Chapter 9

Somatic Motor Neuron

Somatic

• Effector organs - Skeletal Muscle

• Presence of ganglia - No Ganglia

• Number of neurons from CNS to effector - One

• Type of neuromuscular junction - Specialized motor endplate

• Effect of nerve impulse on muscle - Excitatory only

• Type of nerve fibers - Fast conducting, thick, and myelinated

• Effect of denervation - Flaccid paralysis and atrophy

Autonomic Motor Neuron

Autonomic

• Effector organs - Smooth muscle, cardiac muscle, and glands

• Presence of ganglia - Yes ganglia (postganglionic automatic fibers and terminal ganglia)

• Number of neurons from CNS to effector - Two

• Type of neuromuscular junction - No specialization of post synaptic membrane

• Effect of nerve impulse on muscle - Excitatory or inhibitory

• Type of nerve fibers - Slow, thin, not myelinated or slightly myelinated

• Effect of denervation - Muscle tone and function persist, increased sensitivity to neural stimulation

Sympathetic Division of Autonomic Nervous System

• Fight-or-flight

• Mass Activation - many organs are affected simultaneously

• Origin - T1-L2 of spinal cord (thoracolumbar division)

• Preganglionic - Short

• Postganglionic - Long

• Sympathetic ganglia - most preganglionic fibers synapse

• Collateral ganglia - postganglionic fibers innervate digestive, urinary, & reproductive organs

Parasympathetic Division of Autonomic Nervous System

• Rest-and-digest

• No mass activation - not normally activated as a whole

• Origin - brainstem and S2-S4 of spinal cord (craniosacral division)

• Preganglionic - Long

• Postganglionic - Short

• Terminal ganglia - ganglia near or in effector organs

Adrenal Medulla

• Modified sympathetic ganglion

• Innervated only by the sympathetic division

  • by sympathetic preganglionic neuron

  • Chromaffin cells - modified sympathetic postganglionic neurons that secrete hormones

    • Epinephrine 80%

    • Norepinephrine 20%

  • Epinephrine & norepinephrine catecholamines - derived from amino acid tyrosine

  • Stimulated by sympathetic division as a part of mass activation

Parasympathetic (Craniosacral) Division

Preganglionic neurons originating in brainstem run through 4 cranial nerves (3, 7, 9, 10)

• Oculomotor (III) nerve - midbrain origin

  • Effectors - smooth muscles in eye - constrictor muscle of iris & ciliary muscle (adjusts lens for Near vision)

• Facial (VII) nerve - pons origin

  • Effectors - lacrimal, mucous, salivary glands

• Glossopharyngeal (IX) - medulla oblongata origin

  • Effector - parotid salivary gland

• Vagus (X) nerve - medulla oblongata origin

  • Major parasympathetic nerve of body - contains 90% of parasympathetic fibers of body

  • Effectors: heart (Slow HR), lungs (bronchoconstriction), esophagus, stomach pancreas, liver, small intestine, upper half of large intestine (promote digestion)

Preganglionic neurons originating in sacral segments S2-S4 of the spinal cord

• form the pelvic splanchnic nerves

  • Effectors - lower half of large intestine, rectum, urinary bladder, & organs

Branching of Preganglionic Fibers

• Parasympathetic preganglionic fibers - do not branch

  • No mass activation

  • Parasympathetic effects are localized (restricted to a specific part of the body) and discrete (individually separate and distinct)

• Sympathetic preganglionic fibers - extensive branching

  • Mass activation

    • Occurs only in the Sympathetic division

  • Divergence - one preganglionic neuron branches to synapse on several postganglionic neurons at different levels

  • Convergence - one postganglionic neuron receives synaptic input from many preganglionic neurons at different levels

Neurotransmitters Each Division

• Cholinergic: pertaining to acetylcholine (ACh)

• Adrenergic: pertaining to epinephrine and norepinephrine

• Neurotransmitters

  • Preganglionic fibers (all)

    • Acetylcholine (ACh)

  • Postganglionic fibers

    • Sympathetic (most): Norepinephrine

    • Sympathetic (some): Acetylcholine

    • Parasympathetic (all): Acetylcholine

  • Adrenal medulla

    • Epinephrine and Norepinephrine

Protein Receptors Each Division

• All post-ganglionic cells

  • Nicotinic Cholinergic

• Sympathetic division

  • Adrenergic (most)

  • Muscarinic (some)

• All effector cells for parasympathetic division

  • Muscarinic

Adrenergic Receptors

• A1

  • Most sympathetic target tissues, Radial muscle of iris, Smooth muscle of most blood vessels (BVs)

  • More sensitive to NE

  • uses Ca²⁺ as a second messenger system

  • Excitatory, Dilation of pupils, Vasoconstriction

• A2

  • Digestive organs (smooth muscle)

