The Nervous System

Introduction

  • The nervous system is the body's control and communication center, utilizing receptors for monitoring internal and external changes.
  • Key processes:
    • Sensory Input: Collection of information from sensors.
    • Integration: The decision-making process to determine the appropriate response.
    • Motor Output: The physical response carried out by effectors based on the nervous system's determination.

Nervous System Organization

  • The nervous system is complex and divided into:
    • Structural Classification: Divided into Central Nervous System (CNS) and Peripheral Nervous System (PNS).
    • Functional Classification: Focused on the PNS and further divided into sensory (afferent) and motor (efferent) divisions.
Central Nervous System (CNS)
  • Composed of the brain and spinal cord, the CNS acts as the command center for the body.
Peripheral Nervous System (PNS)
  • Encompasses all nerves branching from the CNS, serving as communication lines that carry sensory information to the CNS and motor instructions from the CNS.

Functional Classification

  1. Sensory (Afferent) Division: Carries impulses to the CNS from sensory organs.
  2. Motor (Efferent) Division: Carries impulses from the CNS to effectors (muscles or glands).
    • Somatic Nervous System: Controls skeletal muscle; voluntary.
    • Autonomic Nervous System: Controls smooth and cardiac muscle, and glands; involuntary.
      • Further divided into:
      • Sympathetic Division: Activates during stress or emergency (fight or flight).
      • Parasympathetic Division: Active during rest (rest and digest).

Supporting Cells (Neuroglia)

  • Supporting cells help protect and insulate neurons and include:
    1. Astrocytes: Star-shaped, link neurons and blood vessels.
    2. Microglia: Act as scavengers, removing debris.
    3. Ependymal Cells: Line brain cavities; help circulate cerebrospinal fluid (CSF).
    4. Oligodendrocytes: Form myelin sheaths around CNS axons.
    5. Schwann Cells: Form myelin sheaths in the PNS.
    6. Satellite Cells: Provide cushioning around neuron cell bodies in PNS.

Neurons

  • Definition: Specialized cells transmitting impulses. Includes a cell body, dendrites, and axons.
  • Neuron Structure:
    • Cell Body (Soma): Metabolic center with organelles (not centrioles).
    • Dendrites: Extensions that receive signals.
    • Axons: Single long fibers that transmit impulses away from the cell body. Axons may branch at terminal ends into multiple axon terminals, containing neurotransmitters.
  • Myelin Sheaths:
    • Insulate axons to increase impulse speed. Formed by Schwann cells in the PNS and oligodendrocytes in the CNS. The outer layer is called the neurilemma.

Nerve Impulse Physiology

  • Neurons respond to stimuli (irritability) and transmit impulses (conductivity).
  • At rest, there are more negatively charged ions inside (primarily K+K^+) than outside (primarily Na+Na^+).
  • When a stimulus occurs:
    1. Depolarization: Sodium channels open, Na+Na^+ enters, leading to the inside of the neuron becoming more positive.
    2. If depolarization is significant, an Action Potential (AP) is generated.
    3. Repolarization: After the AP, potassium channels open, K+K^+ exits, restoring a negative internal environment.
    4. The sodium-potassium pump restores original ionic conditions (3 Na+Na^+ out for 2 K+K^+ in).

Signal Transmission at the Synapse

  • When an AP reaches the axon terminal, it opens calcium channels, leading to the fusion of neurotransmitter vesicles with the membrane and release into the synaptic cleft.
  • Neurotransmitters bind to receptors on the receiving neuron, triggering ion channels to open, facilitating signal continuation.
  • The neurotransmitter's effect is brief, cleared by diffusion, reuptake, or enzymatic degradation.

Reflex Physiology

  • Reflex Arcs: Involuntary, predictable responses that involve sensory neurons, interneurons, and motor neurons. Key components:
    1. Sensory Receptor: Detects stimulus.
    2. Sensory Neuron: Transmits signal to CNS.
    3. Interneuron: Processes information in CNS.
    4. Motor Neuron: Conducts command from CNS to effectors.
    5. Effector Organ: Muscle or gland that carries out the response.

Brain Structure and Function

  • The brain comprises multiple regions including:
    • Cerebrum: Higher cognitive functions; divided into lobes.
    • Cerebellum: Coordination and balance.
    • Brain Stem: Controls basic life functions.
Cranial Nerves
  • 12 pairs of cranial nerves serve critical functions in sensory and motor control of the face and neck. Key nerve: Vagus nerve extends into thoracic and abdominal cavities.

Autonomic Nervous System (ANS)

  • Sympathetic System: Mobilizes body for action; effects include increased heart rate, bronchodilation.
  • Parasympathetic System: Restores the body to a calm state; effects include decreased heart rate and enhancement of digestion.
  • Maintaining balance between the two systems is critical for homeostasis.

Protection of the CNS

  • Physical and chemical protections include bones, meninges (dura mater, arachnoid mater, pia mater), cerebrospinal fluid (CSF), and blood-brain barrier, which prevents harmful substances from affecting brain function.
  • CSF acts as a cushion and is involved in the circulation of nutrients and waste removal.

Spinal Cord

  • Extends from the foramen magnum to lumbar vertebrae, and is protected by the meninges and CSF. Contains both gray matter (H or butterfly-shaped arrangement) and white matter (myelinated nerve tracts).
  • 31 pairs of spinal nerves arise from the spinal cord, each consisting of sensory and motor fibers, playing roles in reflexes and communication between body and brain.