Chapter 7 The Peripheral Nervous System: Efferent Division

Outline Autonomic nervous system Somatic nervous System Neuromuscular Junction

Outline Autonomic nervous system –Anatomy Pre and post fibers, sympathetic ganglia chain, collateral ganglia, terminal ganglia –Neurotransmitters Pre = Acetylcholine (ach) Post = adrenaline (epinepherine) noradrenaline (norepinepherine) –Sympathetic and parasympathetic branches Dual innervation Sympathetic dominance “fight or flight” Parasympathetic dominance “rest and digest” –Receptor types Cholinergic, muscarinic, nicotinic Adrenergic.  1  2  1  2 –CNS control

PNS: Efferent Division Communication link by which CNS controls activities of muscles and glands Two divisions of PNS –Autonomic nervous system (ANS) Involuntary branch of PNS Innervates cardiac muscle, smooth muscle, most exocrine glands, some endocrine glands, and adipose tissue –Somatic nervous system Subject to voluntary control Innervates skeletal muscle

Table 7-1, p. 234

ANS Autonomic nerve pathway –Extends from CNS to an innervated organ Ganglion = neuronal cell bodies in the PNS Nuclei = neuronal cell bodies in the PNS CNS –Two-neuron chain Preganglionic fiber (synapses with cell body of second neuron) Postganglionic fiber (innervates effector organ)

ANS Two subdivisions –Sympathetic nervous system Thoracic and lumbar –Parasympathetic nervous system Cervical and sacral Sympathetic Nervous System Parasympathetic Nervous System Fibers originate in thoracic and lumbar regions of spinal cord Fibers originate from cranial and sacral areas of CNS Most preganglionic fibers are short Preganglionic fibers are longer Long postganglionic fibers Very short postganglionic fibers Preganglionic fibers release acetylcholine (Ach) Most postganglionic fibers release noradrenaline (norepinephrine) Postganglionic fibers release acetylcholine

Craniosacral parasym- pathetic nerves Terminal ganglion Collateral ganglion Adrenal medulla Blood E,NE NE ACh Terminal ganglion ACh Sympathetic ganglion chain = Sympathetic system = Parasympathetic system Thoracolumbar sympathetic nerves Spinal cord Brain ACh Effector organs Cardiac muscle Smooth muscle Most exocrine glands and some endocrine glands = Preganglionic fiber = Postganglionic fiber = Acetylcholine = Norepinephrine = Epinephrine = Cell body = Axon Fig. 7-2, p. 235

ANS Most visceral organs innervated by both sympathetic and parasympathetic fibers In general produce opposite effects in a particular organ Dual innervation of organs by both branches of ANS allows precise control over organ’s activity

ANS Sympathetic system dominates in emergency or stressful (“fight-or-flight”) situations –Promotes responses that prepare body for strenuous physical activity Parasympathetic system dominates in quiet, relaxed (“rest-and-digest”) situations –Promotes body-maintenance activities such as digestion

Eye Nasal mucosa Sympathetic Spinal nerves Sympathetic trunk Splanchino nerves Liver Gall bladder Pancreas Adrenal gland Kidney Small intestine Colon Rectum Urinary bladder Genitalia Lung Heart Spinal nerves Cranial nerves Salivary glands Parasympathetic Parotid gland Trachea Lacrimal gland Stomach Spleen Sympathetic preganglionic fiber Parasympathetic postganglionic fiber Parasympathetic preganglionic fiber Sympathetic postganglionic fiber Fig. 7-3, p. 237

Effects of Autonomic Nervous System on Various Organs

ANS Exceptions to general rule of dual reciprocal innervation by the two branches of autonomic nervous system –Most arterioles and veins receive only sympathetic nerve fibers (arteries and capillaries are not innervated) –Most sweat glands are innervated only by sympathetic nerves –Salivary glands are innervated by both ANS divisions but activity is not antagonistic – both stimulate salivary secretion

ANS Adrenal medulla is a modified part of sympathetic nervous system –Modified sympathetic ganglion that does not give rise to postganglionic fibers –Stimulation of preganglionic fiber prompts secretion of hormones into blood About 20% of hormone release is norepinephrine About 80% of hormone released is epinephrine (adrenaline) Broadcast vs. localized

Autonomic Neurotransmitters

Craniosacral parasym- pathetic nerves Terminal ganglion Collateral ganglion Adrenal medulla Blood E,NE NE ACh Terminal ganglion ACh Sympathetic ganglion chain = Sympathetic system = Parasympathetic system Thoracolumbar sympathetic nerves Spinal cord Brain ACh Effector organs Cardiac muscle Smooth muscle Most exocrine glands and some endocrine glands = Preganglionic fiber = Postganglionic fiber = Acetylcholine = Norepinephrine = Epinephrine = Cell body = Axon Fig. 7-2, p. 235