  • More sensitive to NE

  • Inhibits cAMP

  • Inhibitory, Smooth muscle, Relaxation of GI tract, Decreased secretion from pancreas

• B1

  • Cardiac muscle

  • NE = E

  • Activates cAMP

  • Excitatory, Increase HR & force of contraction

• B2

  • Smooth muscle of BVs vessels of skeletal & cardiac muscles (in addition to α1), Bronchioles, & uterus

  • More sensitive to E

  • Activates cAMP

  • Inhibitory, Vasodilation, Bronchodilation, Relaxation of uterus

Cholinergic Receptors

• Nicotinic ACh Receptors

  • All postganglionic neurons & adrenal medulla

• Muscarinic ACh Receptors

  • All parasympathetic effectors

Receptors

Radial Muscle (Iris)

• Sympathetic

  • a1 - contracts (dilate pupil)

Circular Muscle (Iris)

• Parasympathetic

  • m1 - contracts (constrict pupil)

Sinoatrial Node (Heart)

• Sympathetic

  • b1 - increase heart rate

• Parasympathetic

  • m - decrease heart rate

Splanchnic Vessel (Skin)

• Sympathetic

  • a1 - constrict (vasoconstriction)

Blood Vessels of Skeletal Muscles

• Sympathetic

  • a1: vasoconstriction

  • b2: vasodilation

  • n: vasodilation

Bronchiolar Smooth Muscle

• Sympathetic

  • a1 - relaxes (bronchodilation)

• Parasympathetic

  • m - contracts (bronchoconstriction)

Smooth Muscle Walls

• Sympathetic

  • a2, b2 - relaxes

• Parasympathetic

  • m - contracts

Bladder Wall

• Sympathetic

  • b2 - relaxes

• Parasympathetic

  • m - contracts

Pilomotor Smooth Muscle

• Sympathetic

  • a1 - contracts

Thermoregulatory

• Sympathetic

  • m - increases

Drugs that Affect Adrenergic Receptors

Agonist - a substance that binds to & activates a receptor (mimics NT)

Antagonist - a substance that blocks (prevents) the action of NT

Sympathomimetic - drugs that promote release of E & NE or block their reuptake and degradation

• Salbutamol (Albuterol) (mimics E) - selectively activates beta 2 receptors: dilates bronchioles for asthma

Sympatholytic - drugs that block or inhibit the release or activity of catecholamines; suppresses activity of sympathetic division

• Beta blockers - beta-adrenergic receptor antagonists

  • Propranolol - nonselective for beta 1 & 2 receptors

  • Metroprolol and Atenolol - selectively blocks beta 1 adrenergic receptors: lower: heart rate, force of heart contraction, & BP

Dual Innervation

• Innervated by both sympathetic and parasympathetic fibers

• Usually, both divisions are partially active

• Divisions of the ANS are usually reciprocally controlled

  • Increased activity in one division is accompanied by a corresponding decrease in the other

  • Enables precise control over an organ’s activity

Dual Innervation - Effects of The Two Divisions

• Antagonistic Effects - opposite effects

  • Heart rate – sympathetic division ↑, parasympathetic division ↓

  • Digestive functions – sympathetic ↓, parasympathetic division ↑

  • Pupil diameter – sympathetic division dilates, parasympathetic division constricts

• Complementary Effects - both divisions produce similar effects

  • Example - Salivary gland secretion

    • Sympathetic division stimulates thick saliva rich in mucus

    • Parasympathetic division stimulates secretion of water saliva

• Cooperative Effects - produce different effects that work together to promote a single action

  • Example Penis

    • Sympathetic causes ejaculation,

    • Parasympathetic division causes erection

  • Example Urination

    • Sympathetic helps with bladder muscle tone to control urination

    • Parasympathetic division stimulates urinary bladder contraction to void urine

Organs Without Dual Innervation

• Organs innervated only by sympathetic division

  • Adrenal Medulla

  • Arrector pili (piloerector) muscles

  • Sweat glands

  • Most blood vessels

    • Only blood vessels of the penis & clitoris innervated by both divisions

    • Controlled by regulation of “tone” (firing rate) of sympathetic fibers

      • Vasoconstriction: increase firing rate above tone level

      • Vasodilation: decrease firing rate Below tone level

Control of ANS by Higher Brain Centers

• Medulla Oblongata - directly regulates ANS Centers for control of cardiovascular, pulmonary, urinary, reproductive, & digestive functions

• Hypothalamus - major regulatory center of ANS - oversees medulla oblongata

  • Regulates balance between sympathetic & sympathetic activity

  • Centers for control of - body temperature, hunger, thirst, pituitary gland, & emotional states