Neurons, also Chemical Synapse

Question

Neurons are electrically excitable cells that can fire action potentials.

Describe how neurons establish ionic gradients across their plasma membrane, how the membrane becomes electrically charged, and how action potentials work. You should be including lots of details about pumps, channels, and gradients.

The most important structures in the nervous system are the connections between the neurons, the chemical synapses.

Describe how a chemical synapse works. You should include as many details as possible. Start with the action potential arriving at the pre-synaptic axon terminal and trace the steps all the way until the target cell responds.

Answer 1

Neurons have three major components to their structure: neurosoma (control center of the neuron), dendrites (the branches of the neuron that receives the signals from other neurons), and an axon (also called the nerve fiber where a junction/synapse is formed with another neuron to relay the signal). There are three functional classes of neurons: sensory neurons (afferent), interneurons, and motor neurons (efferent). The sensory neurons detect the various stimuli and transmit that information to the central nervous system (brain and spinal cord), the interneurons perform the integrative functions (which are also called association neurons that act as a bridge between the other two functional classes of neurons), and motor neurons relay the signals to effectors (which are muscles and glands). Hence, these are the basic functional definitions and foundation for how neurons work in the body. However, how exactly are those signals started and what happens? The answer to this is involved with action potentials and a key concept called membrane potential.

Before going into the details of how action potentials are created and what exactly happens, we need to go into the electrophysiology of neurons. The Resting Membrane Potential (RMP) occurs when there is a difference in the concentration of charged particles between the extracellular fluid (ECF) and the intracellular fluid (ICF), in this case the membrane of the neuron. The RMP for neurons is approximately -70 mV and these cells naturally have more negative particles on the inside of membrane than the outside. More specifically, the RMP occurs from 3 factors:

1) Ions diffusing down concentration gradient through the membrane

2) Selectively permeable membrane

3) Electrical attraction of cations and anions

Going deeper into the ions associated, potassium (K+) has the greatest influence on the RMP, such that the membrane is more permeable to it. It is about 40x more concentrated in the ICF than ECF. In contrast, sodium (Na+) has a less influence on the RMP and is 12x more concentrated in ECF than ICF. The Na+/K+ Pump is significant in maintaining the "leakage" that may occur, where 3 Na+ are moved out for every 2 K+ brought in. This constant exchange accounts for -3 mV for the overall RMP of -70 mV in neurons. For more specific numbers, at RMP, it is 145 mEq/L Na+ and 4 mEq/L K+ in ECF, and 12 mEq/L Na+ and 150 mEq/L K+ in ICF.

The first step in understanding how action potentials work is to understand local potentials. Local potentials are the changes in the membrane potential that occur near a part of the cell that is stimulated, often dendrites. Important properties of local potentials include: graded, decremental, reversible, either excitatory or inhibitory. The process occurs in these steps:

1) Stimulation occurs to the neuron (different ways: chemicals, light, heat, mechanical disturbance) by binding to the receptor

2) Na+ gates open in the receptor and allows for Na+ ions to enter

3) Depolarization occurs since the entry of the Na+ causes neuron to become less negative (change in membrane potential toward 0 mV)

4) Current travels towards the trigger zone of the neuron, which is the generation of the local potential

Action potentials are more dramatic changes in the membrane polarity and is generated at the trigger zone. Properties of action potentials include: nondecremental, irreversible and adheres to the all-or-none law. If the excitatory local potential is strong enough and reaches the trigger zone, the voltage-gated ion channels can open up. The following steps describe how action potentials work:

1) Local potential/current arrives at axon hillock to depolarize the membrane

2) Threshold must be reached to open the voltage-regulated gates (threshold: -55 mV)

3) Voltage-gated Na+ channels open and Na+ enters the cell by depolarization --> more channels open as positive feedback cycle

4) Once the membrane potential > 0 mV, inactivation of the Na+ channels occur and close (peak voltage: +35 mV)

5) Slow K+ channels open up and K+ outflow causes repolarization of the cell

6) K+ channels continue to be open so that hyperpolarization of the membrane can briefly occur (more negative than RMP)

7) RMP restoration occurs from Na+ leaks in and the extracellular K+ is removed by neuroglia astrocytes