ANS & PNS

I. ORGANIZATION OF THE NERVOUS SYSTEM

A. Structural Division

  • Central Nervous System (CNS): Composed of the brain and spinal cord. Functions primarily as the integration center for information passing to and from peripheral parts of the body.

  • Peripheral Nervous System (PNS): Located outside the CNS. Comprises 12 pairs of cranial nerves and 31 pairs of spinal nerves along with associated ganglia.

  • Important terms:   - Collection of nerve bodies:
         - CNS: Nucleus
         - PNS: Ganglion
      - Bundle of nerve fibers:
         - CNS: Nerve tract
         - PNS: Peripheral nerve

B. Functional Division

Somatic vs. Visceral

  • Somatic Nervous System: Innervates structures of the body wall, including muscles, skin, and mucous membranes. Responsible for voluntary activities.

  • Visceral/Autonomic Nervous System: Composed of portions of both CNS and PNS. Controls activities of smooth muscles, cardiac muscles, glands, and viscera.

Sensory vs. Motor

  • Sensory Division: Transmits information to the CNS from receptors located in peripheral tissues and organs.

  • Motor Division: Carries motor commands from the CNS to peripheral tissues and systems.

II. PERIPHERAL NERVOUS SYSTEM

A. Anatomy of PNS

  • The PNS comprises nerve fibers and cell bodies located outside the CNS.

  • Nerve fiber: Comprised of a single axon, its neurolemma (cell membranes of Schwann cells directly surrounding the axon), and associated endoneurial connective tissue.

  • Neurolemma:   - For unmyelinated fibers: Constructed from Schwann cells which do not form a prominent series and hence do not produce myelin (example: Free nerve endings).   - For myelinated fibers: Arranged in a continuous series of Schwann cells that wrap around individual axons, forming myelin.

  • Nerve: A bundle of nerve fibers surrounded by its connective tissue coverings (epineurium), binding nerve fibers and fascicles together (fascicles are covered by perineurium).

  • Vasa Nervorum: The blood supply to the nerve.

B. Functional Classification

Types of Nerve Fibers

  • Axons in the PNS vary according to their functions:   - Type A (myelinated): Motor & sensory functions (e.g., vibration and proprioception).   - Type B (thinly myelinated): Predominantly preganglionic autonomic fibers.   - Type C (unmyelinated): Primarily sensory functions (e.g., pain and temperature).

Nerve Fiber Classification

  • Nerve fibers can be divided into sensory or motor depending on their diameters and functions. The rule of thumb: the lower the letter classification, the higher the diameter and faster the conduction speed.

C. Composition of a Spinal Nerve

  • 31 pairs of spinal nerves:   - Cervical: 8
      - Thoracic: 12
      - Lumbar: 5
      - Sacral: 5
      - Coccygeal: 1

  • Formation: Each spinal nerve is formed from the conjoining dorsal and ventral roots and exits through the intervertebral foramina.

  • Naming conventions:   - C1 exits above its respective vertebrae, while C2-C7 exit above their respective cervical vertebrae. C8 exits below C7.

  • A total of only one spinal nerve exits each intervertebral foramen.

D. Cranial Nerves

Mnemonic for Cranial Nerves & General Functions

  • Mnemonic: Oh, Oh, Oh To Touch And Feel Very Good Velvet, Such Heaven
      - Cranial Nerve Functions:     - I. Olfactory - Sensory (smell)
        - II. Optic - Sensory (vision)
        - III. Oculomotor - Motor (eye movement)
        - IV. Trochlear - Motor (eye movement)
        - V. Trigeminal - Both
        - VI. Abducens - Motor (eye movement)
        - VII. Facial - Both
        - VIII. Vestibulocochlear - Sensory (balance and hearing)
        - IX. Glossopharyngeal - Both
        - X. Vagus - Both
        - XI. Spinal Accessory - Motor (trap and sternocleidomastoid)
        - XII. Hypoglossal - Motor (tongue movements)

E. Functional Classification of Peripheral Nerves

According to the Direction of Impulse

  • Afferent (Sensory):   - Transmits impulses from sensory organs and receptors to the CNS.   - Comprised of pseudounipolar neurons whose cell bodies are outside CNS in sensory ganglia.

  • Efferent (Motor):   - Multipolar neurons that send impulses from CNS to effector organs (muscles and glands) arising from the anterior root of the spinal cord.

Specialized Classification

  • General vs. Special: Assigned based on quality:   - General: Present in all spinal nerves and some cranial nerves with similar functions.   - Special: Exclusive to certain fibers, e.g. special sense organs (vision, hearing, olfaction).

