A core understanding of functional organization of the nervous system is essential for medical students to grasp the basis of neurological and physiological processes. The nervous system is the central command center of the human body, responsible for perception, movement, cognition, and the regulation of organ systems.
This article will explore the structural and functional hierarchy of the nervous system, mechanisms of neuronal communication, the autonomic nervous system (ANS), and the synthesis, action, and metabolism of neurotransmitters.
1. Overview: Functional Organization of the Nervous System
The nervous system is divided into:
- Central Nervous System (CNS): Comprising the brain and spinal cord.
- Peripheral Nervous System (PNS): Includes all neural elements outside the CNS.
Functionally, the PNS is further divided into:
- Somatic Nervous System: Controls voluntary motor functions.
- Autonomic Nervous System (ANS): Regulates involuntary activities like heart rate, digestion, and gland secretion.
2. Inter-Neuronal Communication and Synapse Structure
Neurons communicate via synapses, which are specialized junctions that allow for the transmission of signals from one neuron to another or to an effector cell (muscle/gland).
A. Synapse Structure
- Presynaptic Terminal: Contains vesicles filled with neurotransmitters.
- Synaptic Cleft: Narrow space through which neurotransmitters diffuse.
- Postsynaptic Membrane: Possesses receptors that bind neurotransmitters to propagate the signal.
B. Synaptic Transmission
- Action potential arrives at the presynaptic terminal.
- Voltage-gated calcium channels open, triggering exocytosis of neurotransmitters.
- Neurotransmitters bind to postsynaptic receptors.
- Ion channels open, leading to depolarization (excitatory) or hyperpolarization (inhibitory).
3. Action Potential: Generation and Mechanism
The action potential is an electrical impulse that facilitates the conduction of signals along neurons.
Phases of Action Potential:
- Resting State: −70 mV membrane potential; Na⁺ and K⁺ channels closed.
- Depolarization: Opening of Na⁺ channels causes influx of sodium ions.
- Repolarization: K⁺ channels open, potassium ions leave the cell.
- Hyperpolarization: Membrane potential becomes temporarily more negative.
- Refractory Period: Neuron is unresponsive to further stimuli until it resets.
4. Functional Organization of the Autonomic Nervous System (ANS)
The ANS regulates involuntary functions such as heart rate, respiration, and digestion. It has three subdivisions:
- Sympathetic (“Fight or Flight”)
- Parasympathetic (“Rest and Digest”)
- Enteric (GI tract-specific regulation)
A. Structural Organization
- Sympathetic: Originates from thoracolumbar spinal segments (T1–L2)
- Parasympathetic: Originates from cranial nerves III, VII, IX, X and sacral nerves (S2–S4)
B. Distribution
- Sympathetic nerves innervate nearly all body organs
- Parasympathetic innervation is more limited, focusing on internal organs
C. Neurotransmitters and Receptors
- Preganglionic neurotransmitter: Acetylcholine (ACh)
- Postganglionic neurotransmitters:
- ACh for parasympathetic
- Norepinephrine (NE) for sympathetic
- Receptors:
- Muscarinic (parasympathetic)
- Adrenergic (α and β, sympathetic)
5. Physiological Role of ANS
The ANS plays a pivotal role in maintaining homeostasis by regulating:
| System | Sympathetic Function | Parasympathetic Function |
|---|---|---|
| Heart | ↑ Heart rate, ↑ Contractility | ↓ Heart rate |
| Lungs | Bronchodilation | Bronchoconstriction |
| GI Tract | ↓ Motility, ↑ Sphincter tone | ↑ Motility, ↑ Secretions |
| Eyes | Pupil dilation (mydriasis) | Pupil constriction (miosis) |
| Bladder | Urinary retention | Promotes urination |
This dual control ensures balance and adaptation to internal and external stimuli.
6. Neurotransmitters of the ANS: Synthesis, Storage, and Action
A. Acetylcholine (ACh)
- Synthesis: Choline + Acetyl-CoA → ACh (via choline acetyltransferase)
- Storage: Synaptic vesicles in presynaptic terminals
- Action: Binds to muscarinic or nicotinic receptors
- Degradation: Rapidly hydrolyzed by acetylcholinesterase
B. Norepinephrine (NE)
- Synthesis: Tyrosine → DOPA → Dopamine → NE
- Storage: Vesicles in sympathetic nerve terminals
- Action: Binds to α and β adrenergic receptors
- Termination: Reuptake by presynaptic neuron or degradation by MAO/COMT
C. Other Modulators
- Dopamine: Precursor to NE; important in CNS
- Nitric Oxide (NO): A non-classical neurotransmitter; mediates vasodilation
7. Higher Control of ANS
Though the ANS operates subconsciously, it is under the influence of higher brain centers:
- Hypothalamus: Master controller of ANS
- Brainstem (medulla, pons): Cardiac, respiratory, and vasomotor centers
- Limbic system: Emotional influence on autonomic responses (e.g., fear → tachycardia)
8. Clinical Correlation
- Autonomic Dysreflexia: Seen in spinal cord injuries; exaggerated sympathetic response
- Horner’s Syndrome: Loss of sympathetic supply to the eye
- Parkinson’s Disease: Involves loss of dopaminergic neurons
- Pharmacologic targeting: Many drugs mimic or block ANS neurotransmitters (e.g., atropine, β-blockers)
Conclusion
The functional organization of the nervous system, especially its autonomic arm, governs critical involuntary activities vital for life. Understanding the intricate mechanisms of neurotransmission, receptor dynamics, and higher control is essential for diagnosing and managing neurological and systemic disorders. For medical students, this foundational knowledge serves as a gateway to advanced clinical neuroscience and pharmacology.