Functional Organization of the Nervous System

Nervous System Functional Organization

• Two Main Divisions:
    - Somatic Nervous System (SNS)
    - Autonomic Nervous System (ANS)
      - Subdivisions:
        - Parasympathetic
        - Sympathetic

Somatic Nervous System (SNS)

• Definition:
    - Consciously perceived or controlled processes.
• Components:
    - Somatic Sensory Portion:
      - Detects signals from special senses (vision, hearing, equilibrium, smell, taste) and from skin and proprioceptors.
    - Somatic Motor Portion:
      - Sends signals from Central Nervous System (CNS) to skeletal muscles.
• Movement Control:
    - Voluntary Movements:
      - Involve the cerebrum.
    - Reflexive Movements:
      - Involve brainstem and spinal cord.

Autonomic Nervous System (ANS)

• Definition:
    - Also called autonomic motor or visceral motor system; processes regulated below the conscious level.
• Function:
    - Transmits signals from CNS to heart, smooth muscle, and glands.
    - Responds to visceral sensory inputs (e.g., from blood vessels) - note that those sensory neurons are not considered part of ANS.
    - Homeostasis:
      - Maintains conditions inside the body within optimal ranges.

Lower Motor Neurons in Nervous System

• Lower Motor Neurons of Somatic vs. Autonomic Nervous System:
    - Somatic Nervous System:
      - Single lower motor neuron extends from CNS to skeletal muscle fibers:
        - Cell body located in brainstem or spinal cord.
        - Myelinated axon with a large diameter releases acetylcholine (ACh) from synaptic knob to excite muscle fiber.
    - Autonomic Nervous System:
      - Utilizes a chain of two motor neurons to reach effector:
        - First Neuron (Preganglionic Neuron):
          - Cell body located within brainstem or spinal cord.
          - Thin, myelinated axon projects to autonomic ganglion in the peripheral nervous system; releases ACh to excite the second motor neuron.
        - Second Neuron (Ganglionic or Postganglionic Neuron):
          - Cell body in autonomic ganglion.
          - Very thin, unmyelinated axon projects to cardiac muscle, smooth muscle, or gland; releases ACh or norepinephrine (NE) to either excite or inhibit the effector.
          - Effectors have varying receptor types.

Central Nervous System Control of ANS

• Regulatory Centers:
    - Primarily regulated by the brain and spinal cord.
    - Hypothalamus:
      - Involved in emotional processes controlled by ANS (e.g., fight-or-flight response).
      - Influenced by cortex, thalamus, and limbic system.
    - Brainstem Nuclei:
      - Mediate visceral reflexes (e.g., blood pressure maintenance).
    - Spinal Cord:
      - Governs some ANS reflexes (e.g., defecation, urination controlled by parasympathetic nervous system).

Functional Differences in ANS Divisions

• Parasympathetic Division:
    - Known as “rest-and-digest.”
    - Functions to conserve energy and replenish nutrients.
• Sympathetic Division:
    - Known as “fight-or-flight.”
    - Functions to prepare the body for exercise, excitement, or emergency situations.

Anatomical Differences in Lower Motor Neurons

• Parasympathetic Division:
    - Craniosacral Division
      - Preganglionic neurons located in brainstem or S2–S4 spinal cord.
      - Long preganglionic axons and short postganglionic axons; ganglia are close to or within the effector organ.
    - Sympathetic Division:
      - Thoracolumbar Division
        - Preganglionic neurons in lateral horns of T1–L2.
        - Short preganglionic axons and long postganglionic axons; ganglia are near the spinal cord, often lateral or anterior to it.
        - Preganglionic axons exhibit many branches.

Comparison of Parasympathetic and Sympathetic Divisions

• Origin:
    - Parasympathetic Division:
      - Craniosacral originating from brainstem and S2-S4 segments.
    - Sympathetic Division:
      - Thoracolumbar originating from lateral horns of T1-L2 segments.
• General Functions:
    - Parasympathetic:
      - Conservation of energy, return body to homeostasis during rest.
    - Sympathetic:
      - Preparing body for emergencies and enhancing alertness.

