Nervous System: Structure, Function, and Organization

B10: Nervous System

Function and Organization

The nervous system is responsible for control and communication within the body, maintaining homeostasis and enabling rapid responses to internal and external stimuli.

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes cranial nerves and spinal nerves.

How the Nervous System Operates

• Sensory Receptors: Detect stimuli (e.g., touch, temperature) and send information to the CNS for interpretation.

• Motor Output: CNS sends instructions to effectors (muscles, glands) to produce a response.

• Types of Sensory Fibers:

  • First Order Neurons: Found in skin, skeletal muscles, or joints; send impulses to the CNS via Sensory Afferent Fibers.

  • Second Order Neurons: Located in the CNS; relay signals to higher centers.

• Types of Motor Fibers:

  • Somatic Motor System: Controls voluntary muscles (except skeletal muscles under reflex control).

  • Autonomic Motor System: Controls smooth muscle, cardiac muscle, and glands (involuntary control). The autonomic system is divided into:

    • Sympathetic Nervous System: Stimulates the effector to produce the "fight or flight" response (increases heart rate, dilates pupils).

    • Parasympathetic Nervous System: Stimulates the effector to produce the "rest and digest" response (slows heart rate, promotes digestion).

Neuronal Structure and Terminology

Neurons are the basic functional units of the nervous system, specialized for communication.

• Parts of a Neuron: Cell body, nucleus, axon, dendrites, axon hillock, Schwann cell, node of Ranvier, axon terminal.

• Types of Neurons:

  • Unipolar Neurons: One stalk at the cell body; axon and dendrites branch from a single process.

  • Multipolar Neurons: Multiple stalks from the cell body; one axon and several dendrites.

  • Bipolar Neurons: Two stalks from the cell body; one axon and one dendrite.

  • Association Neurons (Interneurons): Connect sensory and motor neurons within the CNS.

• Glial Cells: Support and protect neurons.

  • Astrocytes: Star-shaped; act as a barrier, provide nutrients to neurons.

  • Microglial Cells: Remove dead cells and debris.

  • Ependymal Cells: Line cavities; produce cerebrospinal fluid (CSF).

  • Satellite Cells: Surround cell bodies in the PNS.

  • Schwann Cells: Make segments of myelin in the PNS.

• Key Terminology:

  • Synapse: Junction between two neurons or a neuron and an effector.

  • Ganglia: Clusters of neuron cell bodies in the PNS.

  • Nucleus: Cluster of neuron cell bodies in the CNS.

  • Node of Ranvier: Gaps in myelin sheath that facilitate rapid signal transmission.

  • Tract: Bundle of axons in the CNS.

  • Nerve: Bundle of axons in the PNS.

Reflexes and Pathways

Reflexes are rapid, automatic responses to stimuli, mediated by neural pathways called reflex arcs.

• Example: Patellar (knee-jerk) reflex.

• Polysynaptic Reflex Arc: Involves multiple neurons and synapses.

• Withdrawal Reflex: Rapid removal from painful stimuli.

Homeostatic Imbalances

Disorders of the nervous system can disrupt normal function and homeostasis.

• Alzheimer's Disease: Breakdown of neurons, abnormal protein deposits, memory loss.

• Parkinson's Disease: Degeneration of dopamine-producing neurons, tremors, movement difficulties.

• Huntington's Disease: Genetic disorder, progressive deterioration of neurons, movement and cognitive decline.

• Multiple Sclerosis: Demyelination of neurons, muscle weakness, vision problems.

• Syncope: Sudden loss of consciousness due to reduced blood flow to the brain.

Brain Structure and Function

The brain is the control center of the nervous system, divided into several regions with specialized functions.

• Cerebrum: Largest part; divided into left and right hemispheres, connected by the corpus callosum.

  • Lobes: Frontal, Parietal, Temporal, Occipital.

  • Cortex: Outer layer of gray matter; responsible for higher cognitive functions.

  • Gyri and Sulci: Folds and grooves that increase surface area.

  • Functional Areas:

    • Motor Cortex: Controls voluntary movement.

    • Somatosensory Cortex: Processes sensory input.

    • Visual Cortex: Processes visual information.

    • Auditory Cortex: Processes sound.

• Diencephalon: Contains thalamus, hypothalamus, epithalamus, and pineal gland.

  • Thalamus: Relay station for sensory impulses.

  • Hypothalamus: Regulates hunger, thirst, temperature, and emotional responses.

  • Epithalamus: Produces CSF; pineal gland controls circadian rhythms.

  • Pituitary Gland: Releases hormones under hypothalamic control.

• Brain Stem: Includes midbrain, pons, and medulla oblongata; controls vital functions such as breathing and heart rate.

• Cerebellum: Coordinates movement, balance, and posture.

• Spinal Cord: Connects the brain with the PNS; mediates reflexes.

Meninges and Protection of the CNS

The CNS is protected by three layers of meninges and cerebrospinal fluid (CSF).

• Dura Mater: Tough, outermost layer.

• Arachnoid Mater: Middle layer; web-like projections.

• Pia Mater: Innermost layer; adheres closely to the brain and spinal cord.

• Subarachnoid Space: Contains CSF, which cushions and nourishes the CNS.

• Meningitis: Inflammation of the meninges, often caused by infection.

Summary Table: Major Divisions and Functions of the Nervous System

Division	Main Components	Primary Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, processing, and coordination of sensory and motor information
Peripheral Nervous System (PNS)	Cranial Nerves, Spinal Nerves	Transmission of signals between CNS and body
Somatic Nervous System	Motor Neurons, Sensory Neurons	Voluntary control of skeletal muscles
Autonomic Nervous System	Sympathetic & Parasympathetic Divisions	Involuntary control of smooth muscle, cardiac muscle, glands

Key Equations and Concepts

• Resting Membrane Potential: The difference in electrical charge across the neuron's membrane at rest. $V_{rest} \approx -70\ \text{mV}$

• Action Potential: Rapid change in membrane potential that travels along the axon. $V_{m} = V_{rest} + \Delta V$

Additional info: Some terminology and details were expanded for clarity and completeness, including definitions of neuron types, glial cells, and major brain regions.