Term
| Describe the organization of autonomic motor neurons: |
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Definition
| It involves two neurons (instead of one, like the somatic motor neurons) in the efferent pathway. The cell body of the first of the two lies in the grey matter of the brain or spinal cord. Instead of innervating the effector organ, it synapses with a second neuron, which is within an autonomic ganglion. The first neuron is called a preganglionic neuron, the second, a postganglionic neuron. Autonomic ganglia are located in the head, neck and abdomen; chains of autonomic ganglia also parallel the right and left sides of the spinal cord. |
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Term
| Describe how neural regulation of smooth and cardiac muscles differs from neural regulation of skeletal muscles: |
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Definition
| Skeletal muscles enter a state of atrophy and flaccid paralysis when motor nerves are severed. Damage to an autonomic nerve makes its target tissue more sensitive than normal to stimulating agents (denervation hypersensitivity). Skeletal muscles lose tone when it it is not stimulated; autonomic effectors maintain a resting tone in the absence of stimulation, and can even contract rhythmically, due to electrical responses to electrical waves of depolarization. The release of ACh from somatic motor neurons always stimulates the effector organ (skeletal muscle) but some autonomic nerves release transmitters that inhibit the activity of their effectors. |
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Term
| Describe the structure of the sympathetic division: |
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Definition
| Thoracolumbar- Its short preganglionic fibers exit the spinal cord from the first thoracic (T1) to the second lumbar (L2) levels. Most sympathetic fibers synapse with long postganglionic neurons within a double row of sympathetic ganglia (paravertebral ganglia). The ganglia are interconnected, forming a sympathetic chain of ganglia, that parallels each side of the spinal cord. Myelinated preganglionic sympathetic axons exit the spinal cord in the ventral roots, then diverge from the spinal nerves within the white rami communicantes. Axons within each ramus then enter the sympathetic chain, where they can travel to ganglia at different levels and synapse with postganglionic sympathetic neurons. The axons of the postganglionic neurons are unmyelinated, and form the grey rami communicantes as they return to the spinal nerves to their effector organs. The divergence of impulses from the spinal cord to the ganglia and the convergence of impulses within the ganglia results in mass action of almost all the postganglionic neurons, explaining why the sympathetic neurons are usually activated as a unit, affecting all the effector organs at the same time. |
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Term
| Describe the structure of the parasympathetic division: |
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Definition
| Craniosacral- The long preganglionic fibers originate in the brain, and in the 2nd through 4 sacral levels of the spinal column. The preganglionic parasympathetic fibers synapse in ganglia that are located next to- or within- the organs innervated, meaning that they have very short postganglionic fibers. The ganglia are called terminal ganglia. Parasympathetic fibers don't travel within the spinal cord. 4 of the 12 pairs of cranial nerves contain preganglionic parasympathetic fibers- Oculomotor (III) Facial (VII) Glossopharyngeal (IX) and Vagus (X). Parasympathetic fibers within the first 3 synapse in ganglia within the head, fibers in the Vagus nerve synapse in terminal ganglia in widespread regions of the body, providing the major parasympathetic innervation in the body- innervating the heart, lungs, esophagus, stomach, pancreas, small intestine and upper half of the large intestine. Preganglionic fibers from the sacral levels of the spinal cord innervate the lower half of the large intestine, rectum and urinary and reproductive systems. |
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Term
| Explain the relationships between the sympathetic division and adrenal medulla: |
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Definition
| The adrenal medulla could be considered a modified sympathetic ganglion. Like a sympathetic ganglion, the cells of the adrenal medulla are innervated by preganglionic sympathetic fibers. The adrenal medulla secretes epinephrine into the blood in response to this neural stimulation. The effects of epinephrine are complimentary to those of the neurotransmitter norepinephrine, which is released from postganglionic sympatheic nerve endings. The two (sympathetic system and adrenal medulla) are often grouped together as a single sympathoadrenal system. |
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Term
| Identify the neurotransmitters of the sympathetic division: |
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Definition
| In preganglionic neurons, it is always ACh, cholinergic. In most postganglionic neurons it is norepinephrine, adrenergic-- except for sympathetic fibers that innervate sweat glands, and blood vessels in skeletal muscles. |