Degree of Response in ANS

• Parasympathetic Activity:
    - Fairly localized due to long preganglionic axons with limited branches influencing only a few effectors at the same time.
• Sympathetic Activity:
    - Often facilitates mass activation with short preganglionic axons that influence several effectors simultaneously (e.g., during exercise or fear).

Parasympathetic Division Details

• Anatomical Origin:
    - Craniosacral.
    - Ganglia Types:
      - Terminal ganglia located close to the effector.
      - Intramural ganglia located within the wall of the target organ.
• Cranial Components:
    - Cranial Nerves Conveying Parasympathetic Signals:
      - Oculomotor (CN III)
      - Facial (CN VII)
      - Glossopharyngeal (CN IX)
      - Vagus (CN X)
    - Functions include:
      - Oculomotor: controls ciliary muscle and pupil constriction.
      - Facial: controls lacrimal glands, nasal, and salivary glands.
      - Glossopharyngeal: controls the parotid gland.
      - Vagus: controls heart rate, bronchoconstriction, digestive activities.

Pelvic Splanchnic Nerves

• Function:
    - Preganglionic axons from S2–S4 projecting to terminal or intramural ganglia, affecting abdominal and pelvic effectors with functions such as:
      - Smooth muscle contraction, increased secretions in urinary and digestive systems, penile and clitoral erection.

Sympathetic Division Overview

• Function:
    - Motivated by exercise and emergencies.
• Anatomical Origin:
    - Thoracolumbar - T1-L2 segments.
• Ganglia Location:
    - Close to the CNS with complex anatomical pathways.

Organization and Anatomy of the Sympathetic Division

• Preganglionic Neurons:
    - Cell bodies are in the lateral horn of T1–L2.
    - Myelinated axons course through anterior roots and branch off at various ganglia.
• Sympathetic Trunks and Ganglia:
    - Left and right trunks located just lateral to the vertebral column, with multiple ganglia associated with each spinal nerve.
    - Example ganglia:
      - Superior cervical ganglia innervate head, neck, and some thoracic viscera.

Autonomic Plexuses

• Definition:
    - Networks of nerves in thoracic and abdominopelvic cavities containing sympathetic postganglionic axons, parasympathetic preganglionic axons, and visceral sensory axons.
• Examples of Autonomic Plexuses:
      - Cardiac Plexus: regulates heart rate; sympathetic speeds up, parasympathetic slows down.
      - Pulmonary Plexus: regulates bronchodilation (sympathetic) and bronchoconstriction (parasympathetic).
      - Abdominal Aortic Plexus: innervates abdominal organs; consists of celiac, superior mesenteric, and inferior mesenteric plexuses.

Autonomic Reflexes

• Definition:
    - Pre-programmed response to a stimulus facilitated by a reflex arc involving sensory input, CNS integration, and motor output.
• Types of Autonomic Reflexes:
    - Cardiovascular Reflex (e.g., blood pressure regulation).
    - Gastrointestinal Reflex (e.g., defecation).
    - Micturition Reflex (e.g., urination).

Clinical Views in ANS

• Horner Syndrome:
    - Injury to cervical sympathetic trunk results in:
      - Ptosis (drooping eyelid)
      - Miosis (constricted pupil)
      - Anhydrosis (lack of sweating)
      - Facial flushing
• Raynaud Syndrome:
    - Sudden constriction of small arteries in digits, often triggered by cold or emotional stress.
• Autonomic Dysreflexia:
    - Profound blood pressure rise due to sympathetic reflex overactivity, common after spinal cord injuries.

Overview of ANS Neurotransmitters

• Types of Neurotransmitters:
    - Acetylcholine (ACh): Released from all ANS preganglionic neurons, and all parasympathetic ganglionic neurons.
      - Cholinergic receptors include:
        - Nicotinic (excitatory at ganglia).
        - Muscarinic (excitatory or inhibitory, depending on target organ).
    - Norepinephrine (NE): Released by most sympathetic ganglionic neurons, targets adrenergic receptors, which can be further divided into alpha and beta subtypes impacting various body functions.