Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System]

Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] 1 Unit 8 Study Guide The efferent division of the peripheral nervous system is the neural communication system through which the central nervous system transmits signals to control the activities of effector organs (muscles and glands). There are two efferent pathways: the autonomic nervous system and the somatic nervous system. The autonomic nervous system innervates effectors that are controlled involuntarily, such as smooth and cardiac muscles, most exocrine glands, and some endocrine glands. The somatic nervous system innervates the skeletal muscles, which are under voluntary control. In both systems, the efferent axon terminals release neurotransmitters (either acetylcholine or norepinephrine) to induce a response in the effector organs. Flight-or-fight response The term "fight or flight" describes a mechanism in the body that enables humans and animals to mobilize a lot of energy rapidly in order to cope with threats to survival. How does it work? A threat is perceived The autonomic nervous system automatically puts body on alert. The adrenal cortex automatically releases stress hormones. The heart automatically beats harder and more rapidly. Breathing automatically becomes more rapid. Thyroid gland automatically stimulates the metabolism. Larger muscles automatically receive more oxygenated blood. The important thing to take away is that the fight or flight response is an automatic response. Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] 2 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) Preganglionic fibers release acetylcholine (Ach) Most postganglionic fibers release noradrenaline (norepinephrine) Postganglionic fibers release acetylcholine Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] 3 Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] PLEASE NOTE: In the sympathetic system, the preganglionic neurons are short postganglionic neurons are long [called 'cholinergic neurons'] [called 'adrenergic neurons'] While in the parasympathetic system, the preganglionic neurons are long [both are 'cholinergic neurons'] postganglionic neurons are short • Cholinergic neurons – release acetylcholine – all sympathetic and parasympathetic preganglionic neurons – all parasympathetic postganglionic neurons – sympathetic postganglionic neurons that innervate most sweat glands • Adrenergic neurons – release norepinephrine – sympathetic postganglionic neuron 4 Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] 5 Sites of Release Types of Receptors  Cholinergic receptors – Nicotinic receptors are cell bodies of sympathetic and parasympathetic postganglionic neurons – Muscurinic receptors are in effectors • innervated by parasympathetic postganglionic neurons (in smooth muscle, cardiac muscle, and glands) • sweat glands innervated by sympathetic postganglionic neurons o Nicotinic receptors – effect on target cell – Excitatory – depolarization of the post-synaptic cells o Muscarinic receptors – effect on target cell – excitatory or inhibitory – depending on the nature of signal transduction pathway  Adrenergic receptors – receptors activated by E and NE (neurotransmitters) – 2 main types • alpha • beta Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] http://www.youtube.com/watch?v=HkLoHfvCPCk Adrenergic Receptors • • • • alpha 1 activation alpha 2 activation beta 1 beta 2 – excitatory response – inhibitory response – excitatory response – inhibitory response Alpha 1 receptors Alpha 2 receptors - smooth muscle contraction (vasoconstriction) - smooth muscle contraction and neurotransmitter inhibition Beta 1 receptors Beta 2 receptors - heart muscle contraction - smooth muscle relaxation - bronchodilator 6 Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] 7 Some Exceptions • 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 • 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) • Physiological effects of the ANS • Control center is hypothalamus (+ thalamus = diencephalon) • input – emotions and visceral sensations • most organs receive dual innervation • [part of limbic system] • Sympathetic: caused by physical or emotional stress -- “E situations” – emergency, embarrassment, excitement, exercise – Flight-or-fight response • Parasympathetic: Enhance “rest-and-digest” activities – Mechanisms that help conserve and restore body energy during times of rest – SLUDD type responses = salivation, lacrimation, urination, digestion & defecation – 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil – Paradoxical fear when there is no escape route or no way to win causes massive activation of parasympathetic division loss of control over urination and defecation Lesson 1 Chapter 4 – Efferent System [Autonomic Nervous System] • Autonomic reflex controls blood pressure when a person stands up • Orthostatic hypertension (like sitting down and going up, resulting in lower blood pressure momentarily. • Autonomic reflex that isn’t good (aging and diabetes mellitus and dehydration) 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 8