Nervous System Lesson: Signal Flow, Summation, and System Structure

The nervous system is a vast network of specialized cells that communicate through quick, precise electrical and chemical signals. A central part of this communication relies on action potentials-brief electrical impulses that transmit information through neurons. Understanding how action potentials work, how they travel, and how neurons interact is key to grasping how the brain, spinal cord, and nerves function together.

Resting Membrane Potential

Neurons at rest are not inactive. They maintain a resting membrane potential, an electrical difference between the inside and outside of the cell membrane. This is primarily due to the unequal distribution of sodium (Na⁺) and potassium (K⁺) ions.

• More Na⁺ is outside the neuron.
• More K⁺ is inside the neuron.

This is maintained by the sodium-potassium pump, which actively transports 3 Na⁺ out and 2 K⁺ in, and by K⁺ leak channels that allow K⁺ to move back out of the cell.

Hyperpolarization

Hyperpolarization is when the inside of the neuron becomes more negative than the resting potential. It typically occurs after an action potential due to:

• Opening of K⁺ channels, allowing more K⁺ to leave.
• The inside becoming more negative than usual.

This prevents the neuron from firing again immediately and ensures signal directionality.

Graded Potentials and Summation

Neurons respond to stimuli with graded potentials, which are small changes in membrane potential. These changes can add up through:

• Spatial Summation: Multiple neurons stimulate a single postsynaptic neuron at once.
• Temporal Summation: One neuron fires repeatedly in quick succession.

If the combined effect reaches the threshold, an action potential is triggered.

Action Potential: All-or-None

An action potential is triggered when a neuron reaches the threshold voltage (usually around -55 mV). It's an all-or-none event-once it starts, it travels all the way down the axon.

Phases of Action Potential:

1. Depolarization: Na⁺ channels open, Na⁺ rushes in, making the inside positive.
2. Repolarization: K⁺ channels open, K⁺ flows out, returning the inside to negative.
3. Hyperpolarization: K⁺ channels stay open briefly, making the inside too negative.
4. Restoration: Sodium-potassium pump resets the original ion balance.

Refractory Periods

• Absolute Refractory Period: No new action potential can be started. Na⁺ channels are inactivated.
• Relative Refractory Period: A stronger-than-normal stimulus is needed. K⁺ channels are still open.

This ensures that action potentials travel in one direction and do not overlap.

Gated Channels and Neurotransmitters

Neurons have gated ion channels:

• Voltage-gated channels open in response to electrical changes.
• Ligand-gated channels open when neurotransmitters (like acetylcholine) bind.

Example: Acetylcholine binds to a ligand-gated receptor, allowing Na⁺ to enter and trigger depolarization.

Synaptic Transmission

When an action potential reaches the axon terminal:

1. Ca²⁺ enters the terminal.
2. Vesicles release neurotransmitters into the synaptic cleft.
3. These bind to receptors on the postsynaptic neuron.

In neuron diagrams, the presynaptic neuron is the one sending the signal; the postsynaptic neuron receives it.

Nervous System Structure

The central nervous system includes the brain and spinal cord. It does not include spinal nerves or motor neurons, which are part of the peripheral system.

The autonomic nervous system uses a two-neuron pathway:

• Preganglionic neuron originates in the CNS.
• Postganglionic neuron connects to the target tissue or organ.

Direction of Nerve Impulses

• Afferent neurons carry sensory information to the CNS.
• Efferent neurons carry motor instructions from the CNS to muscles and glands.

An easy way to remember this:

• Afferent = Arrive at the CNS
• Efferent = Exit the CNS

Myelination and Speed of Transmission

Signal speed is influenced by the size and insulation of the axon.

Myelin sheaths speed up conduction by allowing the action potential to jump between gaps in the sheath called nodes of Ranvier, a process known as saltatory conduction.

Glial Cells and the Blood-Brain Barrier

Astrocytes are a type of glial cell in the CNS that:

• Maintain the environment around neurons.
• Help form the blood-brain barrier.

The blood-brain barrier is a protective layer that restricts substances from passing from the bloodstream into the brain. It is formed by tight junctions between capillary endothelial cells, supported by astrocytes.