Psychology Neurons, Hormones, and the Brain Neurons: Cells of the Nervous System & Neurotransmitters
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Neurons: Cells of the Nervous System
There are two kinds of cells in the nervous system: glial cells and neurons. Glial cells, which make up the support structure of the nervous system, perform four functions:
- Provide structural support to the neurons
- Insulate neurons
- Nourish neurons
- Remove waste products
The other cells, neurons, act as the communicators of the nervous system. Neurons receive information, integrate it, and pass it along. They communicate with one another, with cells in the sensory organs, and with muscles and glands.
Each neuron has the same structure:
- Each neuron has a soma, or cell body, which is the central area of the neuron. It contains the nucleus and other structures common to all cells in the body, such as mitochondria.
- The highly branched fibers that reach out from the neuron are called dendritic trees. Each branch is called a dendrite. Dendrites receive information from other neurons or from sense organs.
- The single long fiber that extends from the neuron is called an axon. Axons send information to other neurons, to muscle cells, or to gland cells. What we call nerves are bundles of axons coming from many neurons.
- Some of these axons have a coating called the myelin sheath. Glial cells produce myelin, which is a fatty substance that protects the nerves. When an axon has a myelin sheath, nerve impulses travel faster down the axon. Nerve transmission can be impaired when myelin sheaths disintegrate.
- At the end of each axon lie bumps called terminal buttons. Terminal buttons release neurotransmitters, which are chemicals that can cross over to neighboring neurons and activate them. The junction between an axon of one neuron and the cell body or dendrite of a neighboring neuron is called a synapse.
Neurons: Cells of the Nervous System[/caption]
Role of Myelin
People with multiple sclerosis have difficulty with muscle control because the myelin around their axons has disintegrated. Another disease, poliomyelitis, commonly called “polio,” also damages myelin and can lead to paralysis.
Communication Between Neurons
In 1952, physiologists Alan Hodgkin and Andrew Huxley made some important discoveries about how neurons transmit information. They studied giant squid, whose neurons have giant axons. By putting tiny electrodes inside these axons, Hodgkin and Huxley found that nerve impulses are really electrochemical reactions.
The Resting Potential
Nerves are specially built to transmit electrochemical signals. Fluids exist both inside and outside neurons. These fluids contain positively and negatively charged atoms and molecules called ions. Positively charged sodium and potassium ions and negatively charged chloride ions constantly cross into and out of neurons, across cell membranes. An inactive neuron is in the resting state. In the resting state, the inside of a neuron has a slightly higher concentration of negatively charged ions than the outside does. This situation creates a slight negative charge inside the neuron, which acts as a store of potential energy called the resting potential. The resting potential of a neuron is about –70 millivolts.
The Action Potential
When something stimulates a neuron, gates, or channels, in the cell membrane open up, letting in positively charged sodium ions. For a limited time, there are more positively charged ions inside than in the resting state. This creates an action potential, which is a short-lived change in electric charge inside the neuron. The action potential zooms quickly down an axon. Channels in the membrane close, and no more sodium ions can enter. After they open and close, the channels remain closed for a while. During the period when the channels remain closed, the neuron can’t send impulses. This short period of time is called the absolute refractory period, and it lasts about 1–2 milliseconds. The absolute refractory period is the period during which a neuron lies dormant after an action potential has been completed.
The All-or-None Law
Neural impulses conform to the all-or-none law, which means that a neuron either fires and generates an action potential, or it doesn’t. Neural impulses are always the same strength—weak stimuli don’t produce weak impulses. If stimulation reaches a certain threshold, or minimum level, the neuron fires and sends an impulse. If stimulation doesn’t reach that threshold, the neuron simply doesn’t fire. Stronger stimuli do not send stronger impulses, but they do send impulses at a faster rate.
The Synapse
The gap between two cells at a synapse is called the synaptic cleft. The signal-sending cell is called the presynaptic neuron, and the signal-receiving cell is called the postsynaptic neuron.
Neurotransmitters are the chemicals that allow neurons to communicate with each other. These chemicals are kept in synaptic vesicles, which are small sacs inside the terminal buttons. When an action potential reaches the terminal buttons, which are at the ends of axons, neurotransmitter-filled synaptic vesicles fuse with the presynaptic cell membrane. As a result, neurotransmitter molecules pour into the synaptic cleft. When they reach the postsynaptic cell, neurotransmitter molecules attach to matching receptor sites. Neurotransmitters work in much the same way as keys. They attach only to specific receptors, just as certain keys fit only certain locks.
When a neurotransmitter molecule links up with a receptor molecule, there’s a voltage change, called a postsynaptic potential (PSP), at the receptor site. Receptor sites on the postsynaptic cell can be excitatory or inhibitory:
Neurotransmitters
So far, researchers have discovered about 15–20 different neurotransmitters, and new ones are still being identified. The nervous system communicates accurately because there are so many neurotransmitters and because neurotransmitters work only at matching receptor sites. Different neurotransmitters do different things.
| Neurotransmitter | Major functions | Excess is associated with | Deficiency is associated with |
| Acetylcholine | Muscle movement, attention, arousal, memory, emotion | Alzheimer’s disease | |
| Dopamine | Voluntary movement, learning, memory, emotion | Schizophrenia | Parkinsonism |
| Serotonin | Sleep, wakefulness, appetite, mood, aggression, impulsivity, sensory perception, temperature regulation, pain suppression | Depression | |
| Endorphins | Pain relief, pleasure | ||
| Norepinephrine | Learning, memory, dreaming, awakening, emotion, stress-related increase in heart rate, stress-related slowing of digestive processes | Depression | |
| GABA | Main inhibitory neurotransmitter in the brain | ||
| Glutamate | Main excitatory neurotransmitter in the brain | Multiple sclerosis |
Agonists and Antagonists
Agonists are chemicals that mimic the action of a particular neurotransmitter. They bind to receptors and generate postsynaptic potentials.
Nicotine and Receptors
Nicotine is an acetylcholine agonist, which means that it mimics acetylcholine closely enough to compete for acetylcholine receptors. When both nicotine and acetylcholine attach to a receptor site, the nerve fibers become highly stimulated, producing a feeling of alertness and elation.
Antagonists are chemicals that block the action of a particular neurotransmitter. They bind to receptors but can’t produce postsynaptic potentials. Because they occupy the receptor site, they prevent neurotransmitters from acting.
Paralysis and Poison Arrows
Curare is a drug that causes paralysis. As an acetylcholine antagonist, it binds to acetylcholine receptors at nerve-muscle junctions, preventing communication between nerves and muscles. Doctors sometimes use curare to immobilize patients during extremely delicate surgery. South American tribes have long used curare as an arrow poison.
