Neurons, Synapses, and Signaling

Overview: Lines of Communication

The nervous system is responsible for transferring and processing information throughout the body. Neurons are specialized nerve cells that serve as the primary units of communication. They utilize two main types of signals:

• Electrical signals: Used for long-distance communication within the neuron.

• Chemical signals: Used for short-distance communication between neurons or between neurons and other cells.

Interpreting signals in the nervous system involves sorting complex pathways and connections. Information processing can occur in simple clusters of neurons called ganglia or in more complex organizations known as a brain.

Neuron Structure and Function

The structure of a neuron is closely related to its function in information transfer. Neurons have specialized parts:

• Cell body: Contains most of the neuron's organelles.

• Dendrites: Highly branched extensions that receive signals from other neurons.

• Axon: A long extension that transmits signals to other cells. The axon hillock is the cone-shaped base where signals are generated.

• Synaptic terminals: Branched ends of the axon that transmit signals to other cells at junctions called synapses.

At most synapses, chemical messengers called neurotransmitters pass information from the transmitting neuron to the receiving cell.

Neurons are supported by glial cells (or glia), which outnumber neurons in the mammalian brain and provide structural and metabolic support.

Introduction to Information Processing

Nervous systems process information in three main stages:

1. Sensory input: Sensory neurons transmit information about external or internal conditions to processing centers (brain, spinal cord, ganglia).

2. Integration: Interneurons analyze and interpret the sensory input.

3. Motor output: Motor neurons transmit signals to muscles or glands, triggering a response.

Neurons that carry out integration are often organized in the central nervous system (CNS), while those that carry information into and out of the CNS form the peripheral nervous system (PNS). Bundles of neurons form nerves.

Resting Potential and Ion Gradients

Establishing the Resting Potential

The inside of a neuron is negatively charged relative to the outside, creating a membrane potential. The resting potential is the membrane potential of a neuron not sending signals, typically about -60 to -80 mV.

• Potassium (K+) concentration is higher inside the cell, while sodium (Na+) concentration is higher outside.

• Sodium-potassium pumps use ATP to maintain these gradients by pumping 3 Na+ out and 2 K+ in.

• Ion channels are selectively permeable, allowing only certain ions to pass through.

• At rest, many potassium channels are open, allowing K+ to flow out, contributing to the negative charge inside.

Table: Ion Concentrations Inside and Outside Mammalian Neurons

Modeling the Resting Potential

The equilibrium potential for an ion is the membrane voltage at which the net flow of that ion is zero. It can be calculated using the Nernst equation:

For K+, the equilibrium potential is about -90 mV. The actual resting potential is slightly less negative due to a small Na+ influx.

Action Potentials and Signal Transmission

Action Potentials

Action potentials are rapid, large changes in membrane potential that travel along axons. They occur when a depolarization reaches a threshold, opening voltage-gated Na+ channels.

• Graded potentials: Small changes in membrane potential; magnitude varies with stimulus strength and decays with distance.

• Action potential: All-or-none event; constant magnitude and can travel long distances without loss of signal.

Phases of the Action Potential

1. Resting state: Most voltage-gated Na+ and K+ channels are closed.

2. Depolarization: Some Na+ channels open, Na+ enters the cell.

3. Rising phase: Many Na+ channels open, rapid influx of Na+.

4. Falling phase: Na+ channels inactivate, K+ channels open, K+ exits the cell.

5. Undershoot: K+ permeability is higher than at rest, then K+ channels close and resting potential is restored.

After an action potential, a refractory period occurs during which a second action potential cannot be initiated, ensuring one-way signal transmission.

Conduction of Action Potentials

Action potentials are initiated at the axon hillock and travel toward synaptic terminals. The refractory period prevents backward movement.

• Speed of conduction increases with axon diameter.

• In vertebrates, myelin sheaths (produced by oligodendrocytes in the CNS and Schwann cells in the PNS) insulate axons, enabling rapid signal transmission.

• Action potentials jump between nodes of Ranvier in a process called saltatory conduction.

Synaptic Transmission

Chemical Synapses

Most action potentials are not transmitted directly from neuron to neuron. Instead, information is transmitted at synapses, usually via chemical neurotransmitters.

• The presynaptic neuron synthesizes and packages neurotransmitters in synaptic vesicles.

• Arrival of an action potential at the synaptic terminal causes release of neurotransmitter into the synaptic cleft.

• The neurotransmitter binds to receptors on the postsynaptic cell, causing ion channels to open and generating a postsynaptic potential.

Postsynaptic Potentials

• Excitatory postsynaptic potentials (EPSPs): Depolarizations that bring the membrane potential closer to threshold.

• Inhibitory postsynaptic potentials (IPSPs): Hyperpolarizations that move the membrane potential farther from threshold.

A single EPSP is usually insufficient to trigger an action potential. Multiple EPSPs can combine through summation:

• Temporal summation: Multiple EPSPs produced in rapid succession at the same synapse.

• Spatial summation: EPSPs produced simultaneously by different synapses on the same postsynaptic neuron.

• IPSPs can counteract EPSPs; the net effect at the axon hillock determines whether an action potential is generated.

Modulated Signaling at Synapses

Some synapses use metabotropic receptors, where neurotransmitter binding activates a signal transduction pathway involving second messengers. These effects are slower but longer-lasting compared to ligand-gated channels.

Neurotransmitters: Acetylcholine

Acetylcholine is a common neurotransmitter involved in muscle stimulation, memory, and learning. It acts through two major classes of receptors:

• Ligand-gated receptors: Found at vertebrate neuromuscular junctions; binding of acetylcholine opens the channel and generates an EPSP.

• Metabotropic receptors: Involve metabolic steps and second messengers.

Acetylcholine is found in both invertebrates and vertebrates. Some toxins (e.g., nerve gas sarin, botulinum toxin) disrupt acetylcholine signaling, leading to severe physiological effects.

Additional info:

• Other neurotransmitters include amino acids, biogenic amines, neuropeptides, and gases, each with specific roles in the nervous system.