ANS

Autonomic Nervous System Overview

  • Components:

    • Sympathetic

    • Parasympathetic

    • Enteric

Motor Systems

A. Somatic Motor System

  • Central nervous system (CNS)

  • Somatic motor neuron

  • Targets:

    • Skeletal muscle

B. Autonomic Motor System

  • Central nervous system (CNS)

  • Visceral effectors:

    • Smooth muscle

    • Cardiac muscle

    • Gland cells

  • Structure:

    • Preganglionic fiber

    • Postganglionic fiber

Fiber Classification

Somatic Motor Components

  • Sympathetic:

    • Central nervous system (CNS)

    • Peripheral nervous system (PNS)

  • Parasympathetic:

    • Smooth muscle

    • Cardiac muscle

    • Gland cells

  • Fiber Ranges:

    • Preganglionic fibers

    • Autonomic ganglion

    • Postganglionic fibers

Characteristics and Functions of the ANS

  • Involuntary system

  • Anatomical organization:

    • Preganglionic fibers:

      • Cell bodies in the CNS

    • Postganglionic fibers:

      • Cell bodies in a ganglion outside the CNS

  • Ratio of fibers:

    • Sympathetic: 1:10

    • Parasympathetic: 1:3

Neurotransmitter Dynamics

  • Autonomic fibers:

    • Lack pre- and postsynaptic specializations, enabling diffusion over longer distances.

  • Preganglionic autonomic terminals release acetylcholine (ACh):

    • Mediated by nicotinic ACh receptors.

  • Neuropeptides released by both pre- and post-ganglionic terminals:

    • Examples include: neuropeptide Y, substance P, neurotensin, somatostatin, enkephalins.

Neurotransmitter Release and Receptor Types

Sympathetic System

  • Postganglionic terminals release norepinephrine (noradrenaline):

    • Mediated by five types of adrenergic receptors:

      • α1, α2, β1, β2, β3

Parasympathetic System

  • Postganglionic terminals release acetylcholine (ACh):

    • Mediated by three types of muscarinic receptors:

      • M1, M2, M3

Functional Overview

  • Sympathetic (fight or flight) and parasympathetic (rest and digest) systems innervate:

    • Cardiac muscle

    • Smooth muscle

    • Glandular tissue

  • Enteric division functions autonomously, mediating digestive reflexes.

  • Autonomic nervous system includes sensory and motor fibers, informing CNS about visceral homeostasis.

Sympathetic Nervous System Structure

  • Preganglionic sympathetic cells located in T1-L2 intermediolateral gray region.

  • Preganglionic fibers:

    • Exit through the ventral root, join the spinal nerve, and ventral ramus.

    • Form white rami communicantes to enter paravertebral sympathetic ganglia.

Sympathetic Supply - Body Wall and Limbs

  • Preganglionic fibers synapse with sympathetic chain ganglia at varying levels.

  • Postganglionic fibers:

    • Form gray rami communicantes.

    • Reach target organs via dorsal and ventral rami, innervating:

      • Peripheral blood vessels

      • Sweat glands

      • Arrector pili muscles

Sympathetic Supply - Head and Thoracic Viscera

  • Postganglionic sympathetic fibers from cervical ganglia affect:

    • Cranial blood vessels

    • Sweat glands

    • Visceral organs in the head and thorax (heart, lungs).

  • Horner's syndrome can occur due to lesions in the superior cervical ganglion, leading to:

    • Ptosis (drooping eyelid)

    • Miosis (constricted pupil)

    • Anhidrosis (lack of sweating)

    • Enophthalmos (sunken eye).

Sympathetic Supply - Abdominal and Pelvic Viscera

  • Some preganglionic sympathetic fibers pass through ganglia without synapsing:

    • Form splanchnic nerves (greater, lesser, least, lumbar) that synapse in prevertebral ganglia:

      • Celiac

      • Superior mesenteric

      • Inferior mesenteric

  • Postganglionic fibers innervate:

    • GI tract

    • Pancreas

    • Liver

    • Kidneys

    • Bladder

    • Genitalia.

