Fundamentals of the Nervous System and Nervous Tissue

Basic Functions of the Nervous System

The nervous system is responsible for controlling and coordinating the body's activities. It enables rapid communication between different parts of the body and responds to internal and external stimuli.

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpreting sensory input to determine an appropriate response.

• Motor Output: Activating effector organs (muscles and glands) to produce a response.

• Example: Touching a hot object triggers sensory input, integration in the brain/spinal cord, and motor output to withdraw the hand.

Structural and Functional Divisions of the Nervous System

The nervous system is divided into structural and functional components for organization and specialization.

• Structural Divisions:

  • Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and control.

  • Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; connects the CNS to the rest of the body.

• Functional Divisions:

  • Sensory (Afferent) Division: Transmits sensory information to the CNS.

  • Motor (Efferent) Division: Carries commands from the CNS to effectors.

    • Somatic Nervous System: Controls voluntary movements (skeletal muscles).

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

      • Sympathetic Division: Mobilizes body systems during activity.

      • Parasympathetic Division: Conserves energy and promotes housekeeping functions.

Structural Components of a Neuron and Their Functional Roles

Neurons are specialized cells for transmitting electrical signals. Each part has a distinct function.

• Cell Body (Soma): Contains the nucleus and organelles; metabolic center.

• Dendrites: Receive incoming signals and convey them toward the cell body.

• Axon: Conducts electrical impulses away from the cell body to other neurons or effectors.

• Axon Hillock: Initiates action potentials.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

• Example: Motor neuron axon transmits impulses to muscle fibers, causing contraction.

Classification of Neurons by Structure and Function

Neurons are classified based on their shape and their role in the nervous system.

• Structural Classification:

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

  • Bipolar: One dendrite, one axon (found in special senses).

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

• Functional Classification:

  • Sensory (Afferent) Neurons: Carry impulses toward CNS.

  • Motor (Efferent) Neurons: Carry impulses away from CNS to effectors.

  • Interneurons: Connect sensory and motor neurons; found in CNS.

Types and Functions of Neuroglia

Neuroglia (glial cells) support and protect neurons. They are essential for nervous system function.

• Astrocytes: Support neurons, regulate environment, form blood-brain barrier.

• Microglia: Act as immune cells, remove debris and pathogens.

• Ependymal Cells: Line CNS cavities, produce and circulate cerebrospinal fluid.

• Oligodendrocytes: Form myelin sheaths in CNS.

• Schwann Cells: Form myelin sheaths in PNS.

• Satellite Cells: Support neuron cell bodies in PNS.

Structure and Function of the Myelin Sheath

The myelin sheath is a protective covering that insulates axons and speeds up electrical transmission.

• Structure: Layers of lipid-rich membrane formed by oligodendrocytes (CNS) or Schwann cells (PNS).

• Function: Increases conduction velocity, prevents signal loss, and aids in regeneration (PNS).

• Example: Multiple sclerosis is caused by myelin sheath damage in CNS.

Nucleus vs. Ganglion; Nerve vs. Tract

These terms describe collections of neuron cell bodies and axons in different locations.

• Nucleus: Cluster of neuron cell bodies in CNS.

• Ganglion: Cluster of neuron cell bodies in PNS.

• Nerve: Bundle of axons in PNS.

• Tract: Bundle of axons in CNS.

• Example: Dorsal root ganglion (PNS) vs. basal nuclei (CNS).

Types of Membrane Ion Channels

Ion channels regulate the movement of ions across the neuronal membrane, crucial for electrical signaling.

• Leak Channels: Always open; allow passive ion movement.

• Gated Channels: Open or close in response to stimuli.

  • Voltage-Gated: Respond to changes in membrane potential.

  • Chemically-Gated (Ligand-Gated): Open in response to neurotransmitters.

  • Mechanically-Gated: Open in response to physical deformation.

Resting Membrane Potential and Its Electrochemical Basis

The resting membrane potential is the voltage difference across the membrane when the neuron is not transmitting signals.

• Definition: The inside of the neuron is negatively charged relative to the outside, typically about -70 mV.

• Electrochemical Basis: Maintained by ion gradients (mainly Na+ and K+) and selective permeability.

  • Na+/K+ pump moves 3 Na+ out and 2 K+ in per ATP.

• Equation: Additional info: This is the simplified Nernst equation for potassium.

Graded Potentials and Examples

Graded potentials are local changes in membrane potential that vary in size and decrease with distance.

• Characteristics: Can be depolarizing or hyperpolarizing; occur in dendrites and cell bodies.

• Examples: Excitatory postsynaptic potential (EPSP), inhibitory postsynaptic potential (IPSP).

Comparison of Graded Potentials and Action Potentials

Both are electrical signals, but differ in properties and functions.

• Graded Potentials: Variable amplitude, decremental, not self-propagating.

• Action Potentials: All-or-none, constant amplitude, self-propagating along axon.

• Table:

Generation and Propagation of Action Potentials

Action potentials are rapid, all-or-none electrical signals that travel along axons.

• Generation: Triggered when membrane potential reaches threshold; voltage-gated Na+ channels open.

• Propagation: Depolarization spreads, opening adjacent channels, moving the signal down the axon.

• Equation: Additional info: This is Ohm's law for ion flow.

Absolute and Relative Refractory Periods

Refractory periods ensure unidirectional propagation and limit firing frequency.

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

• Relative Refractory Period: Action potential possible with stronger stimulus; some Na+ channels reset, K+ channels still open.

Saltatory vs. Continuous Conduction

Action potentials travel differently depending on myelination.

• Saltatory Conduction: Occurs in myelinated axons; action potential jumps between nodes of Ranvier, increasing speed.

• Continuous Conduction: Occurs in unmyelinated axons; action potential moves along entire membrane, slower.

• Example: Myelinated motor neurons use saltatory conduction for rapid muscle activation.