ANS

Introduction to Nerves and the Autonomic Nervous System

  • The vagus nerve (parasympathetic) decreases heart rate.

  • Sympathetic cardiac nerves increase heart rate and force of contraction.

Learning Objectives

  • Identify each division of the nervous system and describe the independent and interdependent roles of each.

  • Describe the cross-sectional anatomy of the spinal cord and explain the path of sensory and motor nerve impulses into and from the spinal cord, respectively.

  • Define the terms dorsal primary ramus, ventral primary ramus, and white/gray rami communicantes.

  • Compare and contrast the anatomy, tissues innervated by, and functions of the somatic motor, sympathetic, and parasympathetic divisions of the nervous system.

  • Explain the phenomenon known as referred visceral pain.

  • Identify the neurotransmitters used at each synapse of the ANS.

  • Describe the role of each ANS division in sexual/reproductive function.

  • Describe the pathology known as Horner’s syndrome.

Lecture Outline

  • Divisions of the Nervous System

    • I: Anatomical

    • II: Functional

  • Autonomic Nervous System

    • Origin of Fibers

    • Divisions and Anatomy

    • Referred Pain

    • Neurotransmitters

Divisions of the Nervous System

I: Anatomical Division

  • Central Nervous System (CNS)

    • Subdivided into:

    • Spinal cord

  • Peripheral Nervous System (PNS)

    • Comprised of:

    • Motor Neurons

    • Sensory Neurons

II: Functional Division

  • Central Nervous System (CNS)

    • Receives and processes sensory information, initiates responses, conducts signals to and from the brain, controls reflex activities, stores memories, and generates thoughts and emotions.

    • Subdivided into:

    • Peripheral Nervous System (PNS)

      • Motor Neurons

      • Transmit signals from CNS to muscles and glands.

      • Somatic Nervous System

      • Controls voluntary movements.

      • Sensory Neurons

      • Transmit information from sensory organs to CNS.

      • Autonomic Nervous System

      • Controls involuntary responses.

        • Sympathetic Division

        • Engaged in "Fight or Flight" responses.

        • Parasympathetic Division

        • Engaged in "Rest or Digest" functions.

Peripheral Nervous System Summary

  • Posterior root of spinal nerve contains sensory neurons.

  • Motor Neurons are responsible for voluntary movements.

  • Pseudo-unipolar sensory neurons originate from the spinal (posterior root) ganglion.

  • Anterior roots of spinal nerve contain motor neurons transmitting signals away from the spinal cord.

Autonomic Nervous System (ANS)

  • Controls involuntary responses.

  • Sensory Receptors: connect to effector organs including skeletal, smooth, and cardiac muscles, as well as glands.

  • Somatic multipolar motor neuron: controls striated (voluntary) muscle.

  • Autonomic multipolar motor neurons: control involuntary organs.

Distinction between Somatic and Autonomic Nervous Systems

  • Somatic NS: Voluntary/conscious control pertaining to the body wall (limbs).

    • End-organs include skeletal (striated) muscle (motor) and skin or muscle with conscious perception of touch, pain, temperature (somatic sensory).

  • Autonomic NS: Involuntary control of organs within body cavities, functioning as an all-motor system.

Cranial vs. Spinal Nerves

  • Cranial Nerves:

    • 12 paired nerves emerge from the brainstem.

    • 4 carry parasympathetic fibers to the head, neck, thorax, and abdomen (CN III, VII, IX, X).

  • Spinal nerves:

    • 31 pairs that emerge from the spinal cord, named for their vertebral exit level.

    • Sympathetic fibers exit between T1-L2(3).

    • Parasympathetic fibers exit from S2-S4.

Functional Organization of Gray Matter in Spinal Cord

  • Dorsal Horn and Dorsal Roots: Involved in sensory processing.

    • Soma in the Dorsal Root Ganglia (DRG) project through dorsal roots into dorsal horns, present at all levels.

  • Ventral Horn and Ventral Roots: Associated with motor processing.

    • Somatic motor neurons project axons out through ventral roots, present at all levels.

  • Lateral Horn: Involved in visceral motor functions, visible primarily at T1-L2 for sympathetic division, with parasympathetic components found in the lateral aspect of the intermediate zone (S2-S4).

Nerve Fiber Classes

  • A nerve can carry several “flavors” of nerve fibers:

    • Somatic Efferent (GSE)

    • Visceral Efferent (GVE)

    • Somatic Afferent (GSA)

    • Visceral Afferent (GVA)

    • Special Visceral Efferent (SVE) – branchial motor

    • Special Sensory Afferent (SSA) – special senses

Differences between Somatic and Autonomic Fibers

  1. Both types of sensory fibers are processed in spinal or cranial sensory ganglia.

  2. Cell bodies of somatic motor and presynaptic visceral motor neurons reside in the grey matter of the CNS, both fibers leave via ventral roots.

