Autonomic and Somatic Nervous System Overview

Nervous System Overview

  • Composed of two main parts:
    • Central Nervous System (CNS): Includes the brain and spinal cord.
    • Peripheral Nervous System (PNS): Divided into Autonomic and Somatic systems.
Autonomic Nervous System (ANS)
  • Divisions:
    • Sympathetic (Thoracolumbar):
    • Cell bodies of preganglionic fibers located in thoracic and lumbar regions of spinal cord.
    • Parasympathetic (Craniosacral):
    • Cell bodies of preganglionic fibers located in brain (cranial) and sacral regions of spinal cord.
Somatic Nervous System
  • Consists of nerve fibers that innervate striated skeletal muscle only.
  • Lacks ganglions:
    • Cell bodies located in the ventral horns of the spinal cord.

Neurotransmitters of the ANS

  • Acetylcholine (ACh)
  • Norepinephrine (NE)

Receptor Types

Cholinergic Receptors
  • Nicotine Receptors (N)
  • Muscarinic Receptors (M)
Adrenergic Receptors
  • NE can activate:
    • α1, α2
    • β1, β2

Differences Between Autonomic and Somatic Nerves

  • Autonomic:
    • Synapses in ganglia outside the cerebrospinal axis.
    • Spontaneous activity possible without intact innervation.
  • Somatic:
    • Innervates skeletal muscle only.
    • Single fibers: no ganglia.
    • Denervation leads to paralysis and atrophy.

Functions of the Autonomic Division

Parasympathetic Functions:
  • Bradycardia (decreased heart rate)
  • Increased salivation
  • Vasodilation, resulting in reduced blood pressure
Sympathetic Functions:
  • Conservation of energy
  • Increased renal blood flow
  • Increased respiration
  • Increased energy expenditure (Tachycardia)
  • Vasoconstriction, resulting in increased blood pressure
  • Decreased salivation and urine output

Opposing Actions of Parasympathetic and Sympathetic Systems

  • Iris Sphincter Muscle:
    • Parasympathetic (M receptors): Miosis (constriction of pupil)
  • Iris Dilator Muscle:
    • Sympathetic (α1 receptors): Mydriasis (dilation of pupil)

Synergistic Actions of the ANS

Effectors and Responses:
  • Ureter: Increased motility and tone
  • Sex Organs (Male): Ejaculation
  • Skin:
    • Contradiction of pilomotor muscles
    • Increased sweat gland secretion
5 Key Steps in Neurotransmission
  1. Pre-synaptic nerve cell initiates action.
    • Precursors: Choline/Tyrosine
    • Release into synaptic cleft.
  2. Synaptic cleft: neurotransmitter recognition and binding at receptors.
  3. Post-synaptic nerve cell responses.
    • Metabolic disposition of neurotransmitters.

Effects of Stimulating Receptors

Cholinergic Receptors
  • Muscarinic Receptors:
  • Effects include increased salivation, bradycardia, miosis, and increased urination.
Adrenergic Receptors
  • β1: Increases heart rate and contractile force.
  • β2: Causes vasodilation and bronchorelaxation.
  • α1 and α2: Cause vasoconstriction and increased total peripheral resistance (TPR).

Dual Innervation and Autonomic Tone

  • Most organs receive dual innervation with one system predominantly influencing activity:

    • Sympathetic Predominance: Blood vessels, sweat glands
    • Parasympathetic Predominance: Heart, GI tract, bladder, salivary glands

  • Exceptions:

    • Adrenal glands, pilomotor muscles.