6.1.13 The Autonomic Nervous System

Divisions of the Autonomic Nervous System

1. Sympathetic Nervous System (SNS)

• Prepares the body for "fight or flight" responses during stress or emergencies.
• Increases heart rate, dilates pupils, and inhibits digestion.
• Neurotransmitter: Noradrenaline.

2. Parasympathetic Nervous System (PNS)

• Promotes "rest and digest" activities to conserve energy and maintain normal functions.
• Decreases heart rate, constricts pupils, and stimulates digestion.
• Neurotransmitter: Acetylcholine.

Key Features of the ANS

• The ANS uses a two-neurone relay system:
  • A preganglionic neurone originates in the central nervous system (CNS).
  • It synapses with a postganglionic neurone in an autonomic ganglion.
• Sympathetic pathways: Ganglia are located close to the spinal cord.
• Parasympathetic pathways: Ganglia are located near or within the target organs.

Effects of the ANS on Organs

Organ/System	Sympathetic Effect	Parasympathetic Effect
Heart	Increases heart rate	Decreases heart rate
Lungs	Dilates bronchi	Constricts bronchi
Digestive System	Inhibits digestion	Stimulates digestion
Pupils	Dilates	Constricts
Bladder	Relaxes bladder wall	Contracts bladder wall

Nerve Structure in the ANS

Neurones are specialised nerve cells that play a key role in transmitting impulses:

• Dendrites: Conduct impulses towards the cell body.
• Axons: Transmit impulses away from the cell body.
• Myelin sheath: Insulates axons, increasing the speed of action potential conduction.

The Resting Potential

• In the resting state, the neurone membrane is polarised:
  • Outside: Positively charged due to high sodium ion concentration.
  • Inside: Negatively charged due to large anions and fewer positive ions.
• This polarity is maintained by the sodium-potassium pump which actively transports 3 Na⁺ out and 2 K⁺ in.