Chapter 10 Nervous System

Autonomic Nervous System Regulates activities that maintain homeostasis Sends impulses to cardiac and smooth muscle Also called visceral motor system Activities include secretion of digestive enzymes, constriction and dilation of blood vessels for maintenance of blood pressure, and the secretion of hormones. Activities occur without awareness or control (autonomously). Also called visceral motor system because it targets organs.

View animation on “Visceral (somatic) reflex arc” Visceral Reflexes 1 2 3 The autonomic nervous system (ANS) asserts control through visceral reflexes (similar to somatic reflexes but affect an organ instead of a skeletal muscle). All visceral reflexes follow similar steps; this illustration shows the visceral reflex arc responsible for the regulation of blood pressure: Receptors detect a change in body conditions. (Here, pressure receptors in the carotid artery, called baroreceptors, detect a rise in blood pressure.) Afferent neurons transmit information about this change to the central nervous system. (The glossopharyngeal nerve relays this information to the medulla oblongata.) The brain processes the information and transmits a signal along an efferent nerve. (Vagus nerve sends a signal to the heart’s pacemaker to slow its rate.) The effector organ receives the message and responds. (The heart rate slows and the blood pressure drops.) 4 View animation on “Visceral (somatic) reflex arc”

Autonomic vs. Somatic Pathways In the somatic pathway: The neuron’s cell body lies within the central nervous system (either the brain or spinal cord). A single myelinated axon extends from the brainstem or spinal cord to a skeletal muscle. At the target muscle, the neurotransmitter acetylcholine (ACh) is released to cause muscle contraction.

Autonomic vs. Somatic Pathways (cont'd) Autonomic pathway In the autonomic pathway: A myelinated preganglionic neuron extends from the brainstem or spinal cord to a ganglion. In the ganglion, it synapses with a postganglionic neuron and the neurotransmitter acetylcholine (ACh) is released. The axon of the unmyelinated postganglionic neuron extends to the target organ. The neurotransmitter released varies: parasympathetic fibers release ACh, whereas sympathetic fibers release norepinephrine (NE).

Divisions of the Autonomic Nervous System Sympathetic: Prepares the body for activity Parasympathetic: Calms body functions The sympathetic division is called on in times of stress, anger, or fear—“fight or flight” reaction. Note: Chapter 10 of the textbook contains a chart comparing the actions of the sympathetic and parasympathetic divisions. Both the sympathetic and parasympathetic divisions work at the same time, providing a background level of activity called autonomic tone. The balance between sympathetic and parasympathetic activity constantly changes depending on the body’s needs.

View animation on “Structure of the sympathetic division” Sympathetic preganglionic neurons begin within the spinal cord. From the cell bodies, myelinated fibers reach to sympathetic ganglia, most of which exist in chains along both sides of the spinal cord. The preganglionic neurons are short. Not all preganglionic neurons synapse in the first ganglion they encounter. Some travel up or down the chain to synapse with other ganglia at different levels. Others pass through the first ganglion to synapse with another ganglion a short distance away. Unmyelinated postganglionic fibers leave the ganglia and extend to the target organs. Postganglionic fibers tend to be long. Each preganglionic neuron branches and synapses with multiple postganglionic neurons. This allows one preganglionic neuron to excite multiple postganglionic neurons simultaneously, which is why the sympathetic division can ignite such an immediate, widespread response, such as what occurs during “fight or flight.” View animation on “Structure of the sympathetic division”

Adrenal Glands Play a role in sympathetic nervous system Sympathetic preganglionic fibers pass through the outer cortex of the adrenal gland and terminate in the center (the adrenal medulla). When stimulated, the adrenal medulla secretes a mixture of epinephrine and norepinephrine into the bloodstream. Both of these hormones can bind to the receptors of sympathetic effectors, which helps prolong the sympathetic response.

Parasympathetic Division Neurons of the parasympathetic division arise from the brain and sacral region of the spinal cord (which is why this division of the autonomic nervous system is also called the craniosacral division). Parasympathetic fibers leave the brainstem by joining either the oculomotor nerve (cranial nerve III), facial nerve (cranial nerve VII), glossopharyngeal nerve (cranial nerve IX), or vagus nerve (cranial nerve X); the vagus nerve carries about 90% of all parasympathetic preganglionic fibers. The ganglia of the parasympathetic division reside in or near the target organ: preganglionic fibers are long and postganglionic fibers are short. Because the ganglia are more widely dispersed, the parasympathetic division produces a more localized response than that of the sympathetic division. View animation on “Structure of the parasympathetic division”

Sympathetic vs. Parasympathetic Division

Question Which of the following is an action of the parasympathetic division of the autonomic nervous system? Increase heart rate Constrict bronchial tubes Stimulate sweat glands Inhibit intestinal motility

Answer Correct answer: B Rationale: The sympathetic division increases heart rate, stimulates sweat glands, and inhibits intestinal motility.

Neurotransmitters Cholinergic fibers Adrenergic fibers Preganglionic fibers of sympathetic and parasympathetic divisions Postganglionic fibers of parasympathetic division Secrete acetylcholine Include most postganglionic fibers of the sympathetic division Secrete norepinephrine The two divisions of the autonomic nervous system (ANS) tend to exert opposite effects, although there are exceptions (e.g., the sympathetic division dilates blood vessels leading to skeletal muscles and constricts blood vessels leading to most organs). Two factors determine the effect of each division: the neurotransmitter released and the type of receptor on the target cells. The ANS uses two different neurotransmitters: acetylcholine (ACh) and norepinephrine (NE). Fibers that secrete acetylcholine are called cholinergic fibers. Fibers that secrete NE are called adrenergic fibers. The effects of the sympathetic division last longer than those of the parasympathetic division because acetylcholine is disposed of more quickly after release than is norepinephrine.

Cholinergic Receptors Muscarinic receptors Nicotinic receptors Occur within the adrenal medulla and the neuromuscular junction Produce an excited response Occur on glands, smooth muscle, and cardiac muscle cells of organs innervated by cholinergic fibers Produce a variable response Neurotransmitters bind to receptors on the effector cells of target organs: acetylcholine (ACh) binds to cholinergic receptors, and norepinephrine (NE) binds to adrenergic receptors. Cholinergic receptors may be one of two types: nicotinic or muscarinic. The different types of receptors determine the effect produced by a neurotransmitter. The variable response produced by muscarinic receptors allows ACh to stimulate intestinal smooth muscle whereby inhibiting cardiac muscle.

Adrenergic Receptors Cells with alpha-adrenergic receptors are excited by norepinephrine (NE). Cells with beta-adrenergic receptors are inhibited by NE. View animation on “Receptors of the sympathetic and parasympathetic divisions” The binding of NE to alpha-adrenergic receptors in blood vessels causes the blood vessels to constrict. The binding of NE to beta-adrenergic receptors in the heart and skeletal muscles causes the vessels to dilate. Both alpha and beta receptors contain subtypes that can cause exceptions to these principles.

Sympathetic Division This chart summarizes the neurotransmitters and receptors of the sympathetic division of the ANS.

Parasympathetic Division This chart summarizes the neurotransmitters and receptors of the parasympathetic division of the ANS.

Question The effect produced by a neurotransmitter is ultimately determined by: the amount of neurotransmitter released. the type of neurotransmitter released. the type of receptor. the number of receptors.

Answer Correct answer: C Rationale: The amount of neurotransmitter influences the strength (not type) of a response. Although the effect varies according to the type of neurotransmitter, the same neurotransmitter bound to a different receptor will have a different response. Therefore, it is the receptor that ultimately determines the response. The number of receptors influences the strength of the response.