Understanding the Nervous System: Neurons, Synapses, and Action Potentials

Sympathetic Nervous System (SNS) vs. Parasympathetic Nervous System (PNS)

The Parasympathetic Nervous System (PNS) prepares the body for changes, influencing heart rate and respiration. The Sympathetic Nervous System (SNS) restores the body to a resting state.

Nervous System Cells

• Neurons: The basic units of the Central Nervous System (CNS) and the peripheral nervous system.
• Glial cells: Support cells within the nervous system.

Peripheral Nervous System

The peripheral nervous system forms neural networks.

• Soma (neuronal body): Contains the cell nucleus and organelles. It provides energy to the cell. Lysosomes degrade cellular waste.
• Dendrites: Branching extensions that receive signals from other neurons.
• Axon: A long, slender projection that conducts nerve impulses away from the soma to other neurons. It contains synaptic buttons at its terminals, which connect to other neurons.

Axons are often covered by myelin sheaths, which speed up the transmission of nerve impulses.

General Function of Neurons

Neurons create intraneuronal communication by transmitting nerve impulses.

• Afferent (Sensory) Neurons: Transmit impulses from sensory receptors to the central nervous system (spinal cord).
• Association (Interneurons): Located within the central nervous system, they connect sensory neurons to motor neurons.
• Motor (Efferent) Neurons: Send impulses from the central nervous system to effector organs.

Neuron Morphology

• Unipolar: One neuron branches into two extensions (one dendritic and one axonal).
• Bipolar: Two extensions emerge from the soma (two dendrites).
• Multipolar: One axon and one or more dendrites emerge from the soma. These are the most common type in invertebrates.

Nerve Synapse

A synapse is the junction where a neuron communicates with another neuron. Luigi Galvani's experiments with electric organs demonstrated the electrical nature of nerve impulses.

Electric Potential

Differences in electrical charges (+) and (-) between the intracellular and extracellular environments of a neuron can produce changes in the cell membrane.

Resting Potential

When a neuron is inactive, it has a negative charge intracellularly and a positive charge extracellularly. This difference is maintained by the sodium-potassium pump (active transport), which pumps two potassium ions into the cell and three sodium ions out. There is a higher concentration of potassium ions and negatively charged proteins inside the neuron, and a higher concentration of sodium and calcium ions outside.

Action Potential

The neuron transitions from a resting state to an action potential when a nerve impulse is transmitted. This involves a reversal of electrical charges: the intracellular environment becomes positive, and the extracellular environment becomes negative. This depolarization occurs because sodium channels open, allowing sodium ions to enter the cell. A stimulus must reach a certain threshold intensity (all-or-nothing principle) to propagate. This threshold is typically around 30 millivolts. The transmission of an impulse depends not on the intensity of the stimulus but on its frequency.

Axon Diameter

The larger the axon's diameter, the faster the propagation of the impulse.

Impulse Conduction

• Continuous Conduction: Occurs in unmyelinated axons, resulting in slower impulse transmission.
• Saltatory Conduction: Occurs in myelinated axons. The impulse jumps from one node of Ranvier to another, resulting in faster transmission.

Synapses: The Process of Communication Between Neurons

Synaptic communication begins with a chemical discharge that causes an electrical current in the presynaptic neuron's membrane. When the nerve impulse reaches the end of the axon, the neuron secretes a neurotransmitter. This neurotransmitter is released into the synaptic space, the area between the transmitting (presynaptic) and receiving (postsynaptic) neurons. The neurotransmitter then either excites or inhibits the postsynaptic neuron.