BIO111 Anatomy and Physiology 1 Lecture 19 PDF

BIO111 Lecture 19 Anatomy and Physiology 1 1 Autonomic Nervous System (Chapter 14) 14.1 Automatic nervous system (ANS) consists of motor neurons that: – Innervate smooth muscles, cardiac muscle, and glands – Make adjustments to ensure optimal support for body ac...

BIO111 Lecture 19 Anatomy and Physiology 1 1 Autonomic Nervous System (Chapter 14) 14.1 Automatic nervous system (ANS) consists of motor neurons that: – Innervate smooth muscles, cardiac muscle, and glands – Make adjustments to ensure optimal support for body activities § Shunts blood to areas that need it and adjusts heart rate, blood pressure, digestive processes, etc. – Operate via subconscious control Also called involuntary nervous system or general visceral motor system 2 14.1 Autonomic Nervous system (ANS) Versus Somatic Nervous System Both have motor fibers but differ in: – Effectors §Somatic NS innervates skeletal muscles §ANS innervates cardiac muscle, smooth muscle, and glands – Efferent pathways and ganglia §Somatic NS: cell body is in CNS, and a single, thick myelinated axon extends in spinal or cranial nerves directly to skeletal muscle cells §ANS: pathway uses a two-neuron chain Preganglionic neuron: cell body in CNS with lightly myelinated preganglionic axon extending to ganglion Postganglionic (ganglionic) neuron (outside CNS): cell body synapses with preganglionic axon in autonomic ganglion with nonmyelinated postganglionic axon that extends to effector organ (some exceptions) 3 Autonomic Nervous system (ANS) Versus Somatic Nervous System – Target organ responses to neurotransmitters §Somatic NS: release acetylcholine (Ach) and effect is always stimulatory causing contraction of skeletal muscle cells §ANS: Preganglionic fibers (axons) release ACh §Postganglionic fibers (axons) release norepinephrine or ACh at effectors §Effect is either stimulatory or inhibitory, depending on type of receptors 4  Spinal cord Or brain stem Figure 14.2 Comparison of motor neurons in the somatic and autonomic nervous systems. 5  Overlap of Somatic and Autonomic Motor Function Higher brain centers regulate and coordinate both systems Most spinal and many cranial nerves contain both somatic and autonomic fibers Adaptations usually involve both skeletal muscles and visceral organs – Example: Active skeletal muscles require more oxygen and glucose, so ANS nerves speed up heart rate and open airways 6 14.2 Divisions of Autonomic Nervous System Two arms of ANS: – Parasympathetic division: promotes maintenance functions, conserves energy – Sympathetic division: mobilizes body during activity Dual innervation: all visceral organs are served by both divisions, but these divisions cause opposite effects – Dynamic antagonism between two divisions maintains homeostasis 7  Role of the Parasympathetic Division Keeps body energy use as low as possible, even while carrying out maintenance activities – Directs digestion, diuresis, defecation All pre-ganglionic and post-ganglionic fibers release Ach Example: person relaxing and reading after a meal – Blood pressure, heart rate, and respiratory rates are low – Gastrointestinal tract activity is high – Pupils constricted, lenses accommodated for close vision 8 Role of the Sympathetic Division Mobilizes body during activity All pre-ganglionic fibers release Ach; post-ganglionic fibers release norepinephrine (NE) or Ach Exercise, excitement, emergency, embarrassment activates sympathetic system – Increased heart rate; dry mouth; cold, sweaty skin; dilated pupils During vigorous physical activity: – Shunts blood to skeletal muscles and heart by vasoconstrictor control (smooth muscle surrounding blood vessles – Dilates bronchioles – Causes liver to release glucose 9 Key Anatomical Differences Three main differences between sympathetic and parasympathetic divisions: – Sites or origin § Parasympathetic fibers are craniosacral; originate in brain and sacral spinal cord § Sympathetic fibers are thoracolumbar; originate in thoracic and lumbar regions of spinal cord – Relative lengths of fibers § Parasympathetic has long preganglionic and short postganglionic fibers § Sympathetic has short preganglionic and long postganglionic – Location of ganglia § Parasympathetic ganglia are located in or near the their visceral effector organ § Sympathetic ganglia lie close to spinal cord 10 14.3-14.4 Note origin, location of ganglia, length of pre and post-ganglionic fibers, 11 Figure 14.3 Key anatomical differences between ANS divisions. 14.5 Visceral Reflexes Visceral reflex arcs have same components as somatic reflex arcs: receptor, sensory neuron, integration center, motor neuron, and effector, but three main differences: 1. Visceral reflex arc has two consecutive neurons in the motor pathway going to the effector organs because it is ANS 2. Afferents fibers are visceral sensory neurons 3. Effectors are smooth muscle, cardiac muscle, and glands rather than skeletal muscles 12 Figure 14.9 Visceral reflexes. 