Lesson 1.7 · Unit 1 · CED topic 1.3 (part 1)
Neurons and the firing of an action potential
Your brain contains roughly 86 billion neurons, and everything you think or do depends on how they pass messages among themselves. The AP Exam asks about this process of neural communication rather than about memorizing parts, so this lesson concentrates on what a neuron does and how a message moves: how a signal is triggered, how it travels, and what happens when the process is disrupted. The next lesson covers the chemical messengers that carry the signal across the gap between neurons, and the drugs that interfere with them.
Learning objectives.
- 1.3.A. Explain how the structures and functions of typical neurons in the central nervous system affect behavior and mental processes.
- 1.3.B. Explain how the basic process of neural transmission is related to behavior and mental processes.
Essential knowledge.
- 1.3.A.1. Two common types of neural cells in the brain are neurons (neural cells that transmit information) and glial cells (cells that provide structure, insulation, communication, and waste transport). These types of cells form the basis of the nervous system and are the building blocks of all behavior and mental processes.
- 1.3.A.2. In the spinal cord, the reflex arc demonstrates how neurons within the central and peripheral nervous systems work together to respond to stimuli. Three types of neurons work together in the spinal cord to create a reflex arc: sensory neurons, motor neurons, and interneurons.
- 1.3.B.1. The process of neural transmission most commonly occurs in an orderly, systematic way and involves the all-or-none principle, depolarization, refractory period, resting potential, reuptake, and threshold. Disruptions to this process could lead to disorders such as multiple sclerosis or myasthenia gravis. Exclusion Statement: The sodium-potassium pump is outside the scope of the AP Psychology Exam. 1.A 2.A 3.A
Two kinds of cells
The nervous system is built from two types of cells. Neurons are the cells that transmit information. Glial cells do not carry messages themselves but support the neurons: they provide structure, insulate neurons, help with communication, and carry away waste. Together, these cells form the basis of the nervous system and of all behavior and mental processes.
A typical neuron has a cell body (soma) containing the nucleus, branching dendrites that receive messages from other neurons, a long axon that carries the message away from the cell body, and terminal buttons at the end of the axon that send the message on. Many axons are wrapped in a fatty myelin sheath, which insulates the axon and makes the signal travel much faster, like the plastic coating on a wire.
The reflex arc
The spinal cord shows how neurons work together. When you touch a hot pan, you pull your hand back before you consciously feel the pain. This is a reflex arc, an automatic response that goes through the spinal cord without waiting for the brain. Three types of neurons are involved:
- A sensory neuron detects the stimulus (heat) and carries the message to the spinal cord.
- An interneuron in the spinal cord connects the incoming sensory message to the outgoing response. Interneurons are the most numerous neurons in the body, and in the brain they connect neurons to each other.
- A motor neuron carries the command to the muscle, which pulls the hand away.
A message travels to the brain at the same time, which is why you then feel the pain and decide what to do about the burn. The reflex arc is useful because the shortcut saves time when speed matters.
How a neuron fires
Neural communication follows an orderly sequence. A neuron is not simply "on" or "off"; it is an electrical device that can be pushed over a limit.
- Resting potential. When a neuron is not being stimulated, the inside of the cell is slightly negative compared with the outside, with a voltage of about −70 millivolts. The neuron is polarized, "at rest," ready to respond.
- Threshold. Signals from other neurons arrive at the dendrites. Some are excitatory (they make an action potential more likely) and some are inhibitory (they make it less likely). The neuron adds them up. If the total reaches a minimum level of stimulation, called the threshold (about −55 millivolts), the neuron fires. If the total falls short, nothing happens.
- Depolarization and the action potential. At threshold, channels in the axon open and positively charged particles (sodium ions) rush in, making the inside of the cell briefly positive. This sudden change in voltage is the action potential (the neural impulse). The change, called depolarization, triggers the same change in the next section of the axon, so the impulse travels down the axon like a wave. In a myelinated axon the impulse jumps along the gaps in the myelin, which is why myelin makes it faster.
- Release. When the impulse reaches the terminal buttons, the neuron releases chemical messengers (neurotransmitters) into the synapse, the tiny gap between neurons.
- Refractory period. After firing, the neuron needs a short time to reset. During this refractory period it cannot fire again, however strong the stimulus. The membrane returns to its resting potential.
- Reuptake. Neurotransmitter molecules left in the synapse are reabsorbed by the sending neuron, which clears the gap and ends the message. This is reuptake. The next lesson covers what this means for drugs.
The all-or-none principle
The all-or-none principle says that a neuron either fires completely or does not fire at all, and every action potential in a given neuron has the same strength. A stronger stimulus does not make a bigger impulse. Instead, a stronger stimulus makes the neuron fire more often (a higher rate of action potentials) and recruits more neurons to fire, and that is how the nervous system distinguishes a light touch from a hard push. An analogy is a toilet or a gun: pushing the handle or trigger harder does not change what happens once it has been triggered, but you may fire again sooner.
