Neurons: The Brain's Building Blocks

Neurons: Building Blocks of the Nervous System

• The brain is primarily composed of neurons, with tens of billions of them.
• Neurons are the building blocks of the brain and are believed to be the principal cellular elements of intelligence.

The Neuron

• A neuron consists of three basic structures: the cell body, axon, and one or more dendrites.
• Dendrites receive sensory input signals and information from other neurons.
• The cell body determines responses to message inputs.
• The axon carries messages away from the neuron and is insulated with a myelin sheath.

Neuron Structures: Myelin Sheath

• The myelin sheath helps transmit information and is produced by glial cells.
• Damage to the myelin sheath can affect transmission of electrical impulses along the axon.
• Multiple sclerosis is a disease that results from progressive deterioration of the myelin sheath, leading to jerky, uncoordinated movement.

Glial Cells

• Glial cells form the myelin sheath and perform basic housekeeping functions, such as cleaning up cellular debris.
• They also form the blood-brain barrier, which prevents certain substances in the bloodstream from reaching the brain.

Neuron Structures

• The axon divides into several small branches with small bulbs at the end, known as axon terminals.
• These closely approach but do not actually touch other cells.
• The region where the axon terminals of a neuron closely approach other cells is known as the synapse.

Communication Within Neurons

• There are different negatively and positively charged ions in and outside of the cell.
• The inside of the cell has a slight negative charge in relation to the outside of the cell, resulting in a resting potential of -70 millivolts.
• When positive ions enter the cell body, the cell body works to push out the positive ions while retaining the negative ions.
• Graded potentials are a change in the electrical potential on the membrane of a nerve cell in response to a stimulus.
• If the graded potential exceeds the threshold of the neuron, a complex biochemical change will occur, and an action potential will be generated.

Action Potential

• During an action potential, the cell membrane becomes more easily accessible, and positive ions enter the cell.
• The inside of the cell becomes more positively charged than the outside of the cell.
• The change in electrical potential across the cell membrane moves quickly along the neuron.
• The action potential is the basis of everything we sense, think, or do.

Hyperpolarization

• Hyperpolarization occurs when the membrane potential becomes more negative at a particular spot on the neuron's membrane.
• Depolarization is when the membrane potential becomes less negative (more positive).

Communication Between Neurons

• When a neuron fires, the action potential travels along the membrane of the axon to the axon terminals.
• Inside the terminal buttons are many structures known as synaptic vesicles.
• The action potential causes these vesicles to approach the cell membrane and fuse with the membrane, emptying their chemicals into the synapse.
• These chemicals are called neurotransmitters.
• Neurotransmitters travel across the synapse until they reach the specialized receptor cells of the other cell.

Neurotransmitters

• Neurotransmitters are chemical messengers in the body that transmit signals between nerve cells.
• They help regulate bodily functions ranging from heart rate to appetite.
• Neurotransmitters can produce an excitatory effect on the receptors (make the neuron more likely to fire) or an inhibitor effect (make the neuron less likely to fire).
• After the neurotransmitter travels across the synapse, it is either taken back for reuse in the axon terminals of the neuron that released it (reuptake) or broken down by enzymes.

Neurotransmitters Types

• Acetylcholine: found at every junction between the motor and the muscle tissue, involved in muscle action, learning, and memory.
• Norepinephrine: found in the automatic nervous system, involved in control or alertness and wakefulness.
• Dopamine: produced in the substantia nigra, involved in movement, attention, and learning.
• Serotonin: located in the neurons in the brain and spinal cord, involved in mood and in the control of eating, sleep, and arousal.
• GABA (gamma-amino-butyric acid): found throughout the brain and spinal cord, a major inhibitory neurotransmitter in the brain.
• Endorphin: a natural substance produced in the brain, released in response to pain or vigorous exercise to help reduce sensations that can interfere with ongoing behavior.

Drugs and Neurotransmitters

• Drugs can affect our feelings and/or behaviors by altering the process of synaptic transmission.
• This is as a result of the drug being similar to that of the chemical structure in the neurotransmitter.
• In some cases, drugs can mimic the effects of a neurotransmitter.
• In other cases, they can inhibit the effects that are normally produced by the neurotransmitter.
• Some neurotransmitters are agonists, and others are antagonists.

Agonists and Antagonists

• Agonist: a molecule that combines with a receptor on a cell to trigger a physiological reaction.
• Antagonist: a biological structure or chemical agent that interferes with the physiological action of another.
• Examples of agonists are drugs that combine with the cholinergic receptor, and examples of antagonists are drugs that bind to cell receptors that prevent the agonists from eliciting a biological response.