13 14.6 Neurotransmitters Major neurotransmitters of ANS are acetylcholine (ACh) and norepinephrine (NE) – Ach is released by cholinergic fibers (axons) at: § All ANS (PS and S) pre-ganglionic axons and § All parasympathetic postganglionic axons – NE is released by adrenergic fibers (axons) at: § Almost all sympathetic postganglionic axons, except those at sweat glands (release ACh) Effects of neurotransmitter depends on what type of cholinergic receptor or adrenergic receptor 14 Cholinergic Receptors Two types of cholinergic receptors bind Ach: Nicotinic and Muscarinic Nicotinic receptors – Found on: § All (post)ganglionic neurons (sympathetic and parasympathetic) § Hormone-producing cells of adrenal medulla § Sarcolemma of skeletal muscle cells at neuromuscular junction – Effect of ACh at nicotinic receptors is always stimulatory § Opens ion channels, depolarizing the postsynaptic cells (ganglionic neurons, skeletal muscle cells, adrenal 15 medulla cells) Cholinergic Receptors cont’d Muscarinic receptors – Found on: § All effector cells innervated by PS postganglionic cholinergic fibers (axons) – Effect of ACh at muscarinic receptors § Can be either inhibitory or excitatory § Depends on receptor type on target organ – Example: Binding of ACh to cardiac muscle cells slows heart rate, whereas binding to intestinal smooth muscle cells increases motility vias stimulation of smooth muscle cells 16 Adrenergic Receptors Two major classes that respond to NE or epinephrine – Alpha (α) receptors § Divided into subclasses: α1 , α2 § Beta (β) receptors § Divided into subclasses: β1, β2, β3 Effects depend on which subclass of receptor predominates on target organ – Example: NE binding to cardiac muscle β1 receptors causes increase in rate, whereas epinephrine causes bronchial relaxation when bound to β2 receptors 17 When studying make your own tables comparing PS/S divisions and/or cholinergic/adrenergic fibers/receptors Table 14.1 Anatomical and Physiological Differences between the Parasympathetic and Sympathetic Divisions 18 14.7 Parasympathetic and Sympathetic Interactions Most visceral organs have dual innervation Action potentials continually fire down axons of both divisions, producing a dynamic antagonistic interaction – Works to precisely control visceral activity = homeostasis Both ANS divisions are partially active, resulting in a basal sympathetic and parasympathetic tone One division usually predominates, but in a few cases, divisions have a cooperative effect 19 Unique Roles of the Sympathetic Division Adrenal medulla, sweat glands, arrector pili muscles, kidneys, and almost all blood vessels receive only sympathetic fibers Other unique functions of sympathetic division include: – Thermoregulatory responses to heat § When body temperatures rise, sympathetic nerves: 1. Dilate skin blood vessels, allowing heat to escape 2. Activate sweat glands § When body temperatures drop, blood vessels constrict – Release of renin from kidneys § Sympathetic system causes release of renin from kidneys that in turn activates a system that increases blood pressure – Metabolic effects § Increases metabolic rates of cells § Raises blood glucose levels 20 § Mobilizes fats for use as fuels  14.8 Control of ANS Function ANS is under control of CNS centers in: – Brain stem and spinal cord, hypothalamus, and cerebral cortex – Hypothalamus is generally main integrative center of ANS activity Cerebral input may modify ANS but does so subconsciously – Works through limbic system structures on hypothalamic centers – Brain stem and spinal cord controls § Brain stem reticular formation appears to exert influence over ANS § Medullary oblongata centers regulate heart rate and blood vessel diameter, as well as gastrointestinal activities § Midbrain controls muscles of pupil and lens § Spinal cord controls defecation and micturition but are subject to conscious override 21 – Hypothalamic controls § Control may be direct or indirect through reticular system or spinal cord § Centers of hypothalamus controls: – Heart activity, bp, bT, water balance, and endocrine activity – Emotional responses (rage, fear, pleasure) activated through limbic system signal hypothalamus to activate fight-or-flight system 22 Figure 14.10 Levels of ANS control. Most ANS disorders involve abnormalities in smooth muscle control Hypertension Hypertension (high blood pressure) may result from an overactive sympathetic vasoconstrictor response promoted by continuous high levels of stress. Hypertension is always serious because it forces the heart to work harder, which may precipitate heart disease, and increases the wear and tear on artery walls. Hypertension is sometimes treated with adrenergic receptor– blocking drugs that counteract the effects of the sympathetic nervous system on the cardiovascular system. 23