Functions of the Nervous System

Overview

The nervous system is the master control and communication system in the body, responsible for every thought, action, and emotion. It processes sensory information, integrates data, and initiates motor responses.

• Sensory Input: Uses sensory receptors to monitor changes (stimuli) inside and outside the body. - Example: Touching a hot surface activates sensory receptors in the skin.

• Integration: Processes and interprets sensory input to decide what actions are necessary. - Example: The brain analyzes pain and determines to withdraw the hand.

• Motor Output: Activates muscles or glands to respond to the stimulus. - Example: Muscles contract to move the hand away from the hot surface.

Feedback Loop: Nervous system functions are similar to a feedback loop, where information is received, processed, and results in motor response.

Organization of the Nervous System

Structural Classification

• Central Nervous System (CNS): Comprises the brain and spinal cord; acts as the command center, interpreting sensory information and issuing instructions.

• Peripheral Nervous System (PNS): Includes all nerves outside of the CNS; consists of spinal nerves (spinal cord) and cranial nerves (brain) and serves as communication lines between the body and the CNS.

Functional Classification

• Sensory Division (Afferent): Carries impulses from sensory receptors to the CNS; keeps it informed of external and internal changes.

• Motor Division (Efferent): Transmits impulses from the CNS to muscles and glands; causes motor response.

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

  • Autonomic Nervous System (ANS): Manages involuntary functions, with subdivisions:

    • Sympathetic: "Fight or Flight"

    • Parasympathetic: "Rest and Digest"

Neuroglia Cells of the Nervous System

Central Nervous System (CNS) Neuroglia

• Astrocytes: Support neurons, maintain environment, and remove debris.

• Ependymal Cells: Line brain and spinal cord cavities, produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Wrap around CNS nerve fibers, forming myelin sheaths that insulate and increase speed of nerve transmission.

Peripheral Nervous System (PNS) Neuroglia

• Schwann Cells: Form myelin sheaths around PNS nerve fibers; essential for rapid transmission of nerve impulses.

• Satellite Cells: Cushion and protect neuron cell bodies in the PNS; regulate chemical environment.

Myelin Sheath

Structure and Function

The myelin sheath is a whitish, fatty material that covers most long nerve fibers and has a waxy appearance.

• Protection and Insulation: Prevents electrical impulses from leaking out.

• Increased Speed of Transmission: Insulation increases the speed at which nerve impulses travel.

• Structure: In the PNS, Schwann cells wrap around axons, forming the sheath; in the CNS, oligodendrocytes perform this function.

• Nodes of Ranvier: Gaps in the myelin sheath crucial for rapid transmission of impulses.

Functions of Neurons/Nerve Cells

Overview

Neurons are specialized cells in the nervous system responsible for transmitting messages throughout the body.

• Dendrites: Branching fibers that convey incoming electrical signals towards the cell body.

• Axon: Long fiber that carries impulses away from the cell body.

Multiple Sclerosis (MS)

Cause and Effects

MS is an autoimmune disease where the immune system attacks the myelin sheaths in the CNS, disrupting nerve impulse transmission.

• Formation of hardened areas (scleroses) disrupts electrical pulse flow.

• Results in vision/speech disturbances, muscle control loss, and increased disability.

Additional info: The exact cause of MS is unknown, but it involves the immune system attacking a protein component of the myelin sheath.

Key Nervous System Terms

Definitions

• Ganglia: Small collections of neuron cell bodies outside the CNS and inside PNS; serve as relay stations.

• Nuclei: Clusters of neuron cell bodies within the CNS; carry out essential metabolic functions.

• Tracts: Bundles of nerve fibers in the CNS; transmit signals within the CNS.

• Nerves: Bundles of nerve fibers in the PNS; serve as communication lines.

• White Matter: Dense collections of myelinated nerve fibers (tracts); found in the CNS.

• Grey Matter: Mostly unmyelinated fibers and neuron cell bodies; found in regions such as cerebral cortex and basal nuclei.

Functional Classification of Nerves

Sensory Division (Afferent)

• Carry impulses toward the CNS from sensory receptors.

• Somatic Sensory Fibers: Deliver impulses from skin, skeletal muscles, and joints.

• Visceral Sensory Fibers: Transmit impulses from visceral organs.

Motor Division (Efferent)

• Carry impulses from the CNS to effector organs (muscles and glands).

• Somatic Nervous System: Controls voluntary movements.

• Autonomic Nervous System: Regulates involuntary activities.

  • Sympathetic: "Fight or Flight"

  • Parasympathetic: "Rest and Digest"

Structural Classification of Neurons

Types of Neurons

• Multipolar Neurons: Most common type; several processes including multiple dendrites and one axon.

• Bipolar Neurons: Rare; found in special sense organs (e.g., eyes, nose); two processes (one axon, one dendrite).

• Unipolar Neurons: Typically found in sensory neurons within the PNS; single process divides into two branches.

Nerve Cell Functional Properties

Irritability and Conductivity

• Irritability: Ability to respond to a stimulus by generating a nerve impulse.

• Conductivity: Ability to transmit the impulse to other neurons, muscles, or glands.

Action Potential: Irritability leads to the generation of an action potential (nerve impulse).

Action Potential: Steps and Mechanism

Phases of Action Potential

1. Resting Membrane Potential: The inside of the neuron is negatively charged compared to the outside due to the distribution of ions (mainly Na+ and K+).

2. Stimulus Initiates Local Depolarization: A stimulus changes the membrane permeability, allowing Na+ ions to enter, making the inside more positive.

3. Depolarization and Generation of Action Potential: If the stimulus is strong enough, depolarization reaches a threshold, and an action potential is generated.

4. Propagation of Action Potential: The action potential travels along the axon as more Na+ channels open sequentially.

5. Repolarization: K+ ions diffuse out, restoring the negative charge inside the cell.

6. Initial Ionic Conditions Restored: The Na+-K+ pump restores original ion concentrations.

All-or-Nothing Response: An action potential either occurs fully or not at all.

Key Equation: Resting Membrane Potential

The resting membrane potential is maintained by the sodium-potassium pump:

Table: Neuroglia Cells and Their Functions

Table: Structural Classification of Neurons

Additional info: These notes expand on the original content with definitions, examples, and tables for clarity and completeness.