The Peripheral Nervous System: Efferent Division Chapter 7 The Peripheral Nervous System: Efferent Division

Outline Autonomic nervous system Somatic nervous System Neuromuscular Junction

Outline Autonomic nervous system Anatomy Neurotransmitters 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. a1 a2 b1 b2 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

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

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 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

Figure 7.2: Autonomic nervous system. ACh ACh Autonomic effectors Terminal ganglion Nicotinic receptor Muscarinic receptor Cardiac muscle α receptor Sympathetic ganglion chain NE Adrenal Medulla E NE Blood E Nicotinic receptor Smooth muscle β1 receptor E Most endocrine glands and some NE Collateral ganglion Nicotinic receptor E β2 receptor Figure 7.2: Autonomic nervous system. The sympathetic nervous system, which originates in the thoracolumbar regions of the spinal cord, has short cholinergic (acetylcholine-releasing) preganglionic fibers and long adrenergic (norepinephrine-releasing) postganglionic fibers. The parasympathetic nervous system, which originates in the brain and sacral region of the spinal cord, has long cholinergic preganglionic fibers and short cholinergic postganglionic fibers. In most instances, sympathetic and parasympathetic postganglionic fibers both innervate the same effector organs. The adrenal medulla is a modified sympathetic ganglion, which releases epinephrine and norepinephrine into the blood. Nicotinic cholinergic receptors are located in the autonomic ganglia and adrenal medulla and respond to ACh released by all autonomic preganglionic fibers. Muscarinic cholinergic receptors are located at the autonomic effectors and respond to ACh released by parasympathetic postganglionic fibers. α1, α2, β1, β2 adrenergic receptors are variably located at the autonomic effectors and differentially respond to norepinephrine released by sympathetic postganglionic fibers and to epinephrine released by the adrenal medulla. Adipose tissue ACh ACh KEY Terminal ganglion Nicotinic receptor Muscarinic receptor Parasympathetic preganglionic fiber Parasympathetic postganglionic fiber Sympathetic preganglionic fiber Sympathetic postganglionic fiber ACh NE E Acetylcholine Norepinephrine Epinephrine Fig. 7-2, p. 241

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

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

Table 7-02a p244

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

Table 7-1 p242

Figure 7.2: Autonomic nervous system. ACh ACh Autonomic effectors Terminal ganglion Nicotinic receptor Muscarinic receptor Cardiac muscle α receptor Sympathetic ganglion chain NE Adrenal Medulla E NE Blood E Nicotinic receptor Smooth muscle β1 receptor E Most endocrine glands and some NE Collateral ganglion Nicotinic receptor E β2 receptor Figure 7.2: Autonomic nervous system. The sympathetic nervous system, which originates in the thoracolumbar regions of the spinal cord, has short cholinergic (acetylcholine-releasing) preganglionic fibers and long adrenergic (norepinephrine-releasing) postganglionic fibers. The parasympathetic nervous system, which originates in the brain and sacral region of the spinal cord, has long cholinergic preganglionic fibers and short cholinergic postganglionic fibers. In most instances, sympathetic and parasympathetic postganglionic fibers both innervate the same effector organs. The adrenal medulla is a modified sympathetic ganglion, which releases epinephrine and norepinephrine into the blood. Nicotinic cholinergic receptors are located in the autonomic ganglia and adrenal medulla and respond to ACh released by all autonomic preganglionic fibers. Muscarinic cholinergic receptors are located at the autonomic effectors and respond to ACh released by parasympathetic postganglionic fibers. α1, α2, β1, β2 adrenergic receptors are variably located at the autonomic effectors and differentially respond to norepinephrine released by sympathetic postganglionic fibers and to epinephrine released by the adrenal medulla. Adipose tissue ACh ACh KEY Terminal ganglion Nicotinic receptor Muscarinic receptor Parasympathetic preganglionic fiber Parasympathetic postganglionic fiber Sympathetic preganglionic fiber Sympathetic postganglionic fiber ACh NE E Acetylcholine Norepinephrine Epinephrine Fig. 7-2, p. 241

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 Adrenergic 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 Adrenergic receptors – bind to norepinephrine and epinephrine G protein coupled Alpha (α) receptors - Ca second messenger system - a 1= stimulates cyclic amp, sympathetic tissues, vessel constriction, contraction of smooth muscle, constrictor - a 2 blocks cyclic amp, decreases smooth muscle contraction in the digestive tract, inhibitory to the effector organ Beta (β) receptors - cyclic amp - b1 Found in the heart, increases contractility and rate - b2 Found in the arterioles and airways, dilator

Table 7-3 p246

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

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 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

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

Outline Neuromuscular junction Anatomy Ach release 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

Muscle fibers innervated by red motor neuron Axons of two efferent motor neuron Muscle fibers innervated by red motor neuron Muscle fibers innervated by red motor neuron Spinal cord (section) Axon terminals Axon terminals Terminal buttons Neuro- muscular junction Muscle Figure 7.4: Motor neuron innervating skeletal muscle cells. The cell body of a motor neuron originates in the ventral horn of the spinal cord. The axon (somatic efferent fiber) exits through the ventral root and travels through a spinal nerve to the skeletal muscle it innervates. When the axon reaches a skeletal muscle, it divides into many axon terminals, each of which forms a neuromuscular junction with a single muscle cell (muscle fiber). The axon terminal within a neuromuscular junction further divides into fine branches, each of which ends in an enlarged terminal button. Note that the muscle fibers innervated by a single axon terminal are dispersed throughout the muscle, but for simplicity they are grouped together in this figure. Muscle fibers Muscle fibers Terminal button Neuromuscular junction Fig. 7-4, p. 251

Axon terminal of motor neuron Action potential propagation in motor neuron Myelin sheath 1 Terminal button Voltage-gated Na+ channel Vesicle of acetylcholine Voltage-gated Ca2+ channel Plasma membrane of muscle fiber Action potential propagation in muscle fiber Ca2+ Na+ 8 2 8 6 7 7 Na+ 3 6 Figure 7.5: Events at a neuromuscular junction. 1. An action potential in a motor neuron is propagated to the axon terminal (terminal button). 2. This local action potential triggers the opening of voltage-gated Ca2+ channels and the subsequent entry of Ca2+ into the terminal button. 3. Ca2+ triggers the release of acetylcholine (ACh) by exocytosis from a portion of the vesicles. 4. ACh diffuses across the space separating the nerve and muscle cells and binds with receptor-channels specific for it on the motor end plate of the muscle cell membrane. 5. This binding brings about the opening of these nonspecific cation channels, leading to a relatively large movement of Na+ into the muscle cell compared to a smaller movement of K+ outward. 6. The result is an end-plate potential. Local current flow occurs between the depolarized end plate and the adjacent membrane. 7. This local current flow opens voltage-gated Na+ channels in the adjacent membrane. 8. The resultant Na+ entry reduces the potential to threshold, initiating an action potential, which is propagated throughout the muscle fiber. 9. ACh is subsequently destroyed by acetylcholinesterase, an enzyme located on the motor end-plate membrane, terminating the muscle cell’s response. K+ Acetylcholinesterase 4 9 5 Acetylcholine-gated receptor-channel (for nonspecific cation traffic) Na+ Motor end plate Contractile elements within muscle fiber Fig. 7-5, p. 252

Comparison of Somatic and Autonomic Nervous System

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

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