F. Functional Component of Nerves

  • Functional Components of Peripheral Nerves: Classifications:

Three-Letter System

First Letter

Second Letter

Third Letter

G (General)

S (Somatic)

A (Afferent)

S (Special)

V (Visceral)

E (Efferent)

Combination of Classifications

  • Functional Components:   - Sensory (Afferent):     - Somatic sensory [General Somatic Afferent (GSA)]
        - Visceral sensory [General Visceral Afferent (GVA)]
      - Motor (Efferent):     - Somatic motor [General Somatic Efferent (GSE)]
        - Visceral motor [General Visceral Efferent (GVE)]

Functional Components of Cranial Nerves

  • Functional Components Breakdown:   - Sensory (Afferent):     - GSA, GVA, SSA (vision, hearing), SVA (taste)
      - Motor (Efferent):     - GSE, GVE, SVE (branchial motor fibers)

G. Composition of a Spinal Nerve

  • Overview: Formed by anterior (ventral) and posterior (dorsal) roots with both motor and sensory fibers.

  • Spinal nerve pathways: Mixed nerves which branch into anterior and posterior rami, supplying varying body parts including skin and muscles in a segmental distribution.

III. AUTONOMIC NERVOUS SYSTEM

A. Somatic vs. Autonomic Nervous System

  • Somatic Nervous System (SNS): Contains a single neuron pathway from the CNS directly to skeletal muscles; heavily myelinated fibers release acetylcholine (ACh) causing excitation.

  • Autonomic Nervous System (ANS): Composed of a two-neuron chain (preganglionic and postganglionic neurons) which synapse at autonomous ganglia. Includes:   - Preganglionic neurons: Lightly myelinated (Type B).   - Postganglionic neurons: Unmyelinated (Type C).   

B. Hypothalamus: Higher Control

  • The hypothalamus is pivotal in regulating the ANS and integrates autonomic and neuroendocrine functions vital for maintaining homeostasis.

  • Stimulation Effects:   - Anterior region influences parasympathetic responses.   - Posterior part drives sympathetic responses.

C. Major Divisions of the ANS

  • Two Major Divisions: Sympathetic and Parasympathetic

  • Differentiation: Based on the location of preganglionic cell bodies, autonomic ganglion locations, and neurotransmitter types.

  • Cooperative Effects: Both divisions are not merely antagonistic but can work synergistically in certain conditions.

Impacts on Physiological Processes

  • Sympathetic: Increases heart rate, blood pressure, and energy mobilization ( fight/flight).

  • Parasympathetic: Decreases heart rate, promotes digestion ( rest/digest).

Physiological Effects Summary Table

Physiologic Process

Sympathetic Stimulation

Parasympathetic Stimulation

Pupil Diameter

+

-

Heart Rate

+

-

Gut Peristalsis

-

+

Salivary Gland Flow

-

+

IV. EFFERENT FIBER SYSTEM OF THE SYMPATHETIC PATHWAY

A. Sympathetic Preganglionic Fibers

  • Preganglionic fibers’ cell bodies are located in the intermediolateral gray matter of spinal segments T1-L2.

  • Somatotropic organization gives specificity to the innervation such that distinct regions correspond to specific spinal segments.

B. Sympathetic Chain Ganglia

Pathways Through the Sympathetic Chain

  • Preganglionic fibers from T1-L2 can:   1. Synapse at the same level.   2. Ascend or descend to synapse with postganglionic neurons in nearby ganglia.   3. Pass through the chain ganglion to synapse with prevertebral ganglia.

  • Gray vs. White Rami: Gray rami communicantes carry unmyelinated postganglionic fibers; white rami carry myelinated preganglionic fibers for the sympathetic pathway from the CNS to sympathetic chain.

V. PARASYMPATHETIC NERVOUS SYSTEM

A. Cranial Outflow

  • Major cranial outflow through CN III (Oculomotor), CN VII (Facial), CN IX (Glossopharyngeal), and CN X (Vagus) provides parasympathetic innervation to various visceral organs and glands.

B. Sacral Outflow

  • Arises from pelvic splanchnic nerves (S2-S4) supplying hindgut and pelvic viscera, including bladder control.

VI. VISCERAL AFFERENT

  • General Visceral Afferent Fibers: Key components of the ANS that inform the CNS about visceral conditions. They connect with afferent pathways to relay sensations like distension and blood pressure.

VII. CLINICAL CORRELATION

A. Bladder Dysfunction

  • Types of bladder dysfunction based on pathway disruptions leading to varying control of micturition.

B. Referred Pain

  • Occurs when visceral pain signals are perceived through the same spinal segments as somatic pain, leading to confusion in brain interpretation.

C. Horner Syndrome

  • Symptoms include miosis, ptosis, and anhidrosis, resulting from disruption within sympathetic pathways to the face.

VIII. REVIEW QUESTIONS

IX. FORMATIVE QUESTIONS

X. REFERENCES

  • Include all sources and materials referenced throughout the presentations and study materials including textbooks and notes from lectures.

XI. APPENDIX

  • Additional diagrams and supplementary information pertinent to the PNS and ANS that assist in understanding complex systems of neuronal function and integration.