When the process is disrupted
Because neural communication depends on an orderly process, disruptions cause disorders. Multiple sclerosis (MS) is a disease in which the body's immune system attacks the myelin sheath of neurons in the central nervous system. Without healthy myelin, signals slow down or fail, and people experience problems with vision, balance, muscle control, and sensation. The next lesson describes a second example, myasthenia gravis, which involves the chemical step of the process. You are not required to know how the sodium-potassium pump works; just remember the sequence above and what happens when it breaks down.
In the nineteenth century people believed that nerve messages traveled almost instantly, like thought. The German scientist Hermann von Helmholtz tested that idea by stimulating a frog's nerve at two different distances from the muscle and measuring the time it took the muscle to twitch. The difference in time divided into the difference in distance gave the speed of the impulse. It was far slower than electricity in a wire, on the order of tens of meters per second in that preparation. Later work found that myelinated axons in humans conduct impulses at more than 100 meters per second.
Reading the research: a controlled laboratory experiment in which the independent variable was the distance from the muscle and the dependent variable was the time to twitch. It established that neural transmission is a physical process that takes time. A limitation is that the early measurements used a frog nerve and were later refined for humans.
A neuron has a resting potential of −70 mV and a threshold of −55 mV. Four stimuli shift its voltage to −62 mV, −56 mV, −54 mV, and −20 mV. Which stimuli cause an action potential, and how do the resulting impulses compare?
Step 1, compare each level with the threshold of −55 mV. A voltage of −62 mV is below (more negative than) the threshold, so no firing. −56 mV is also below, so no firing.
Step 2, −54 mV has crossed the threshold, so the neuron fires. So does −20 mV.
Step 3, compare the impulses: by the all-or-none principle, the action potentials at −54 mV and −20 mV have the same size. The stronger stimulus (−20 mV) does not make a larger impulse.
Step 4, what changes with stimulus strength: the stronger stimulus would make the neuron fire more frequently, and would likely activate more neurons.
You step on a sharp piece of glass. Describe the path of the message and name the neurons.
Step 1, a sensory neuron in the foot detects the stimulus and fires, sending the impulse along its axon to the spinal cord.
Step 2, in the spinal cord an interneuron receives the signal and passes it to a motor neuron without waiting for a decision from the brain.
Step 3, the motor neuron's impulse travels to the leg muscles, and they contract to lift the foot.
Step 4, at the same time the message goes up the spinal cord to the brain, which interprets it as pain. This is why you pull away first and feel the pain a moment later.
A graph of a neuron's voltage over time shows a flat line at −70 mV, a rise to about −55 mV, a sharp spike to about +40 mV, a drop below −70 mV, and a return to −70 mV. Identify each phase.
Step 1, the flat line at −70 mV is the resting potential.
Step 2, the rise to −55 mV is the stimulus bringing the neuron to threshold. The point where the sudden spike begins is the threshold.
Step 3, the spike to +40 mV is depolarization and the action potential.
Step 4, the drop below −70 mV and slow return is the refractory period, when the neuron cannot fire again until it resets.
Step 5, if a second stimulus arrived during the refractory period, the neuron would not fire, however strong the stimulus.
- Neurons transmit information; glial cells support, insulate, and clean up. A reflex arc uses a sensory neuron, an interneuron, and a motor neuron in the spinal cord.
- The sequence: resting potential, then enough excitation to reach threshold, then depolarization (action potential), then refractory period, then reuptake of the neurotransmitter.
- All-or-none principle: every action potential has the same size; stimulus strength changes the rate of firing and the number of neurons.
- Multiple sclerosis (damaged myelin) is an example of disrupted neural transmission. The sodium-potassium pump is outside the scope of the exam.
- Name the three neuron types in a reflex arc and give the job of each.
Show answer
Sensory neurons carry the message from the stimulus to the spinal cord; interneurons connect the sensory message to the response within the spinal cord; motor neurons carry the command to the muscle.
- Explain the all-or-none principle. How can a neuron signal that a stimulus is stronger?
Show answer
A neuron either fires with full strength or does not fire. A stronger stimulus makes it fire more frequently, and more neurons are activated, rather than making a stronger single impulse.
- What is the refractory period, and why is it important?
Show answer
It is the brief time after an action potential when the neuron cannot fire again while it resets. It keeps impulses moving in one direction and limits how fast the neuron can fire.
- How does damage to the myelin sheath, as in multiple sclerosis, affect behavior?
Show answer
Without myelin the impulses travel more slowly or fail, so signals do not reach their targets properly. This produces problems with movement, balance, vision, and sensation.
- A neuron's threshold is −55 mV. Will a stimulus that moves its voltage from −70 mV to −58 mV make it fire? Explain.
Show answer
No. The voltage did not reach the threshold of −55 mV, so there is no action potential, and by the all-or-none principle there is no partial impulse.
- What is the function of the glial cells?
Show answer
They support neurons: providing structure, insulation (including myelin), communication, and waste removal.