Autonomic Neurotransmitter Receptors Tissues innervated by autonomic nervous system have one or more of several different receptor types for postganglionic chemical messengers –Alter tissue/cell response instead of chemical message –Alter the distribution (localized – varicosity, broadcast – adrenal medulla) –Cholinergic receptors – bind to ACh Nicotinic receptors – found on postganglionic cell bodies of all autonomic ganglia –nicotine Muscarinic receptors – found on effector cell membranes –Mushroom poison –Andrenergic receptors – bind to norepinephrine and epinephrine Alpha (α) receptors Beta (β) receptors

Autonomic Neurotransmitter Receptors –Cholinergic receptors – bind to ACh Nicotinic receptors – found on postganglionic cell bodies of all autonomic ganglia –Nicotine –Respond to Ach, opens Na and K channels, triggers AP Muscarinic receptors – found on effector cell membranes –Mushroom poison –Smooth muscle, cardiac muscle, glands –5 subtypes –G protein linked

Autonomic Neurotransmitter Receptors –Andrenergic receptors – bind to norepinephrine and epinephrine G protein coupled Alpha (α) receptors - Ca second messenger system –  1 = stim cyclic amp, sympathetic tissues, vessel constriction, contraction of smooth muscle, constrictor –  2 blocks cyclic amp, decreased smooth muscle contraction in the digestive tract, inhibitory to the effector organ Beta (β) receptors - cyclic amp   Found in the heart, increases contractility and rate   Found in the arterioles and airways, dilator

Spinal cord Sympathetic preganglionic fiber Adrenal medulla Blood Sympathetic postganglionic fiber both = Acetylcholine = Norepinephrine = Epinephrine Fig. 7-4, p. 239 epinorepi

Autonomic Agonists and Antagonists Agonists –Bind to same receptor as neurotransmitter –Elicit an effect that mimics that of neurotransmitter Antagonists –Bind with receptor –Block neurotransmitter’s response

Regions of CNS Involved in Control of Autonomic Activities Can be influenced by prefrontal association complex through its involvement with emotional expression characteristic of individual’s personality Hypothalamus plays important role in integrating autonomic, somatic, and endocrine responses that automatically accompany various emotional and behavioral states Medulla within brain stem is region directly responsible for autonomic output Some autonomic reflexes, such as urination, defecation, and erection, are integrated at spinal cord level

Distinguishing Characteristics of Sympathetic and Parasympathetic Nervous Systems

Outline Somatic nervous system –Anatomy Motor neurons Continuous to effector Voluntary Final common pathway ALS = Amyotropic lateral sclerosis, death of motor neurons

Somatic Nervous System Consists of axons of motor neurons of motor neurons that originate in spinal cord or brain stem and end on skeletal muscle Motor neuron releases neurotransmitter, Ach, which stimulates muscle contraction Motor neurons are final common pathway by which various regions of CNS exert control over skeletal muscle activity –These areas of CNS include spinal cord, motor regions of cortex, basal nuclei, cerebellum, and brain stem

Comparison of Somatic and Autonomic Nervous System

Spinal cord Sympathetic preganglionic fiber Adrenal medulla Blood Sympathetic postganglionic fiber Target organs = Acetylcholine = Norepinephrine = Epinephrine Fig. 7-4, p. 239

Outline Neuromuscular junction –Anatomy Muscle fiber, terminal button “motor end plate” –Ach release Presynaptic vesicular Postsynaptic intracellular –Ach activation of end plate potentials Activation of nicotinic receptors Inflow of sodium and potassium Ap propagated along the muscle fiber –Ach breakdown AChe

Neuromuscular Junction Axon terminal of motor neuron forms neuromuscular junction with a single muscle cell Signals are passed between nerve terminal and muscle fiber by means of neurotransmitter ACh Released ACh binds to receptor sites on motor end plate of muscle cell membrane Binding triggers opening of specific channels in motor end plate Ion movements depolarize motor end plate, producing end- plate potential Local current flow between depolarized end plate and adjacent muscle cell membrane brings adjacent areas to threshold Action potential is initiated and propagated throughout muscle fiber

Neuromuscular Junction Acetylcholinesterase –Inactivates ACh –Ends end-plate potential and the action potential and resultant contraction Neuromuscular junction is vulnerable to chemical agents and diseases –Black widow spider venom causes explosive release of ACh –Botulism toxin blocks release of ACh –Curare blocks action of ACh at receptor sites –Organophosphates prevent inactivation of ACh –Myasthenia gravis inactivates ACh receptor sites

Axon of motor neuron Myelin sheath Axon terminal Terminal button Vesicle of acetylcholine Acetylcholine receptor site Acetycholinesterase Plasma membrane of muscle fiber Voltage-gated Na + channel Chemically gated cation channel Motor end plate Contractile elements within muscle fiber Voltage-gated calcium channel Action potential propagation in motor neuron Action potential propagation in muscle fiber Fig. 7-6, p. 245

Neuromuscular conditions and compounds ALS –Lou Gehrig’s Disease death of motor neurons –Black widow venom Explosive release of Ach Paralysis of the diaphram –Botulinum toxin Inhibits Ach release –Curare Ach receptor blocker (chloinergic) –Organophosphates Ache inhibition –Mysanthia Gravis Autoimmune, body attacks Ach receptors –Rigor mortis