Integration of the Nervous System: Structure, Function, and Physiology

Major Control Systems for Homeostasis

Nervous System vs. Endocrine System

The body maintains homeostasis through two major control systems: the nervous system and the endocrine system. Each system has distinct mechanisms and effects.

• Nervous System: Acts rapidly, with short-lived effects. Utilizes action potentials transmitted via neurons.

• Endocrine System: Responds more slowly, but effects are longer-lasting. Uses hormones transported through the bloodstream to target organs.

Example: The nervous system controls muscle contraction within milliseconds, while the endocrine system regulates growth over months or years.

Functions of the Nervous System

Key Roles

The nervous system is essential for integrating and responding to internal and external stimuli.

• Receiving sensory input: Monitors changes in the environment.

• Integrating information: Processes sensory input and initiates responses.

• Controlling muscles and glands: Directs movement and secretion.

• Maintaining homeostasis: Regulates physiological balance.

• Establishing mental activity: Responsible for consciousness, thinking, memory, and emotion.

Divisions of the Nervous System

CNS and PNS

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. Main site for integration of information.

• Peripheral Nervous System (PNS): All nerves outside the CNS. Subdivided into:

  • Afferent (sensory): Brings information to CNS from receptors.

  • Efferent (motor): Carries information from CNS to effectors (muscles, glands).

Divisions of PNS

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

• Autonomic Nervous System (ANS): Regulates involuntary functions (smooth muscle, cardiac muscle, glands). Subdivided into:

  • Sympathetic: Prepares body for activity ("fight or flight").

  • Parasympathetic: Regulates resting or vegetative functions ("rest and digest").

  • Enteric: Controls digestive tract functions.

Organization of the Nervous System

Pathway of Information

Information flows from receptors to sensory neurons to CNS, then to motor neurons and effectors.

• Receptor: Detects stimulus (e.g., temperature, pain).

• Sensory NS: Transmits input to CNS.

• CNS: Integrates and processes information.

• Motor NS: Sends output to effectors.

Cells of the Nervous System

Neurons and Neuroglia

The nervous system contains two main cell types: neurons and neuroglia.

• Sensory receptors: Specialized endings or cells that detect stimuli.

• Nerves: Bundles of axons connecting CNS to receptors and effectors.

• Neuroglia: Support, protect, and nourish neurons.

• Neurons: Transmit action potentials. Organized into:

  • Cell body (soma): Contains nucleus and organelles.

  • Dendrites: Receive input.

  • Axons: Send output.

Parts of the Neuron

Structure and Function

• Cell Body (Soma): Site of protein synthesis.

• Dendrites: Highly branched, receive signals.

• Axon: Conducts impulses away from cell body; may form collaterals.

• Axon Hillock: Initiates action potentials.

• Axon Terminals: Release neurotransmitters.

Types of Neurons

Functional and Structural Classification

• Functional:

  • Sensory (afferent): Toward CNS.

  • Motor (efferent): Away from CNS.

  • Interneurons: Within CNS.

• Structural:

  • Multipolar: Many dendrites, one axon (most CNS neurons).

  • Bipolar: One dendrite, one axon (sensory organs).

  • Unipolar: Single process (most sensory neurons).

CNS Neuroglial Cells

Types and Functions

• Astrocytes: Form blood-brain barrier, maintain chemical environment.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid (CSF).

• Microglia: Phagocytic, respond to injury/infection.

• Oligodendrocytes: Form myelin sheaths in CNS.

Neuroglia of the PNS

Schwann Cells and Satellite Cells

• Schwann Cells: Myelinate axons in PNS, aid in repair.

• Satellite Cells: Support neuron cell bodies in ganglia.

Myelination

Myelinated vs. Unmyelinated Axons

• Myelinated Axons: Insulated, rapid transmission, gaps called Nodes of Ranvier allow saltatory conduction.

• Unmyelinated Axons: Slower transmission, not all axons are myelinated.