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Term
| Identify the neurotransmitters of the parasympathetic division: |
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Definition
| In the preganglionic neurons, it is always ACh, cholinergic. In all postganglionic neurons it is also ACh, cholinergic. |
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Term
| Identify the hormones released by the adrenal medulla: |
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Definition
| 85% epinephrine, 15% norepinephrine |
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Term
| Briefly decribe the different types of adrenergic receptors: |
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Definition
| Adrenergic stimulation has both excitatory and inhibitory effects. These effects can be produced in different tissues by the same neurotransmitter, but the responses must depend on the characteristics of the cell. This is due to different membrane proteins, Alpha (a) and Beta (B) adrenergic receptors. They have two subtypes of each: a1, a2, B1 and B2. They all work through G-proteins; if an a subunit dissiociates, its a1 or a2 subtype (or B1 or B2 subtype) will result in the characteristic response of the tissue to its neurotransmitter. |
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Term
| Give an example of an a2 receptor at work: |
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Definition
| a2 receptors are located in the presynaptic axon terminals, and cause a decrease in the amount of norepinephrine released. However, vascular smooth muscle cells have a2 receptors on the postsynaptic membrane, and are activated to cause vasoconstriction, raising blood pressure. However, drugs that activate a2 receptors are used to lower blood pressure. This is because they stimulate a2 receptors in the brain, which somehow reduces activity of the entire sympathetic nervous system. |
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Term
| Describe the effects of parasympathetic nerve regulation: |
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Definition
| Parasympathetic nerves secrete ACh from their postganglionic axon terminals which stimulate the muscarinic ACh receptors. These effects are usually excitatory, but sometimes inhibitory. ex. The cholinergic effects of the postganglionic fibers innervating the heart slows the heart rate, because the muscarinic ACh receptors cause a hyperpolarization by causing K+ channels to open. The cholinergic effects of parasympathetic nerves are promoted by the drug muscarine, and inhibited by atropine. |
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Term
| How does the drug atropine and other related drugs affect parasympathetic neural control? |
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Definition
| The muscarinic effects of ACh are inhibited by atropine, causing pupil dilation, reduction of respiratory secretions, decreased spasmodic contractions of the lower GI tract and inhibited stomach acid secretions. |
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Term
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Definition
| It is transmission at synapses in which the neurotransmitter released is norepinephrine, as in most postganglionic sympathetic nerve fibers with effector cells. |
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Term
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Definition
| It is the transmission at synapses in which the neurotransmitter released is ACh, as in all preganglionic fibers and most parasympathetic postganglionic nerve fibers with effector cells. |
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Term
| Explain how the effects of the sympathetic and parasympathetic systems can be antagonistic: |
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Definition
ex. Pacemaker region of the heart: Sympathetic and parasympathetic fibers innervate the same cells. Adrenergic stimulation from sympathetic fibers increases the heart rate, release of ACh from parasympathetic fibers decreases heart rate. |
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Term
| Explain how the effects of the sympathetic and parasympathetic systems can be complimentary: |
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Definition
ex. Salivary glands: The secretion of watery saliva and other secretions in the digestive tract are stimulated by parasympathetic nerves. Sympathetic nerves stimulate the constriction of blood vessels throughout the digestive tract, as well as to the salivary glands. The resultant decrease in blood flow to the salivary glands causes thicker, more viscous saliva to be produced. |
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Term
| Explain how the effects of the sympathetic and parasympathetic systems can be cooperative: |
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Definition
ex. Erection and ejaculation: Erection is due to vasodilation resulting from parasympathetic nerve stimulation; ejaculation is due to stimulation through sympatheic nerves. |
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