2.1 The Neuron & Nerve Impulse

Why the Neuron Matters on Biology 30

Unit A is worth about 25% of the diploma exam, and the neuron is its foundation. Expect both multiple-choice and numerical-response questions on membrane voltages, ion movement, and the order of events in an impulse. Examiners reward students who can read a graph of an action potential and match each phase to the ion crossing the membrane.

A neuron is a cell specialized to carry electrochemical signals. The dendrites receive incoming signals and carry them toward the cell body (soma), which contains the nucleus and integrates inputs. The axon then conducts the impulse away from the cell body toward the axon terminals.

Neuron Structure & Myelin

Many axons are wrapped in a myelin sheath, a fatty insulating layer formed by Schwann cells in the peripheral nervous system. Gaps between adjacent Schwann cells are the nodes of Ranvier, where the axon membrane is exposed.

Myelin matters because it insulates the axon and lets the impulse "jump" from node to node. The three functional regions map to three jobs:

• Dendrites — receive stimuli and graded signals
• Cell body — houses the nucleus; sums incoming signals
• Axon — conducts the action potential toward the terminals
• Myelin sheath / nodes of Ranvier — insulate and speed conduction

Neurons are also classed by role: sensory (afferent) neurons carry signals from receptors to the central nervous system, motor (efferent) neurons carry signals out to muscles and glands, and interneurons connect the two inside the CNS.

Resting Potential: -70 mV

A neuron at rest is polarized: the inside of the membrane is about -70 mV relative to the outside. This difference is the resting membrane potential, and two factors create it.

First, the sodium-potassium pump uses ATP to actively move 3 Na+ out of the cell for every 2 K+ in, against their concentration gradients. Because more positive ions are pumped out than in, the inside becomes relatively negative.

Second, the membrane is more permeable to K+ than to Na+ at rest, so K+ leaks back out. Large negatively charged proteins trapped inside add to the negative interior. The result is a cell that is electrically "loaded" and ready to fire.

The Action Potential: All-or-None

An action potential is a rapid, self-propagating reversal of membrane voltage. It follows the all-or-none principle: a stimulus either reaches threshold (about -55 mV) and triggers a full impulse, or it does not fire at all. A stronger stimulus does not make a bigger action potential — it makes them fire more frequently.

During depolarization, voltage-gated Na+ channels open and Na+ floods in, making the inside positive. During repolarization, K+ channels open and K+ exits, restoring the negative interior. The Na+/K+ pump then re-establishes the original gradients.

Saltatory Conduction

In a myelinated axon the impulse cannot cross the insulated segments, so it regenerates only at the nodes of Ranvier, appearing to leap from node to node. This is saltatory conduction (from the Latin saltare, "to jump"). Because the action potential skips the myelinated stretches, it travels much faster than in an unmyelinated axon, where every patch of membrane must depolarize in sequence. Larger diameter axons also conduct faster.

The Synapse & Neurotransmitters

Neurons do not touch; they meet at a synapse separated by a tiny gap called the synaptic cleft. When the action potential reaches the axon terminal, voltage-gated Ca2+ channels open and calcium enters. This triggers synaptic vesicles to fuse with the membrane and release a neurotransmitter by exocytosis.

The neurotransmitter — commonly acetylcholine — diffuses across the cleft and binds receptors on the postsynaptic membrane, changing its permeability and starting a new impulse. The signal must then be cleared: the enzyme acetylcholinesterase breaks acetylcholine down, preventing continuous firing. Transmission is therefore one-way, because only the presynaptic terminal stores and releases the transmitter.

The Reflex Arc

A reflex arc is the fastest involuntary response pathway and a frequent exam target. It bypasses conscious thought in the brain. The classic knee-jerk sequence is:

1. Receptor detects the stimulus (the tap/stretch)
2. Sensory (afferent) neuron carries the signal to the spinal cord
3. Interneuron in the spinal cord integrates the signal
4. Motor (efferent) neuron carries the command outward
5. Effector (a muscle or gland) produces the response

Because the spinal cord, not the brain, processes the signal, the response is rapid and protective. You feel the action a moment after it happens, which shows the message reaches the brain separately and later.

For the exam, be able to put these five components in order and identify which neuron type appears at each step. A frequent trap is reversing the sensory and motor neurons: the sensory (afferent) neuron always carries the signal toward the CNS, and the motor (efferent) neuron always carries it away toward the effector. The interneuron sits between them inside the spinal cord.