The Process of Neural Transmission (College Board AP® Psychology): Study Guide

Neural transmission

• Neural transmission is the process by which information is communicated within and between neurons

  • It is the biological basis of every thought, feeling, and behavior

• Neural transmission involves two stages:

  • Electrical transmission — the signal travels along the length of a single neuron as an electrical impulse

  • Chemical transmission — the signal crosses the gap between neurons (the synapse) via chemical messengers called neurotransmitters

• This process occurs in an orderly, systematic way and involves six key concepts:

  • resting potential

  • threshold

  • depolarization

  • the all-or-none principle

  • the refractory period

  • reuptake

Stage 1: Electrical transmission — the action potential

• Resting potential

  • When a neuron is not firing, it is in a resting state - this electrical charge is known as the resting potential

    • The neuron is described as polarized in this state as it is negatively charged on the inside, positively charged on the outside

• Threshold

  • For a neuron to fire, it must receive enough incoming stimulation to reach a critical level of charge known as the threshold

    • If the combined input from other neurons is sufficient to push the charge to approximately −55 millivolts, the threshold is reached and the neuron fires

    • If the threshold is not reached, the neuron does not fire

• Depolarization

  • Once the threshold is reached, positive ions rush into the cell, rapidly changing the membrane potential from negative to positive

    • This change in charge travels down the length of the axon as an action potential — the electrical impulse that carries information along the neuron

    • Depolarization spreads progressively down the axon toward the terminal buttons

• The all-or-none principle

  • A neuron either fires completely or does not fire at all; there is no in-between

    • The strength of the action potential is always the same regardless of the strength of the stimulus

    • E.g. a louder noise does not produce a stronger action potential in a single neuron, but it may cause more neurons to fire more frequently

      • This principle means neural communication is reliable and consistent

• The refractory period

  • Immediately after firing, the neuron cannot fire again for a brief period while it resets to its resting potential

    • During the absolute refractory period, no amount of stimulation can trigger another action potential

    • During the relative refractory period that follows, the neuron can fire again but requires stronger-than-normal stimulation to do so

    • The refractory period limits the rate at which a neuron can fire and ensures signals travel in one direction only

Stage 2: Chemical transmission — across the synapse

• When the action potential reaches the terminal buttons at the end of the axon, it triggers the release of neurotransmitters from small storage sacs called vesicles

• Neurotransmitters are released into the synaptic cleft

  • This is the gap between the terminal buttons of the presynaptic neuron and the dendrites of the postsynaptic neuron

• Neurotransmitters diffuse across the synaptic cleft and bind to receptor sites on the postsynaptic membrane

  • Receptor sites are specific to particular neurotransmitters, like a lock-and-key mechanism

• Binding at the postsynaptic receptor either:

  • Excites the postsynaptic neuron

    • This makes an action potential more likely by pushing the charge toward threshold

  • Inhibits the postsynaptic neuron

    • This makes an action potential less likely by pushing the charge away from threshold

• Whether the postsynaptic neuron fires depends on the sum of all excitatory and inhibitory signals it receives at any given moment

  • This is known as summation

• After neurotransmitters have been released and have bound to receptor sites, they are cleared from the synaptic cleft

  • Most neurotransmitters are reabsorbed back into the presynaptic neuron through a process called reuptake, where they can be recycled and used again

  • Some are broken down by enzymes in the synaptic cleft

  • Reuptake is essential for terminating the signal and preventing continuous, uncontrolled firing of the postsynaptic neuron

When neural transmission goes wrong

• Disruptions to the process of neural transmission can have significant consequences for behavior and mental processes

• Multiple sclerosis (MS) is a disorder in which the immune system attacks and deteriorates the myelin sheath surrounding axons

  • Without adequate myelination, the electrical signal slows down or fails to travel efficiently along the axon

  • This disrupts motor control, coordination, and sensation depending on which neurons are affected

• Myasthenia gravis is a disorder in which the immune system attacks the receptor sites at the neuromuscular junction, blocking the neurotransmitter acetylcholine from binding

  • Because acetylcholine is responsible for triggering muscle contractions, this disruption leads to muscle weakness and fatigue

  • This illustrates how disrupted chemical transmission at the synapse directly affects behavior and physical functioning

Neurotransmitters affecting behavior

• Each neurotransmitter has specific functions related to behavior and mental processes

  • How a neurotransmitter functions depends on its location in the nervous system

• Neurotransmitters communicate either:

  • excitatory messages (making an action potential more likely)

  • inhibitory messages (making an action potential less likely)

• The same neurotransmitter can have different effects depending on:

  • where in the nervous system it is acting

  • which receptor it binds to

• Below are eight key neurotransmitters affecting behavior:

Key distinctions

• Glutamate is the brain's primary excitatory neurotransmitter

  • It makes action potentials more likely

• GABA (gamma-aminobutyric acid) is the brain's primary inhibitory neurotransmitter

  • It makes action potentials less likely

  • It is essentially glutamate's counterpart

• Endorphins are the body's natural painkillers

  • They are released during physical exercise, injury, or excitement and reduce the perception of pain

• Acetylcholine was the first neurotransmitter ever identified

  • It plays a dual role in both the central nervous system (memory) and the peripheral nervous system (muscle contraction)