Nervous System Structure and Function: Neurons, Synapses, and Membrane Potentials

Functions of the Nervous System

Overview of Nervous System Functions

The nervous system is a complex network responsible for coordinating the body's response to internal and external stimuli. It achieves this through rapid communication and integration of information.

• Sensory Input: The nervous system gathers information from sensory receptors that monitor changes inside and outside the body.

• Integration: The collected sensory information is processed and interpreted, allowing the body to make decisions.

• Motor Output: The nervous system sends commands to muscles or glands (effectors) to elicit a response.

• Interaction with the Endocrine System: The nervous system works with the endocrine system to regulate long-term physiological processes.

Neuronal Structure and Information Transmission

How Neurons Transmit Information

Neurons are specialized cells that transmit information via electrical and chemical signals. The basic structure of a neuron includes dendrites, a cell body, and an axon.

• Dendrites: Receive input from other neurons or sensory receptors.

• Cell Body (Soma): Contains the nucleus and integrates incoming signals.

• Axon: Conducts electrical impulses away from the cell body toward other neurons or effectors.

• Synapse: The junction where the axon terminal of one neuron communicates with another cell via neurotransmitters.

Example: In the cockroach Periplaneta americana, sensory neurons detect vibrations, transmit signals through interneurons, and activate motor neurons to produce a startle response.

Neural Circuits and Reflexes

Reflex Arcs and Neural Pathways

Reflexes are rapid, automatic responses to stimuli, mediated by specific neural circuits. These circuits often involve sensory neurons, interneurons, and motor neurons.

• Sensory Neurons: Detect environmental changes (e.g., sound waves or air currents).

• Interneurons: Relay and process information within the central nervous system (CNS).

• Motor Neurons: Transmit signals to muscles or glands to produce a response.

Example: The startle response in the cockroach involves a neural circuit where sensory neurons synapse with giant interneurons, which then activate motor neurons to trigger muscle contraction and escape behavior.

Neuronal Characteristics

Key Properties of Neurons

Neurons possess several unique characteristics that enable efficient communication within the nervous system.

• Specificity: Neurons direct information to specific target cells.

• Speed: Neuronal signals can travel at speeds up to 150 meters per second, allowing for rapid responses.

• Versatility: Neurons can integrate multiple inputs and generate multiple or single outputs.

Types of Neurons and Supporting Cells

Classification of Neurons

Neurons are classified based on their function within the nervous system.

• Sensory Neurons: Collect information from sensory organs (e.g., eyes, ears, nose, mouth).

• Motor Neurons: Transmit signals from the CNS to effectors (muscles or glands).

• Interneurons: Connect sensory and motor neurons, facilitating communication within the CNS.

Supporting Cells (Glia)

Glial cells provide structural and metabolic support for neurons and outnumber neurons by 10–50:1.

• Astrocytes: Connect neurons to capillaries and provide metabolic support.

• Microglial Cells: Act as immune cells, removing debris and pathogens via phagocytosis.

• Oligodendrocytes (CNS) and Schwann Cells (PNS): Form myelin sheaths around axons, increasing the speed of action potential conduction.

• Ganglion: A cluster of neuron cell bodies in the peripheral nervous system.

Synaptic Structure and Function

Components of the Synapse

The synapse is the site of communication between neurons or between a neuron and another cell type. It consists of several specialized structures:

• Presynaptic Terminal: Contains synaptic vesicles filled with neurotransmitters.

• Synaptic Cleft: The small gap between the presynaptic and postsynaptic cells.

• Postsynaptic Element: Contains receptors for neurotransmitters.

• Microtubules and Neurofilaments: Provide structural support and transport materials within the neuron.

• Mitochondria: Supply energy for synaptic transmission.

Electrical Properties of Neurons

Membrane Potentials

Neurons maintain a voltage difference across their plasma membrane, known as the membrane potential. This is essential for the generation and propagation of electrical signals.

• Resting Membrane Potential: The steady-state voltage across the membrane when the neuron is not transmitting a signal, typically around -70 mV.

• Ion Gradients: The membrane potential is established by differences in ion concentrations (mainly K+, Na+, and Cl-) across the membrane.

• Selective Permeability: The membrane is more permeable to K+ than to Na+ or Cl- at rest.

• Na+/K+ ATPase: This pump maintains ion gradients by moving 3 Na+ out and 2 K+ into the cell, contributing to the negative resting potential.

Equilibrium Potential and the Nernst Equation

The equilibrium potential for a particular ion is the membrane voltage at which there is no net movement of that ion across the membrane. It can be calculated using the Nernst equation:

• Nernst Equation:

$E_{ion} = 62\ \text{mV} \times \log\left(\frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}}\right)$

• Example: The equilibrium potential for K+ (EK) is negative, while for Na+ (ENa) it is positive.

Ion Concentrations in Mammalian Neurons

The following table summarizes typical ion concentrations inside and outside mammalian neurons:

Ion	Intracellular Concentration (mM)	Extracellular Concentration (mM)
Potassium (K+)	140	5
Sodium (Na+)	15	150
Chloride (Cl-)	10	120
Large anions (A-, e.g., proteins)	100	Not applicable

The Goldman Equation

The Goldman equation calculates the membrane potential by considering the relative permeabilities and concentrations of multiple ions:

$V_m = 58\ \text{mV} \times \log\left(\frac{P_{K}[K^+]_{out} + P_{Na}[Na^+]_{out} + P_{Cl}[Cl^-]_{in}}{P_{K}[K^+]_{in} + P_{Na}[Na^+]_{in} + P_{Cl}[Cl^-]_{out}}\right)$

• Relative Permeability Example: $P_{K^+} = 1$, $P_{Na^+} = 0.04$, $P_{Cl^-} = 0.01$

Action Potentials and Voltage-Gated Channels

Generation and Propagation of Action Potentials

An action potential is a rapid, transient change in membrane potential that travels along the axon. It is the fundamental signal of the nervous system.

• Voltage-Gated Channels: Na+ and K+ channels open and close in response to changes in membrane voltage.

• Depolarization: Rapid opening of Na+ channels causes an influx of Na+, making the inside of the cell more positive.

• Repolarization: K+ channels open more slowly, allowing K+ to exit the cell and restore the negative membrane potential.

• Propagation: The action potential travels undiminished along the axon.

Refractory Periods

After an action potential, the neuron experiences periods during which it is less excitable:

• Absolute Refractory Period: Lasts about 1 ms after repolarization; Na+ channels are inactivated and cannot reopen.

• Relative Refractory Period: Lasts 1–3 ms longer; K+ channels remain open, causing hyperpolarization and requiring a stronger stimulus for another action potential.

• Unidirectional Propagation: Inactivation of Na+ channels prevents the action potential from traveling backward.

Summary Table: Key Ion Concentrations in Neurons

Ion	Inside (mM)	Outside (mM)
K+	140	5
Na+	15	150
Cl-	10	120
Large anions (A-)	100	Not applicable

Additional info: Some explanations and context have been expanded for clarity and completeness, including the summary of the startle response circuit and the detailed explanation of the Goldman equation.