Autonomic Nervous System

Divisions of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord.
• Peripheral Nervous System (PNS): Cranial and spinal nerves.
  • Afferent division: Carries information to the CNS.
  • Efferent division: Carries information from the CNS.
  • Enteric Nervous System: Involved in digestion.

Efferent Division

• Somatic/motor: Voluntary movement, skeletal muscle activation.
• Autonomic Nervous System (ANS): Involuntary functions, innervates organs, glands, smooth muscle.
  • Sympathetic Nervous System: "Fight or flight".
  • Parasympathetic Nervous System: "Rest and digest".

Autonomic Nervous System (ANS)

• Involuntary homeostasis maintenance.
• Regulates body temperature, cardiovascular, respiratory, and digestive functions.
• Two branches: Sympathetic and Parasympathetic.

Sympathetic Division

• Fight or flight: Prepares the body for action.
• Activated by exercise, excitement, anxiety, fear, embarrassment.
• Effects: Adrenaline release, increased heart rate, dry mouth, cold/sweaty skin, dilated pupils.
• During exercise: Blood diverted to muscles/heart, bronchioles dilate, liver releases glucose.

Parasympathetic Division

• Rest and digest: Conserves energy.
• Directs digestion, diuresis, defecation.
• Activated by relaxation/pleasure.
• Effects: Lower blood pressure/heart rate/respiratory rates, increased GI activity, glucose uptake, constricted pupils.

Anatomy of the Autonomic Nervous System

• Two neurons in efferent pathway: preganglionic and postganglionic.
• Ganglion: Collection of cell bodies outside the CNS.

Key Anatomical Differences Between ANS Divisions

• Innervate most organs.

• Two neurons between CNS and effector.

  1. Sites of origin: Parasympathetic (brain and sacral spinal cord), Sympathetic (thoracic and lumbar regions).
  2. Fiber lengths: Parasympathetic (long preganglionic, short postganglionic), Sympathetic (short preganglionic, long postganglionic).
  3. Ganglia location: Parasympathetic (near effector organ), Sympathetic (close to spinal cord).

• Sympathetic NS innervates adrenal medulla (releases adrenaline/noradrenaline as hormones).

Sympathetic Innervation of Adrenal Medulla

• Receives input from sympathetic preganglionic neuron.
• Activation releases adrenaline/noradrenaline into bloodstream.
• Creates slower, long-lasting effects.

Neurotransmitters and Receptors of the ANS

• Major neurotransmitters: Acetylcholine (ACh) and Noradrenaline (NA).
• Acetylcholine:
  • Released by all preganglionic axons and parasympathetic postganglionic axons.
• Noradrenaline:
  • Released by sympathetic postganglionic axons (except sweat glands).
• Effects depend on receptor type.

Acetylcholine Receptors

1) Nicotinic Receptors

• Ligand-gated ion channels permeable to sodium ions.
• Always stimulatory (depolarizes postsynaptic cell).
• Found on: Postganglionic neurons (sympathetic/parasympathetic), adrenal medulla cells, skeletal muscle cells.

2) Muscarinic Receptors

• G protein-coupled receptors.
• Can be inhibitory or excitatory (depends on receptor type).
• Found on: Cells stimulated by parasympathetic postganglionic neurons, sweat glands innervated by sympathetic neurons.

Adrenergic Receptors

• Respond ONLY to noradrenaline and adrenaline (G protein-coupled receptors).
  • Alpha (\alpha) adrenoceptors: {\alpha}1, {\alpha}2
  • Beta (\beta) adrenoceptors: {\beta}1, {\beta}2, {\beta}3
• Effects depend on receptor subclass.

ANS Receptors as Drug Targets

• Beta blockers: Block {\beta}1 receptors to slow heart rate.
• {\beta}2 activating drugs: Dilate bronchi in lungs for asthma.
• Muscarinic receptor inactivating drugs: Reduce nausea/vomiting.
• {\alpha}1 activating drugs: Cause vasoconstriction for nasal congestion.

Control of the Autonomic Nervous System

• Controlled by brainstem (directly) and hypothalamus (via brainstem/hormonally).
• Brain regulates ANS via control centers for body temperature, water balance, food intake, cardiovascular system, respiration.
• Spinal cord controls autonomic reflexes (defecation/urination).

Control by Higher Brain Regions

• Input from periphery and higher brain regions to brainstem/hypothalamus.
• Limbic system activates emotional responses (anger, fear, pleasure).
• Negative emotions activate fight or flight response.
• Voluntary control: Breathing, biofeedback.