3.3 The Nervous System

Structure of a Neuron

• Fundamental Unit: Neuron, specialized for information transmission.
• Parts:
  • Cell Body (Soma): Contains nucleus, metabolic center.
  • Dendrites: Receive signals, transmit towards cell body.
  • Axon: Carries impulses away from cell body.
  • Myelin Sheath: Fatty insulation (Schwann cells), with Nodes of Ranvier; increases speed (saltatory conduction).
  • Axon Terminals: Branched endings, release neurotransmitters into synapse.

Divisions of the Nervous System

• Central Nervous System (CNS): Brain and Spinal Cord (command center).
• Peripheral Nervous System (PNS): Nerves branching from CNS to rest of body.
  • Somatic Nervous System: Voluntary control (skeletal muscles).
  • Autonomic Nervous System (ANS): Involuntary functions (heart rate, digestion).
    • Sympathetic: "Fight or flight."
    • Parasympathetic: "Rest and digest."

The Brain: Key Parts

• Cerebrum: Largest part; intelligence, memory, voluntary actions.
• Cerebellum: Coordinates muscle movements, posture, balance.
• Medulla Oblongata: Controls vital involuntary functions (heartbeat, breathing).
• Thalamus: Sensory relay station.
• Hypothalamus: Regulates temperature, hunger, thirst, links to endocrine system.
• Pons: Relays signals, role in sleep.

The Spinal Cord

• Long bundle of nervous tissue from medulla oblongata.
• Functions:
  • Pathway for nerve impulses to/from brain.
  • Center for many reflexes.

Actions: Voluntary and Involuntary

• Voluntary Actions: Under conscious control (e.g., walking, talking); initiated by cerebrum.
• Involuntary Actions: Not under conscious control (e.g., heartbeat, digestion); controlled by ANS.

Reflex Action

• Rapid, involuntary, stereotyped response to a stimulus.
• Reflex Arc: Neural pathway mediating a reflex.
  • Receptor → Sensory Neuron → Interneuron (CNS) → Motor Neuron → Effector.
• Types: Natural (innate, e.g., blinking) vs. Acquired (conditioned, e.g., salivating).

Sense Organs: The Eye

• Structure: Cornea, Iris, Pupil, Lens, Retina (photoreceptors), Optic Nerve.
• Vision:
  • Accommodation: Lens changes shape to focus.
  • Adaptation: Adjust to light levels.
  • Stereoscopic Vision: Depth perception.
• Defects: Myopia (nearsightedness), Hyperopia (farsightedness), Presbyopia, Astigmatism, Cataract.

Sense Organs: The Ear

• Parts:
  • Outer Ear: Pinna, Auditory Canal.
  • Middle Ear: Tympanic Membrane (eardrum), Ossicles (Malleus, Incus, Stapes).
  • Inner Ear: Cochlea (hearing), Vestibular System (balance).
• Functions: Hearing (sound waves to nerve impulses) and Balance (detects head movement).

Nerve Impulse Transmission

• Resting Potential: Neuron polarized (-70mV inside), maintained by Na+/K+ pump.
• Action Potential (All-or-None): Stimulus above threshold causes rapid depolarization (Na+ influx).
• Repolarization: K+ efflux restores negative charge.
• Hyperpolarization: Brief period more negative than resting potential.
• Refractory Period: Neuron cannot fire another action potential.
• Synaptic Transmission: Action potential triggers neurotransmitter release into synapse, binding to next neuron.