Example: Multiple sclerosis involves demyelination in CNS.

Organization of Nervous Tissue

Gray Matter and White Matter

• Ganglion: Neuron cell bodies outside CNS.

• Nucleus: Neuron cell bodies inside CNS.

• Plexus: Network of axons and cell bodies.

• Gray Matter: Unmyelinated axons, dendrites, cell bodies.

• White Matter: Myelinated axons.

Electrical Signals in Neurons

Action Potentials

• Action potentials: Rapid changes in membrane potential, propagate signals.

• Electrically excitable membranes: Neurons and muscle cells.

• Signal transmission: Often involves neurotransmitters.

Concentration Differences Across the Membrane

Ionic Gradients

• High K+ inside cell, high Na+, Ca2+, Cl- outside.

• Maintained by Na+/K+ pump and selective permeability.

Establishing Resting Membrane Potential

Mechanisms

• Resting potential typically -70 mV.

• Maintained by Na+/K+ ATPase and leak channels.

Equation:

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

Additional info: This is the Nernst equation for potassium ions.

Generating the Action Potential

Phases

• Depolarization: Na+ channels open, Na+ enters cell.

• Repolarization: K+ channels open, K+ exits cell.

• Hyperpolarization: Membrane becomes more negative than resting potential.

Effect of Ca2+ on Action Potentials

Calcium's Role

• Voltage-gated Na+ channels are sensitive to extracellular Ca2+ concentration.

• Low Ca2+ increases excitability; high Ca2+ decreases excitability.

Example: Hypocalcemia can cause muscle spasms (Trousseau's sign).

Graded Potentials

Characteristics

• Result from ligand binding, changes in membrane permeability.

• Can summate to trigger action potentials.

Characteristic	Description
Amplitude	Proportional to stimulus strength
Summation	Can add together
Decay	Decrease with distance

Action Potentials

All-or-None Principle

• Triggered when graded potential reaches threshold.

• Depolarization followed by repolarization.

• Do not decay with distance.

Characteristic	Description
Threshold	Must be reached to trigger AP
Propagation	Self-propagating along axon
Refractory Period	Limits frequency

Refractory Period

Types

• Absolute: No new AP can be generated.

• Relative: AP possible with stronger stimulus.

Action Potential Frequency

Determinants

• Strength of stimulus: Stronger stimulus increases frequency.

• Absolute refractory period: Limits maximum frequency.

Propagation of Action Potentials

Mechanisms

• Depends on myelination and axon diameter.

• Saltatory conduction in myelinated axons (jumps between nodes).

The Synapse

Types and Function

• Electrical synapses: Direct current flow via gap junctions.

• Chemical synapses: Neurotransmitter release across synaptic cleft.

Chemical Synapse

Mechanism

• Presynaptic terminal releases neurotransmitter.

• Neurotransmitter binds to postsynaptic membrane, opens ion channels.

Neurotransmitter Removal

Mechanisms

• Enzymatic breakdown: e.g., acetylcholinesterase splits ACh.

• Reuptake: Neurotransmitter taken back into presynaptic cell.

• Diffusion: Away from synaptic cleft.

Types of Postsynaptic Potentials

EPSP and IPSP

• Excitatory postsynaptic potential (EPSP): Depolarizes membrane, increases likelihood of AP.

• Inhibitory postsynaptic potential (IPSP): Hyperpolarizes membrane, decreases likelihood of AP.

Presynaptic Inhibition and Facilitation

Modulation of Synaptic Transmission

• Presynaptic inhibition: Reduces neurotransmitter release.

• Presynaptic facilitation: Increases neurotransmitter release.

Neuronal Pathways and Circuits

Organization in CNS

• Convergent pathways: Many inputs, one output.

• Divergent pathways: One input, many outputs.

• Oscillating circuits: Multiple synapses, repeated activation.

Example: Reflex arcs use convergent and divergent pathways for rapid response and integration.