Splanchnic Nerve Overview

  • Some fibers bypass prevertebral ganglia to innervate the adrenal medulla:

    • Releases epinephrine and norepinephrine into circulation.

Preganglionic Neuron Overview

  • Locations include:

    • Spinal cord (intermediolateral gray matter)

    • Ventral root, paravertebral chain ganglion.

  • Sympathetic nerve trunk connects to postganglionic fibers targeting peripheral autonomic effector organs.

Parasympathetic Nervous System Structure

  • Preganglionic cells situated in:

    • Cranial nerves III, VII, IX, X

    • Intermediolateral gray of S2-4.

  • Brainstem locations:

    1. Edinger-Westphal nucleus (III)

    2. Superior salivatory nucleus (VII)

    3. Inferior salivatory nucleus (IX)

    4. Dorsal motor nucleus and nucleus ambiguus (X)

Parasympathetic Preganglionic Fiber Functions

  • Oculomotor nerve (III) synapses in ciliary ganglion.

  • Facial nerve (VII) synapses in:

    • Pterygopalatine ganglion

    • Submandibular ganglion.

  • Glossopharyngeal nerve (IX) synapses in otic ganglion.

  • Vagus nerve (X) synapses with diffuse thoracic and abdominal ganglia.

Parasympathetic Postganglionic Fiber Functions

  • Ciliary ganglion fibers:

    • Innervate sphincter pupillae and ciliary muscles.

  • Pterygopalatine ganglion fibers:

    • Innervate lacrimal gland and serous/mucous glands of nasopharynx.

  • Submandibular ganglion fibers:

    • Innervate submandibular and sublingual salivary glands.

  • Otic ganglion fibers:

    • Innervate parotid salivary gland.

Parasympathetic Production and Targets

  • Diffuse ganglia fibers:

    • Innervate heart, esophagus, lungs, stomach, liver, gallbladder, pancreas, and gut derivatives.

  • Fibers from S2,3,4 intermediolateral column:

    • Innervate descending/sigmoid colon and pelvic viscera via pelvic splanchnic nerves.

Enteric Nervous System Overview

  • Function:

    • Controls GI tract, pancreas, gallbladder.

  • Enteric sensory neurons respond to:

    • Gut wall tension

    • Chemical changes in the gut.

  • Enteric motor neurons work with:

    • Smooth muscle

    • Vasculature

    • Secretion of the gut.

  • Composed of:

    • Myenteric (Auerbach’s) plexus - between smooth muscle layers.

    • Submucosal (Meissner's) plexus - between smooth muscle layer and mucosa.

  • Highly autonomous functioning.

Enteric Plexus Structure

  • Myenteric (Auerbach's) plexus:

    • Located between external longitudinal and circular muscle layers.

  • Submucosal (Meissner's) plexus:

    • Situated between circular muscle layer and mucosa.

Sensory Component of the ANS

  • Nerves involved:

    • Facial nerve: visceral chemosensory info about taste.

    • Glossopharyngeal nerve: innervates head and neck viscera.

    • Vagus nerve: innervates thoracic and abdominal viscera.

Integration and Response

  • Sensory information converges at the solitary nucleus:

    1. Direct autonomic control via reflexes (vagus and spinal cord nuclei).

    2. Projections to the reticular formation and periaqueductal gray, regulating:

      • Breathing

      • Blood pressure

      • Heart rate.

    3. Projections to the hypothalamus and then to thalamus, amygdala, and cingulate cortex.

Hypothalamic Integration

  • Exerts control over:

    • Brainstem and spinal cord nuclei regulating:

      • Temperature

      • Heart rate

      • Blood pressure

      • Respiration

      • Feeding/metabolic rate.

  • Releases hormones influencing autonomic functions.