Motor Nervous System Overview

Somatic Motor System

  • Involves a 1-neuron system that innervates only skeletal muscle for voluntary and reflexive movements.

  • Efferent neurons: Somatic efferent neurons make up this system.

    • Effector cells include skeletal muscle.

    • Neurotransmitter involved: Acetylcholine (ACh).

Visceral Motor System (Autonomic Nervous System)

  • Involves a 2-neuron system (within and outside the CNS) to stimulate involuntary muscles and secretory cells (glandular).

  • Efferent neurons: Can be further categorized into sympathetic and parasympathetic systems:

    • Sympathetic Division:

    • Pre-ganglionic neurons synapse with post-ganglionic neurons in sympathetic ganglia.

    • Involves neurotransmitters such as ACh at the synapse and norepinephrine (NE) or epinephrine at the target organs.

    • Parasympathetic Division:

    • Preganglionic fibers are long, postganglionic fibers are short, located near or in target organs.

    • Involves ACh as the primary neurotransmitter in both synapses.

Parasympathetic Nervous System Overview

  • General Features:

    • Preganglionic cell bodies located in brain stem and sacral spinal cord (S2-S4).

    • Long preganglionic axons with short postganglionic axons, with ganglia located near or within effector organs.

    • Effects are restricted to head and body cavities, not extremities or body wall.

  • Cranial Outflow includes:

    • Oculomotor Nerve (CN III): pupillary constriction and lens reshaping.

    • Facial Nerve (CN VII): various facial glands.

    • Glossopharyngeal Nerve (CN IX): parotid salivary gland.

    • Vagus Nerve (CN X): leading nerve for thorax and abdominal viscera.

  • Sacral Outflow: Includes pelvic splenic nerves targeting distal colon and urogenital system.

Summary of Parasympathetic and Sympathetic Systems

Parasympathetic (Craniosacral) Nervous System

  • Presynaptic neurons arise from specific locations in the CNS and exit through distinct routes.

  • Postsynaptic neurons reside in minuscule ganglia throughout the body:

    • Scattered about in the trunk or integrated within the effector organs.

Sympathetic (Thoracolumbar) Nervous System

  • Presynaptic neurons found in the grey matter (lateral horns) of spinal cord T1-L2(L3).

  • All sympathetic axons exit spinal cord and enter the trunk (rami communicantes).

  • Characteristics include:

    • Short preganglionic and long postganglionic fibers, which synapse with paravertebral (sympathetic chains) or prevertebral ganglia.

    • The adrenal medulla acts like sympathetic nervous tissue, secreting hormones like epinephrine and norepinephrine into the bloodstream through preganglionic axons.

Anatomy of Sympathetic Pathways

  • Sympathetic pathway utilizes the rami communicantes for nerve fiber travel from the CNS to effector organs. All presynaptic sympathetic fibers travel through the same pathway to synapse in sympathetic ganglia. The options for reaching their targets involve:

    • Ascension, descent, or passage without synapsing below the diaphragm.

Referred Visceral Pain

  • Visceral pain afferents travel back to spinal cord along with autonomic motor efferents.

  • This causes excitation of somatic sensory neurons at the same spinal level, leading to the perception of pain occurring in the associated dermatome (e.g., T2 to T5).

  • This creates the clinical phenomenon of pain being referred to body surfaces that correspond with the spinal level affected.

Neurotransmitters in the ANS

  • Somatic Nervous System (SNS): Uses Acetylcholine as the neurotransmitter at the neuromuscular junction for skeletal muscle stimulation.

  • Autonomic Nervous System (ANS):

    • Sympathetic Division: uses Norepinephrine at postganglionic sites and Acetylcholine at preganglionic sites.

    • Parasympathetic Division: predominantly uses Acetylcholine throughout both preganglionic and postganglionic synapses.

Summary Comparison of Sympathetic and Parasympathetic Systems

  • Origin of Fibers:

    • Sympathetic: Thoracolumbar region (T1-L2).

    • Parasympathetic: Brainstem and sacral spinal cord (S2-S4).

  • Length of Fibers:

    • Sympathetic: Short preganglionic fibers and long postganglionic fibers.

    • Parasympathetic: Long preganglionic fibers and short postganglionic fibers.

  • Location of Ganglia:

    • Sympathetic: Close to the spinal cord.

    • Parasympathetic: In or near the visceral effector organs.

Conclusion

  • The ANS is entirely a MOTOR system (specifically, the visceral subdivision of the motor nervous system).

  • It consists of two neurons between the CNS and the effector organ (pre- & postsynaptic).

  • The system controls involuntary muscle (modified cardiac and smooth) and glands.

  • Functions at a subconscious level to regulate visceral body functions in cooperation with the endocrine system to maintain homeostasis across physiological systems including cardiovascular, respiratory, digestive, urinary, and reproductive functions.

Acknowledgment

  • Thank you for attending!