Overview of the Nervous System

The nervous system is a body‑wide network of nerve cells that gathers information from the environment, processes it, and initiates actions by sending electrochemical signals to organs and muscles.
Two major divisions:
- Central Nervous System (CNS)
- Peripheral Nervous System (PNS)

Central Nervous System (CNS)

CNS – the “command center” that performs complex processing; includes the brain and spinal cord.
Brain – site of conscious thought and much unconscious processing.
Spinal cord – conduit between brain and body; also capable of simple reflexes (e.g., knee‑jerk).
The CNS integrates sensory input and coordinates motor output; reflexes can occur without being processed by the brain.

PNS – a body‑wide network of messenger neurons that relay sensory information to the CNS and carry motor instructions away from it.
Afferent (sensory) neurons – transmit data to the CNS.
Efferent (motor) neurons – transmit data from the CNS.
The PNS splits into two functional systems:
- Division: Somatic Nervous System (SNS)
- Primary Target: Skeletal muscles
- Voluntary?: ✅ Voluntary
- Typical Example: Raising your arm
- Division: Autonomic Nervous System (ANS)
- Primary Target: Internal organs & glands
- Voluntary?: ❌ Involuntary
- Typical Example: Heart rate regulation

ANS – regulates internal organs automatically, without conscious input.
It further divides into two opposing branches that together maintain homeostasis.
- Sympathetic Division (Fight‑or‑Flight)
- Parasympathetic Division (Rest‑and‑Digest)

Activation: stress, danger, intense activity.
Primary neurotransmitter: noradrenaline (norepinephrine).
Physiological effects:
- ↑ Heart rate & force of contraction
- ↑ Breathing rate
- Dilation of pupils (mydriasis)
- ↓ Digestive activity
Purpose: mobilize energy and prepare the body for rapid action.

Activation: calm, post‑meal, recovery states.
Primary neurotransmitter: acetylcholine.
Physiological effects:
- ↓ Heart rate
- ↓ Breathing rate
- Constriction of pupils (miosis)
- ↑ Digestive activity (stimulates peristalsis, enzyme release)
Purpose: conserve energy, promote maintenance and repair.

Homeostasis – the dynamic balance maintained by the coordinated actions of the sympathetic and parasympathetic divisions.
Under normal conditions, the two branches offset each other, keeping internal variables (heart rate, blood pressure, digestion, etc.) within optimal ranges.

The enteric nervous system (often called the “gut brain”) contains ≈ 100 million nerve cells — roughly the same number found in a cat’s brain.
- Number expressed as: ext{≈ }10^{8} nerve cells.
These cells help process and digest food independently of the CNS, illustrating the extensive reach of the nervous system throughout the body.
Typical triggers and responses (from the transcript):
- Typical trigger: Stress, danger, exercise ➜ activates sympathetic outputs
- Relaxation, digestion ➜ activates parasympathetic outputs
Neurotransmitters involved (in this context):
- Noradrenaline (norepinephrine)
- Acetylcholine
Integrated effects across systems (summary):
- Heart: Increases rate & force (sympathetic) / Decreases rate (parasympathetic)
- Respiration: Increases rate (sympathetic) / Decreases rate (parasympathetic)
- Pupil size: Dilates (sympathetic) / Constricts (parasympathetic)
- Digestive system: Inhibits (sympathetic) / Stimulates (parasympathetic)
Overall:
- Fight‑or‑flight (sympathetic dominant)
- Rest‑and‑digest (parasympathetic dominant)

The CNS–PNS organization forms a closed loop: sensory input → CNS processing → motor output to muscles and glands.
Reflexes (e.g., knee‑jerk) demonstrate rapid, automatic responses that can bypass higher brain processing, illustrating spinal cord function as a reflex arc.
Understanding autonomic balance is key in physiology and medicine (e.g., stress management, autonomic disorders, anesthesia considerations).
The enteric nervous system highlights the degree to which the body maintains independent neural control in addition to CNS oversight, with implications for digestion, gut health, and neurogastroenterology.

Central Nervous System (CNS): brain and spinal cord; primary processing and command center.
Peripheral Nervous System (PNS): all nerves outside CNS; connects CNS to body.
Afferent neurons: carry sensory information to the CNS.
Efferent neurons: carry motor commands from the CNS to effectors.
Somatic Nervous System (SNS): voluntary control of skeletal muscles.
Autonomic Nervous System (ANS): involuntary control of internal organs and glands; maintains homeostasis.
Sympathetic division: prepares body for action (fight‑or‑flight).
Parasympathetic division: promotes conservation and restoration (rest‑and‑digest).
Mydriasis: pupil dilation.
Miosis: pupil constriction.
Enteric Nervous System: gut‑brain; neural network governing digestion.
Homeostasis: dynamic equilibrium of internal states.
Reflex arc: neural pathway that mediates an immediate, automatic response.

Enteric nervous system cell count: ext{≈ }10^{8} nerve cells.
Major neurotransmitters:
- Noradrenaline (norepinephrine) — sympathetic outputs
- Acetylcholine — parasympathetic outputs
Typical autonomic responses (sympathetic vs parasympathetic):
- Heart rate: ↑ (sympathetic) vs ↓ (parasympathetic)
- Breathing rate: ↑ (sympathetic) vs ↓ (parasympathetic)
- Pupil size: Dilates (sympathetic) vs Constricts (parasympathetic)
- Digestive activity: Inhibits (sympathetic) vs Stimulates (parasympathetic)
Typical triggers:
- Stress, danger, exercise → sympathetic activation
- Relaxation, digestion → parasympathetic activation