Anatomy & Physiology: The Nervous System – Structure, Function, and Electrical Signaling

7.1 Overview of the Nervous System

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting electrical and chemical signals. It is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information from sensory organs and determines responses to internal and external stimuli.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS. It connects the CNS to limbs and organs, facilitating communication throughout the body.

The PNS is further subdivided into:

• Somatic Nervous System: Controls voluntary movements by innervating skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion) and is divided into sympathetic and parasympathetic branches, which often have opposing effects on organs.

Key Functions:

• Receiving sensory input

• Integrating and processing information

• Coordinating motor output

• Maintaining homeostasis

Example: When you touch a hot surface, sensory neurons send a signal to the CNS, which processes the information and sends a response via motor neurons to withdraw your hand.

7.2 Cells of the Nervous System

Neurons and Glial Cells

The nervous system contains two main types of cells: neurons and glial cells.

• Neurons: The primary signaling cells, specialized for transmitting electrical impulses (action potentials). Each neuron typically consists of a cell body (soma), dendrites (receive signals), and an axon (sends signals).

• Glial Cells: Supportive cells that outnumber neurons and provide structural, metabolic, and protective support. Types include astrocytes, oligodendrocytes (CNS myelination), Schwann cells (PNS myelination), microglia, and ependymal cells.

Neurogenesis: The process of generating new neurons. Once thought to occur only during development, it is now known that neurogenesis can occur in certain adult brain regions (e.g., hippocampus).

Key Terms:

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

• Dendrites: Branch-like extensions that receive input from other neurons.

• Axon: Long projection that transmits electrical impulses away from the cell body.

• Synapse: Junction where an axon terminal communicates with another cell (neuron, muscle, or gland).

Example: Motor neurons in the spinal cord send signals to muscles to initiate movement.

Structural and Functional Classification of Neurons

• Structural Classification: Based on the number of processes extending from the cell body:

  • Multipolar: One axon, multiple dendrites (most common in CNS).

  • Bipolar: One axon, one dendrite (e.g., retina, olfactory epithelium).

  • Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons).

• Functional Classification: Based on the direction of impulse transmission:

  • Sensory (Afferent) Neurons: Carry information from sensory receptors to the CNS.

  • Motor (Efferent) Neurons: Transmit commands from the CNS to effectors (muscles/glands).

  • Interneurons: Connect neurons within the CNS; involved in processing and integration.

7.3 Establishment of the Resting Membrane Potential

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest, typically ranging from -60 to -70 mV in neurons. This potential is essential for the generation and propagation of electrical signals.

• Created by the unequal distribution of ions (mainly Na+, K+, and Cl-) across the membrane and selective permeability to these ions.

• Maintained by the sodium-potassium pump (Na+/K+ ATPase), which actively transports 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

• Potassium ions (K+) have the greatest influence due to higher membrane permeability.

Key Equation: The Nernst equation calculates the equilibrium potential for a single ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

where $E_{ion}$ is the equilibrium potential, $R$ is the gas constant, $T$ is temperature, $z$ is the ion charge, and $F$ is Faraday's constant.

Example: The equilibrium potential for K+ is typically around -90 mV, while for Na+ it is about +60 mV.

7.4 Electrical Signaling Through Changes in Membrane Potential

Ion Channels and Membrane Permeability

Neuronal signaling depends on the opening and closing of ion channels, which alter membrane permeability and thus membrane potential.

• Leak Channels: Always open; maintain resting membrane potential.

• Ligand-Gated Channels: Open in response to binding of a chemical messenger (neurotransmitter).

• Voltage-Gated Channels: Open or close in response to changes in membrane potential; essential for action potentials.

• Mechanically Gated Channels: Open in response to mechanical deformation (e.g., touch receptors).

Graded Potentials

• Small, localized changes in membrane potential that vary in magnitude and decay with distance from the stimulus site.

• Can be depolarizing (membrane potential becomes less negative) or hyperpolarizing (more negative).

• Summation of graded potentials can trigger an action potential if threshold is reached.

Action Potentials

• Large, rapid, all-or-none electrical signals that propagate along the axon without decrement.

• Initiated when membrane potential reaches threshold, causing voltage-gated Na+ channels to open, followed by K+ channel opening for repolarization.

• Phases: Depolarization (Na+ influx), Repolarization (K+ efflux), Hyperpolarization (K+ channels remain open briefly).

• Refractory Period: Time during which a second action potential cannot be generated (absolute and relative phases).

Key Equation: The Goldman-Hodgkin-Katz equation estimates the membrane potential considering multiple ions:

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Propagation of Action Potentials

• Action potentials travel along axons by local current flow, depolarizing adjacent membrane regions.

• Unmyelinated Axons: Continuous conduction; slower due to ion channel opening along the entire axon.

• Myelinated Axons: Saltatory conduction; action potentials jump between nodes of Ranvier, increasing conduction velocity.

Example: Motor neurons use saltatory conduction to rapidly transmit signals to muscles.

7.5 Maintaining Neural Stability

Homeostasis and Neural Function

Neural stability is essential for proper function and involves maintaining ion gradients, membrane potentials, and synaptic balance.

• Disruption of ion gradients or channel function can lead to neurological disorders.

• Glial cells play a critical role in maintaining the extracellular environment and supporting neurons.

Example: In diabetes, peripheral neuropathy can result from damage to nerves due to chronic high blood sugar, affecting neural stability and function.

Table: Comparison of Neuron Types (Structural Classification)

Type	Structure	Location	Function
Multipolar	One axon, multiple dendrites	CNS (brain, spinal cord)	Motor neurons, interneurons
Bipolar	One axon, one dendrite	Retina, olfactory epithelium	Sensory (special senses)
Unipolar (Pseudounipolar)	Single process splits into two branches	Sensory ganglia of PNS	Sensory neurons

Additional info: Some content was inferred and expanded for clarity and completeness, including the table and equations.