Nervous System Overview

Main Components of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and processing sensory information. It consists of the central nervous system (CNS) and peripheral nervous system (PNS).

• Brain and Spinal Cord: Main components of the CNS, responsible for processing and integrating information.

• Receptors of Sense Organs: Specialized cells that detect changes in the environment and send signals to the nervous system.

• Nerves: Connect the CNS to other body systems, allowing communication and control.

Types of Nervous Cells

Neurons vs. Neuroglia

Nervous tissue contains two main cell types: neurons and neuroglia.

• Neurons: Functional units of the nervous system, actively sending electrical signals.

• Neuroglia (Glial Cells): Support neurons, maintain homeostasis, and provide structural support.

Organization of the Nervous System

Functional and Structural Divisions

The nervous system is divided into categories based on function and structure.

• Central Nervous System (CNS): Brain and spinal cord; processes information and coordinates responses.

• Peripheral Nervous System (PNS): Nerves outside the CNS and ENS; transmits sensory input and motor commands.

  • Afferent Division: Sends sensory information from receptors to the CNS.

    • Special Sensory Receptors: Senses (vision, hearing, taste, smell).

    • Visceral Sensory Receptors: Internal organs.

    • Somatic Sensory Receptors: Skeletal muscles, joints, skin.

  • Efferent Division: Sends motor commands from CNS to effectors.

    • Somatic Nervous System: Skeletal muscle control.

    • Autonomic Nervous System: Involuntary and subconscious control.

      • Parasympathetic Division: Rest and digest.

      • Sympathetic Division: Fight or flight.

  • Enteric Nervous System: Gut brain, not directly connected to CNS.

Neuron Structure and Function

Parts of a Neuron

Neurons have specialized structures for receiving and transmitting signals.

• Soma: Cell body containing the nucleus.

• Nissl Bodies: Protein synthesis of neurotransmitters.

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

• Axon Hillock: Integrates signals and initiates action potentials.

• Axon: Conducts action potentials away from the soma.

• Neurofilament: Transport and support cables, cytoskeleton of the soma.

• Axolemma: Excitable membrane of the axon, regulates ion flow.

• Axon Transport: Motor proteins carry vesicles of neurotransmitters.

• Axon Terminals: Transmit signals to postsynaptic cells.

• Synapse: Junction where neurons meet and communicate via neurotransmitters.

Types of Neurons (Structural Classification)

Types of Neurons (Functional Classification)

• Interneuron (Multipolar): Processes information from sensory neurons and communicates with motor neurons.

• Sensory Neuron (Unipolar): Picks up information from receptors and sends it to the CNS.

• Motor Neuron (Multipolar): Sends commands from CNS to effectors (muscles, glands).

Neuroglia (Glial Cells)

Central Nervous System Neuroglia

• Astrocytes: Abundant, maintain blood-brain barrier, recycle neurotransmitters, repair processes.

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

• Oligodendrocytes: Myelinate CNS axons, provide structural framework.

• Microglia: Clean up debris, migrate through neural tissue, remove pathogens by phagocytosis.

Peripheral Nervous System Neuroglia

• Satellite Cells: Surround cell bodies in ganglia, regulate environment.

• Schwann Cells: Surround all axons in PNS, myelinate peripheral axons, repair after injury.

Gray Matter vs. White Matter

• Gray Matter: Regions where nuclei of neurons are congregating.

• White Matter: Insulates axons, making them appear white.

Myelinated vs. Unmyelinated Neurons

• Myelinated Neurons: Axons surrounded by myelin, signal speed increased.

• Unmyelinated Neurons: Axons not surrounded by myelin, slower signal transmission.

Neuron Injury and Repair

• Fragmentation of axon and myelin occurs in the distal stump.

• Schwann cells divide and form cord, guiding axon regrowth.

• Macrophages engulf degenerating axon and myelin.

• Axon sends buds into Schwann cell network, growing along cord.

• Axon continues to grow into the distal stump, enclosed by Schwann cells.

Membrane Transport and Resting Potential

Types of Solute Movement

• Passive Transporters:

  • Open channels: driven by concentration gradient.

  • Voltage-sensitive gated channels: require a stimulus to open.

• Active Transporters: Move against gradient, require energy (ATP).

• Simple Diffusion: Movement through lipid bilayer.

Sodium-Potassium Pump

• Actively transports 3 sodium ions out and 2 potassium ions in.

• Maintains concentration gradients, essential for resting membrane potential.

Membrane Potential

• Resting Membrane Potential: Inside of neuron is negatively charged compared to outside, typically -70 mV.

• Electrochemical Gradient: Combined effect of ion concentration and electrical charge.

• Resulting Charge Difference: Maintains readiness for action potential generation.

Graded Potentials and Action Potentials

Graded Potentials

• Small, localized changes in membrane potential.

• Caused by opening of ligand/chemically gated or mechanically gated ion channels.

• Can be depolarizing or hyperpolarizing.

Action Potentials

• Generated at the axon hillock when threshold is reached.

• Voltage-gated sodium channels open, sodium rushes in, depolarizing the membrane.

• Potassium channels open, potassium exits, repolarizing the membrane.

• Action potential propagates along the axon like a wave.

Propagation of Action Potentials

• Depolarization at one segment of the axon triggers the next to reach threshold.

• Action potential moves down the axon.

Key Equations

• Sodium-Potassium Pump:

• Resting Membrane Potential:

Summary Table: Types of Neuroglia

Additional info:

• These notes expand on brief points to provide a self-contained study guide suitable for exam preparation in Anatomy & Physiology.