The Nervous System: Autonomic & Motor Systems

Chapter 11: The Nervous System – Autonomic & Motor Systems

Overview of the Nervous System

The nervous system is responsible for coordinating and regulating bodily functions through electrical and chemical signaling. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS is further subdivided into sensory (afferent) and motor (efferent) divisions. The motor division includes the somatic nervous system and the autonomic nervous system (ANS).

• Central Nervous System (CNS): Composed of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS.

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions such as heart rate, digestion, and respiratory rate.

Divisions of the Autonomic Nervous System (ANS)

The ANS is divided into the sympathetic and parasympathetic divisions, which often have opposing effects on target organs. Most visceral organs receive dual innervation from both divisions, allowing for fine-tuned regulation of bodily functions.

• Sympathetic Division: Prepares the body for 'fight or flight' responses during stressful situations.

• Parasympathetic Division: Promotes 'rest and digest' activities that conserve and restore energy.

• Dual Innervation: Most organs are innervated by both divisions, which typically exert opposite effects.

Central Control of Autonomic Function

Autonomic functions are regulated by specific areas of the brain, including the hypothalamus, pons, and medulla oblongata.

• Hypothalamus: The main integration center for autonomic functions.

• Pons: Contains respiratory centers.

• Medulla Oblongata: Contains cardiovascular and additional respiratory centers.

Anatomy of the Autonomic Nervous System

The ANS uses a two-neuron chain to reach its effectors:

• Preganglionic Neuron: Originates in the CNS and synapses in an autonomic ganglion.

• Postganglionic Neuron: Extends from the ganglion to the effector organ (smooth muscle, cardiac muscle, or glands).

Sympathetic Division (Thoracolumbar Division)

• Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord.

• Short preganglionic fibers, long postganglionic fibers.

• Neurotransmitters: Acetylcholine (ACh) at preganglionic synapses; norepinephrine (NE) or epinephrine (Epi) at most postganglionic synapses.

• Prepares the body for emergency situations ('fight or flight').

Parasympathetic Division (Craniosacral Division)

• Preganglionic neurons originate in the brainstem and sacral spinal cord.

• Long preganglionic fibers, short postganglionic fibers.

• Neurotransmitter: Acetylcholine (ACh) at both pre- and postganglionic synapses.

• Promotes maintenance activities and conserves energy ('rest and digest').

Major Parasympathetic Nerves

• Cranial Nerves: Oculomotor (III), Facial (VII), Glossopharyngeal (IX), and Vagus (X).

• Vagus Nerve (X): Provides extensive parasympathetic innervation to thoracic and abdominal organs.

• Pelvic Nerves: Innervate lower abdominal and pelvic organs.

Functional Roles of the Sympathetic and Parasympathetic Divisions

• Sympathetic Effects: Increases heart rate, dilates bronchioles, mobilizes energy stores, redirects blood flow to muscles.

• Parasympathetic Effects: Decreases heart rate, stimulates digestive activity, constricts pupils, promotes glandular secretion.

• Organs with Only Sympathetic Innervation: Adrenal medulla, arrector pili muscles, sweat glands, some blood vessels.

Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (spinal cord T1-L2)	Craniosacral (brainstem & sacral spinal cord)
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Location of Ganglia	Close to spinal cord	Near or within effector organs
Main Neurotransmitter (Postganglionic)	NE (norepinephrine)	ACh (acetylcholine)

Neurotransmitters and Receptors in the ANS

• Cholinergic Fibers: Release acetylcholine (ACh).

• Adrenergic Fibers: Release norepinephrine (NE) or epinephrine (Epi).

Cholinergic Receptors

• Nicotinic Receptors: Found on all postganglionic neurons and skeletal muscle motor end plates; always excitatory.

• Muscarinic Receptors: Found on all parasympathetic target organs and some sympathetic targets (e.g., sweat glands); can be excitatory or inhibitory depending on the organ.

Adrenergic Receptors

• Alpha (α) Receptors: Usually excitatory; higher affinity for NE.

• Beta (β) Receptors: Can be excitatory or inhibitory; subtypes include β1 (heart, kidneys), β2 (lungs, some blood vessels), and β3 (adipose tissue).

Summary Table: Effects of Sympathetic vs. Parasympathetic Stimulation

Organ/System	Sympathetic Effect	Parasympathetic Effect
Pupil (Iris)	Dilation (distance vision)	Constriction (close vision)
Salivary Glands	Thick, viscous saliva	Watery saliva
Heart	Increases rate and force	Decreases rate and force
Bronchioles	Dilation	Constriction
Digestive Tract	Decreases motility and secretions	Increases motility and secretions
Urinary Bladder	Decreased urine production, retention	Increased urine production, voiding

Autonomic Neuroeffector Junctions

Communication between autonomic neurons and their effectors occurs at neuroeffector junctions. The process involves neurotransmitter release from varicosities along the axon, binding to receptors on the effector cell, and subsequent cellular response.

1. Action potential arrives at varicosity.

2. Voltage-gated Ca2+ channels open.

3. Ca2+ triggers exocytosis of neurotransmitter.

4. Neurotransmitter binds to receptors on effector organ.

5. Response in effector organ occurs.

6. Neurotransmitter is degraded or diffuses away.

Somatic Nervous System vs. Autonomic Nervous System

• Somatic Nervous System: Controls voluntary movements via a single motor neuron that releases ACh at the neuromuscular junction, stimulating skeletal muscle contraction.

• Autonomic Nervous System: Controls involuntary functions via a two-neuron chain (preganglionic and postganglionic neurons) and can use ACh or NE as neurotransmitters.

Neuromuscular Junction (Somatic)

1. Action potential arrives at the motor neuron terminal.

2. Voltage-gated Ca2+ channels open.

3. Ca2+ triggers release of ACh.

4. ACh diffuses across the synaptic cleft and binds to nicotinic receptors on the muscle end plate.

5. Na+ channels open, causing depolarization and muscle contraction.

6. ACh is broken down by acetylcholinesterase, terminating the signal.

Key Equations

• Resting Membrane Potential: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

• Action Potential Propagation: $I = g(V_m - E_{ion})$

Additional info: Some details, such as the specific cranial nerves involved in parasympathetic innervation and the stepwise process at the neuromuscular junction, were expanded for clarity and completeness.