The Nervous System: Structure, Function, and Neurophysiology

Nervous System Overview

Introduction to the Nervous System

The nervous system is the master control and communication system of the body, responsible for integrating and coordinating bodily functions. It is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS), each with distinct roles in processing and transmitting information.

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for integration and coordination of information.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; facilitates communication between the CNS and the rest of the body.

Divisions of the Nervous System

• Autonomic Nervous System (ANS): Controls involuntary functions; subdivided into the parasympathetic and sympathetic divisions.

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Sensory Division: Brings information from receptors to the CNS.

Histology of Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue contains two main cell types: neurons and neuroglia. Neurons are specialized for communication, while neuroglia support and protect neurons.

• Neurons (Nerve Cells): Conduct electrical messages, have extreme longevity, are amitotic, and possess a high metabolic rate.

• Neuroglia (Glial Cells): Support, protect, and regulate neurons; do not conduct impulses.

Basic Neuron Structure

Neurons have a distinct structure that enables their function:

• Cell body (soma): Biosynthetic center containing the nucleus and organelles.

• Dendrites: Branched extensions that receive signals.

• Axon: Single, long process that conducts impulses and forms synapses at its terminal end.

Classes of Neurons

• Structural Types: Unipolar, bipolar, multipolar.

• Functional Types: Sensory, motor, interneuron (association).

Neuroglia (Glial Cells)

Neuroglia are classified based on their location:

• CNS: Astrocytes (blood-brain barrier), ependymal cells (cerebrospinal fluid), microglia (phagocytic), oligodendrocytes (myelin).

• PNS: Schwann cells (myelin), satellite cells (protect somas).

Gray Matter vs. White Matter

Definitions and Properties

The nervous system is organized into gray and white matter, each with distinct functions and cellular compositions.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons.

• White Matter: Dominated by myelinated axons; myelin is a fatty tissue that insulates axons.

• Ganglia: Collections of neuron cell bodies in the PNS.

• Nuclei: Collections of neuron cell bodies in the CNS.

• Nodes of Ranvier: Gaps between Schwann cells in the PNS.

Neurophysiology

Membrane Potentials

Neurons maintain a polarized plasma membrane, resulting in a membrane potential measured in millivolts (mV). The extracellular fluid is rich in Na+ and Cl-, while the intracellular fluid contains more K+ and negatively charged proteins.

• Resting Membrane Potential: Maintained by ion channels and the Na+/K+ exchange pump.

• Na+/K+ Pump: Pumps 3 Na+ out for every 2 K+ in; requires ATP.

Equation:

$\text{Na}^+/\text{K}^+ \text{ pump:} \, 3\, \text{Na}^+ \rightarrow \text{out}, \, 2\, \text{K}^+ \rightarrow \text{in}$

Types of Ion Channels

• Leak Channels: Always open; allow passive movement of ions.

• Voltage-Gated Channels: Open/close in response to changes in membrane potential.

• Ligand-Gated Channels: Open when a specific chemical binds to the channel.

Changes in Membrane Potential and Action Potentials

Neurons are excitable cells that generate action potentials (AP) when voltage-gated channels open. The process involves depolarization, repolarization, and hyperpolarization.

• Depolarization: VG Na+ channels open, Na+ rushes in, membrane becomes less negative.

• Repolarization: VG K+ channels open, K+ exits, membrane returns to resting state.

• Hyperpolarization: Brief period where membrane potential becomes more negative than resting.

• Na+/K+ Pump: cAll-or-None Principle: Action potentials only occur if threshold is reached.

Refractory Period: Time during which another AP cannot be generated.

Propagation of Action Potentials

Action potentials travel along axons via two mechanisms:

• Continuous Propagation: AP moves in small steps along unmyelinated axons.

• Saltatory Propagation: AP jumps between nodes of Ranvier in myelinated axons, increasing speed and efficiency.

Benefit of Saltatory Propagation: Faster transmission of nerve impulses.

Synaptic Transmission

The Synapse

A synapse is the junction between a neuron and another cell. There is no physical connection; instead, neurotransmitters (NT) are released into the synaptic cleft to transmit information.

• Presynaptic Neuron: Releases NT.

• Postsynaptic Neuron: Receives NT.

Synaptic Transmission Process

• AP reaches axon terminal.

• VG Ca2+ channels open.

• NT released by exocytosis.

• NT binds to receptors on postsynaptic membrane, triggering changes in membrane potential.

Neurotransmitters and Neuronal Pools

Neurotransmitters (NT)

Neurotransmitters can be excitatory or inhibitory, depending on the receptors they bind to. The same NT may have different effects at different synapses.

• Excitatory NTs: Acetylcholine (ACh), Norepinephrine (NE).

• Inhibitory NTs: GABA, serotonin.

• Other NTs: Dopamine (motor function, reward), nitric oxide, histamine.

Postsynaptic Neuron Response

• Multiple presynaptic neurons can synapse with one postsynaptic neuron.

• Excitatory NTs increase likelihood of AP; inhibitory NTs decrease likelihood.

• Mixture of excitatory and inhibitory inputs determines local potential.

Divergence vs. Convergence in Neuronal Pools

• Divergence: One neuron spreads stimulation to multiple neurons.

• One neuron spreads stimulation to multiple neurons.

• Multiple neurons provide input to a single neuron.

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