Comprehensive Notes on the Nervous System and Its Divisions

Afferent vs. Efferent Nervous System

• Afferent Division: Carries sensory signals from the body to the CNS.

  • Components:
  • Somatic sensory pathways (e.g. from skin, special senses)
  • Visceral sensory pathways (e.g. from internal organs)

• Efferent Division: Carries motor signals from the CNS to the body.

  • Components:
  • Somatic motor nervous system (voluntary control of skeletal muscles)
  • Autonomic motor nervous system (involuntary control over heart, smooth muscles, glands)

Functional Organization of the Nervous System

Somatic Nervous System (SNS)

• Processes: Consciously perceived or controlled processes.
• Sensory Portion:
  • Detects signals from special senses: vision, hearing, equilibrium, smell, taste.
  • Receives information from skin and proprioceptors to sense body position.
• Motor Portion:
  • Elicits voluntary movements through the cerebrum.
  • Reflexive movements involve brainstem and spinal cord.

Autonomic Nervous System (ANS)

• Definition: Also known as the autonomic motor or visceral motor system.
• Functions: Transmits signals from the CNS to:
  • Heart
  • Smooth muscle
  • Glands
• Homeostasis: Maintains internal body conditions within optimal ranges through involuntary control.

Lower Motor Neurons

Somatic Nervous System

• Composed of:
  • A single lower motor neuron structure extending from CNS to skeletal muscle fibers.
  • Cell body within brainstem/spinal cord.
  • Myelinated axon releasing acetylcholine (ACh) at neuromuscular junction for muscle excitation.

Autonomic Nervous System

• Two-Neuron Chain:

  • Preganglionic Neuron:
  • Cell body in brainstem/spinal cord, it releases ACh to excite ganglionic (postganglionic) neuron.
  • Thin, myelinated axon travels to autonomic ganglion.
  • Ganglionic Neuron:
  • Cell body within autonomic ganglion, it can release either ACh or norepinephrine (NE) to influence target cell activity.

• Neuronal Convergence & Divergence:

  • Convergence: Multiple preganglionic neurons synapsing on one ganglionic neuron enhances processing.
  • Divergence: A single preganglionic neuron synapsing with numerous ganglionic neurons for widespread control.

CNS Control of the Autonomic Nervous System

• Hierarchy:
  • Hypothalamus: Key integration center for both divisions of ANS; mediates emotional responses and regulatory processes like fight-or-flight.
  • Brainstem: Mediates visceral reflexes (e.g., blood pressure regulation).
  • Spinal Cord: Governs reflexes like defecation and urination, especially by the parasympathetic system.

Functional Differences: Divisions of ANS

Parasympathetic Division

• Function: "Rest-and-digest"; conserves energy, replenishes nutrients.

Sympathetic Division

• Function: "Fight-or-flight"; prepares body for exercise, excitement, and emergencies.

Anatomic Differences

• Parasympathetic:

  • Originates from brainstem and sacral spinal cord (S2-S4).
  • Long preganglionic axons and short postganglionic axons.
  • Ganglia are located nearby or within the target organs.

• Sympathetic:

  • Originates from lateral horns of T1-L2.
  • Short preganglionic axons and long postganglionic axons.
  • Ganglia located close to the spinal cord (sympathetic trunk).

Degree of Response

• Parasympathetic activity is localized, targeting few effectors at once due to limited branching.
• Sympathetic activity often facilitates mass activation, influencing multiple effectors simultaneously due to extensive branching.

Parasympathetic Pathways

• Cranial Nerves: Four main cranial nerves convey parasympathetic signals:
  • Oculomotor (CN III): Controls eye focuses and pupil constriction.
  • Facial (CN VII): Controls lacrimal and salivary glands.
  • Glossopharyngeal (CN IX): Controls parotid salivary gland.
  • Vagus (CN X): Innervates heart, lungs, and visceral organs, regulating heart rate and digestive functions.

Sympathetic Pathways

• Preganglionic Neurons:

  • Originate in the lateral horn of the spinal cord from T1 to L2.
  • Their axons enter the sympathetic trunk and synapse in ganglia.

• Postganglionic Effects: Sympathetic effects vary among systems, such as:

  • Cardiovascular: Increases heart rate and dilates coronary arteries.
  • Respiratory: Dilates bronchi for increased airflow.
  • Digestive: Reduces motility and secretion processes.

Enteric Nervous System

• Overview: The "second brain" consists of neurons within the GI tract.
• Function: Coordinates autonomic functions like peristalsis and digestion through two major plexuses (submucosal and myenteric).

Neurotransmitters in ANS

• Acetylcholine (ACh): Released from cholinergic neurons (all preganglionic autonomic neurons and postganglionic parasympathetic neurons).
• Norepinephrine (NE): Released from adrenergic neurons (most sympathetic neurons).
• Receptors:
  • Cholinergic Receptors: Nicotinic (located on ganglionic neurons) and muscarinic (target organs).
  • Adrenergic Receptors: Found on target organs affected by norepinephrine.