Neurons and the Nervous System

Overview of Neurons

Neurons are specialized cells of the nervous system responsible for transmitting information using electrical impulses known as action potentials. These signals allow rapid communication throughout the body.

• Neurons: Specialized cells that carry information using patterns of electrical signals.

• Nerves: Bundles of neuron fibers wrapped in connective tissue, forming cable-like structures for signal transmission.

• Neuroglial cells (glia): Support, protect, and nourish neurons.

Neuron Structure

Neurons have distinct structural components that enable their function:

• Cell body (soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Receive incoming messages (stimuli) and direct them towards the cell body.

• Axon: Conducts electrical signals (action potentials) away from the cell body.

• Nissl substance: Rough endoplasmic reticulum specialized for making neurotransmitters (proteins or peptides).

• Axon hillock: Region where action potentials are initiated.

• Myelin sheath: Insulating layer formed by Schwann cells (in the peripheral nervous system) that speeds up signal transmission.

• Node of Ranvier: Small gaps between myelin segments that facilitate rapid signal propagation.

The Axon and Signal Transmission

Axon Function and Myelination

The axon transmits electrical signals to other neurons or effector cells. Myelination increases the speed and efficiency of this transmission.

• Myelin sheath: Insulates the axon, allowing action potentials to travel faster by preventing ion leakage.

• Schwann cells: Produce myelin in the peripheral nervous system.

• Nodes of Ranvier: Gaps in myelin where action potentials "jump" (saltatory conduction).

• Multiple Sclerosis (MS): Disease that damages the myelin sheath, slowing or blocking signal transmission.

Membrane Potential

Ion Concentrations and Membrane Potential

The membrane potential is the electrical difference across a neuron's membrane, driven by ion concentration gradients.

• Ion concentrations: [Na+] and [K+] differ inside and outside the cell.

• Resting membrane potential: The inside of the neuron is more negative than the outside.

Key equation:

• High [Na+] outside, high [K+] inside.

• Membrane potential is negative at rest.

Ion Channels and Pumps

Ion channels and pumps regulate the movement of ions, affecting the membrane potential.

• Voltage-gated ion channels: Open or close in response to changes in membrane potential.

• Ion pumps: Actively move ions against their concentration gradients using ATP.

• Sodium-potassium pump (Na+/K+ ATPase): Moves 3 Na+ out and 2 K+ in per ATP, maintaining the resting potential.

Action Potentials

Definition and Initiation

An action potential is a rapid, temporary change in membrane potential that travels along the axon, allowing neurons to communicate over long distances.

• Stimulus: Triggers a change in membrane potential at the axon hillock.

• Threshold: Minimum membrane potential required to initiate an action potential.

• Depolarization: Membrane potential becomes less negative (more positive).

Stages of an Action Potential

Action potentials occur in distinct phases:

1. Stimulation: Na+ ions diffuse into the cell, raising membrane potential to threshold.

2. Rising phase (Depolarization): More Na+ channels open; Na+ floods into the neuron.

3. Peak: Na+ channels close; K+ channels open.

4. Falling phase (Repolarization): K+ ions move out; membrane potential becomes negative again.

5. Recovery (Refractory phase): Na+/K+ pump restores ion concentrations and resting potential.

Summary Table: Stages of an Action Potential

Action Potential Propagation

How Action Potentials Travel

For signals to reach the end of the axon, the action potential must propagate along the membrane. This occurs as voltage-gated Na+ channels open sequentially along the axon.

• Propagation: Action potential "hops" from one node of Ranvier to the next (saltatory conduction).

• Myelin sheath: Speeds up propagation by insulating the axon and forcing the action potential to jump between nodes.

Summary of Propagation Events

• Section A: Depolarization

• Section B and C: At rest

• Propagation continues as each section undergoes the stages of the action potential.

Clinical Application

Muscle Fatigue and Ion Pumps

During muscle fatigue, sodium-potassium pumps may stop working due to lack of ATP, causing improper ion distribution and impaired action potential transmission.

• Example: Sodium-potassium pump in skeletal muscle requires ATP to function; without ATP, Na+ and K+ ions accumulate in the wrong places, preventing proper action potentials.

Additional info: The notes also briefly mention electrical synapses (gap junctions), which allow direct electrical communication between neurons, but do not elaborate. In most vertebrates, chemical synapses are more common for neuron-to-neuron communication.