Nervous System: Structure, Function, and Signal Transmission

Nervous System

Major Parts of a Neuron and Their Functions

The neuron is the fundamental unit of the nervous system, specialized for the transmission of electrical and chemical signals.

• Dendrite: Receives incoming signals from other neurons and transmits them toward the cell body.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals and generates outgoing signals to the axon.

• Axon: Conducts electrical impulses (action potentials) away from the cell body toward the axon terminals.

• Axon Hillock: The region where the axon originates from the cell body; site where action potentials are initiated.

• Axon Terminal: The endpoint of an axon where neurotransmitters are released to communicate with other neurons or effector cells.

• Synapse: The junction between two neurons or a neuron and another cell, where signal transmission occurs via neurotransmitters.

• Myelin: A fatty insulating layer surrounding some axons, produced by glial cells (Schwann cells in the PNS, oligodendrocytes in the CNS); increases the speed of action potential conduction.

• Nodes of Ranvier: Gaps in the myelin sheath along the axon; sites where action potentials are regenerated, enabling rapid signal transmission via saltatory conduction.

Cell Membrane Structure and Selective Permeability

The cell membrane is a selectively permeable barrier that controls the movement of substances into and out of the cell.

• Composition: Primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Impermeable Molecules: Large, polar molecules and ions (e.g., Na+, K+, Cl-, glucose) cannot easily pass through the hydrophobic core of the membrane.

• Importance of Selective Permeability: Maintains homeostasis by regulating the internal environment, allowing essential nutrients in, and keeping harmful substances out.

Active vs. Passive Transport

Transport across the cell membrane can be passive or active, depending on energy requirements and direction relative to concentration gradients.

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., simple diffusion, facilitated diffusion via channel or carrier proteins).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually from ATP); involves specific transport proteins such as pumps (e.g., Na+/K+ ATPase).

ATP: Structure and Importance

Adenosine triphosphate (ATP) is the primary energy currency of the cell.

• Structure: Composed of adenine, ribose, and three phosphate groups.

• Function: Provides energy for cellular processes, including active transport, muscle contraction, and biosynthesis.

• Hydrolysis Reaction: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Membrane Potential and Action Potential

The membrane potential is the voltage difference across a cell's plasma membrane, resulting from unequal distribution of ions.

• Resting Membrane Potential: Typically around -70 mV in neurons; inside of the cell is negative relative to the outside.

• Action Potential: A rapid, temporary change in membrane potential that propagates along the axon, enabling signal transmission.

Development of an Action Potential

Action potentials are generated and propagated by the coordinated opening and closing of voltage-gated ion channels.

• Resting State: Voltage-gated Na+ and K+ channels are closed; Na+/K+ pump maintains ion gradients.

• Depolarization: Voltage-gated Na+ channels open, Na+ enters the cell, membrane potential becomes more positive.

• Repolarization: Na+ channels inactivate, voltage-gated K+ channels open, K+ exits the cell, membrane potential returns toward resting value.

• Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative than resting.

• Ion Concentration Differences: High Na+ outside, high K+ inside; during action potential, these gradients drive ion movement.

Refractory Periods

After an action potential, the neuron experiences a period during which it is less excitable.

• Absolute Refractory Period: No new action potential can be initiated because Na+ channels are inactivated.

• Relative Refractory Period: A stronger-than-normal stimulus is required to initiate another action potential due to ongoing K+ efflux and hyperpolarization.

Role of Myelin and Nodes of Ranvier

Myelin and Nodes of Ranvier work together to increase the speed and efficiency of action potential propagation along axons.

• Myelin Sheath: Insulates axon segments, preventing ion leakage.

• Nodes of Ranvier: Exposed gaps where voltage-gated channels are concentrated; action potentials "jump" from node to node (saltatory conduction), greatly increasing conduction velocity.

Conversion of Action Potentials to Chemical Signals

At synapses, electrical signals are converted to chemical signals for communication between neurons.

• Neurotransmitter Release: Arrival of an action potential at the axon terminal opens voltage-gated Ca2+ channels; Ca2+ influx triggers vesicle fusion and neurotransmitter release into the synaptic cleft.

• Signal Clearance: Neurotransmitters are removed from the synaptic cleft by reuptake into the presynaptic cell, enzymatic degradation, or diffusion away from the synapse.

Inhibitory vs. Excitatory Action Potentials

Neurons can produce different effects on their targets depending on the type of neurotransmitter released and the receptors present.

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing the likelihood of an action potential (e.g., glutamate).

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the postsynaptic membrane, decreasing the likelihood of an action potential (e.g., GABA).

Effects of Neurotoxins and Drugs on Neuron Signal Transmission

Neurotoxins and drugs can alter neuronal signaling, leading to various physiological effects.

• Blockers of Na+ Channels: Prevent action potential initiation and propagation (e.g., tetrodotoxin), potentially causing paralysis.

• Inhibitors of Neurotransmitter Reuptake: Prolong neurotransmitter action, which may lead to seizures or overstimulation.

• Disruptors of Synaptic Vesicle Release: Can prevent neurotransmitter release (e.g., botulinum toxin), leading to paralysis.

• Example: A neurotoxin that blocks K+ channels would prolong repolarization, potentially causing excessive neuronal firing and seizures.

Temperature Regulation

Additional info: The document only lists the heading "Temperature regulation" without further content. In general biology, temperature regulation refers to the processes by which organisms maintain their internal temperature within certain boundaries, even when the surrounding temperature is different. This is crucial for enzyme function and overall homeostasis.