different neurons and thus integrate information from a number of different neurons, even those located in different areas of the brain

There are three major classes of neurons:

• Sensory neurons - which are located in the skin and in various sense organs, respond to a specific type of stimulus from the outside world and send this information to the brain.
• Motor neurons send their axons out of the brain stem and spinal cord to effector cells, such as muscle and gland cells, and control the activity of those cells
• Interneurons - the most numerous class of neurons in the brain, serve as relays between sensory and motor neurons

Four phases of nerve cell signaling:

• First phase - 1791, Galvani discovers electrical activity in animals - show that nervous activity has a scientific basis. In 1859, Helmholtz showed that electricity in an axon travels much slower than in a copper wire (up to 90 feet/second vs 186,000 miles/sec), but their strength does not decrease as they move.
• Second phase - 1920s, Adrian looks at the form of the signal and its role in encoding information. It is called an "action potential" and is a constant, all-or-none signal. The intensity results from the frequency
• Third phase - Bernstein and the membrane hypothesis
• Fourth phase - Hodgkin and Huxley and the ionic hypothesis

Excitation and inhibition The major excitatory neurotransmitter in the brain is the amino acid glutamate, while the main inhibitory transmitter is the amino acid GABA (gamma-aminobutyric acid). A variety of tranquilizing drugs - benzodiazepines, barbiturates, alcohol, and general anesthetics - bind to GABA receptors and produce a calming effect on behavior by enhancing the receptors' inhibitory function.

We now know that serotonin acts on as many as eighteen different types of receptors throughout the brain and that LSD seems to produce its hallucinatory action by simulating one of these receptors, located in the frontal lobe of the brain.