15.02 Nervous Communication

Overview of the Nervous System

1. General Functions

• Sensory Input: Detects stimuli from the environment.
• Integration: Processes and interprets sensory input in the CNS.
• Motor Output: Responds to stimuli by activating effectors (muscles/glands).

2. Divisions of the Nervous System

A. Central Nervous System (CNS)

• Components: Brain + Spinal Cord.
• Functions:
  • Brain: Controls higher functions (e.g., cognition, coordination, autonomic regulation).
    • Regions: Cerebrum, cerebellum, brainstem (specific roles).
  • Spinal Cord:
    • Functions: Reflex actions, communication between brain and PNS.

B. Peripheral Nervous System (PNS)

• Components:
  • Cranial nerves (attached to brain, 12 pairs).
  • Spinal nerves (attached to spinal cord, 31 pairs).

• Subdivisions:

1. Somatic Nervous System (SNS): Voluntary control (skeletal muscles).
2. Autonomic Nervous System (ANS): Involuntary control (internal organs).
   • Sympathetic Division: “Fight or flight” response.
   • Parasympathetic Division: “Rest and digest” response.

3. Types of Neurones (Nerve Cells)

• Neurones: Transmit nerve impulses at high speeds.
• Types:

1. Sensory Neurones:
   • Transmit impulses from sensory organs to CNS.
   • Input: Receptors (e.g., photoreceptors, mechanoreceptors).
2. Motor Neurones:
   • Transmit impulses from CNS to effectors (muscles or glands).
   • Controls skeletal (voluntary), smooth, or cardiac muscles (involuntary).
3. Intermediate/Relay Neurones:
   • Connect sensory neurones to motor neurones.
   • Located entirely within CNS (spinal cord, brain).

4. Structure of Neurones

A. General Structure

• Common Features:
  • Cell body (contains nucleus and cytoplasm).
  • Dendrites (receive signals).
  • Axon (transmits signals).
  • Axon terminals (contain vesicles with neurotransmitters).
• Synapse:
  • Junction between two neurones or between neurones and effectors.
• Neurotransmitters: Facilitate impulse transmission (e.g., acetylcholine).

B. Motor Neurone

• Cell Body:
  • Contains nucleus.
  • Located in brain or spinal cord.
  • Cytoplasm: Rough endoplasmic reticulum for protein synthesis.
• Dendrites:
  • Short, highly branched.
  • Provide a large surface area for connections.
• Axon:
  • Long extension for impulse transmission (e.g., spinal cord → toe).
  • Insulated by myelin sheath (formed by Schwann cells).
• Axon Terminals:
  • Contain mitochondria + vesicles with neurotransmitters (for signaling).
• Schwann Cells: Surround axon, aid in impulse conduction.
• Node of Ranvier:
  • Gaps between Schwann cells.
  • Enable saltatory conduction (faster transmission).

C. Sensory Neurone

• Features:
  • Single, long axon.
  • Cell body: Near stimulus source or in ganglion (nerve swelling).
• Dendron: May not apply to all sensory neurones.

D. Relay Neurone

• Location: Found only in CNS.
• Function: Connect sensory and motor neurones.

5. Impulse Transmission

• Myelin Sheath:
  • Insulates axon; formed by Schwann cells (PNS) or oligodendrocytes (CNS).
• Role: Speeds up conduction of impulses.
• Saltatory Conduction: Impulses jump between Nodes of Ranvier.
• Depolarization: Change in axon membrane potential during impulse transmission.

6. Specialized Reflex Arcs

• Reflex involves all three types of neurones:
• Sensory → Relay → Motor.
• Function: Automatic, rapid response to stimuli (e.g., knee-jerk reflex).

7. Key Terms

• CNS: Brain + Spinal Cord.
• PNS: Cranial and spinal nerves.
• Motor Neurone: Transmits impulses CNS → effectors.
• Sensory Neurone: Transmits impulses sensory organs → CNS.
• Relay Neurone: Connects sensory and motor neurones within CNS.
• Synapse: Junction where signals are transmitted via neurotransmitters.
• Myelin Sheath: Insulating layer for faster impulse conduction.