Human Physiology Unit 2 Exam
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OpenStax_Physiology_CH15.pptxphysiology
Chapter 15 THE AUTONOMIC NERVOUS SYSTEM
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Autonomic Nervous system
Fight or Flight? Rest and Digest?
Though the threats that modern humans face are not large predators, the modern world presents stimuli that trigger the same response.
The autonomic nervous system is adapted to this type of stimulus by controlling cardiac and smooth muscle and glandular tissue through involuntary responses.
(credit: Vernon Swanepoel)
Divisions of the Autonomic NS
Sympathetic Division of the Autonomic Nervous System
Sensory information arrives at the central nervous system. This information is processed and integrated. Then efferent responses are sent to target effectors throughout the body. These responses exit the CNS along the thoracic and lumbar spine.
This division is responsible for the Fight or Flight response.
Divisions of the Autonomic NS
Parasympathetic Division of the Autonomic Nervous System
Sensations arrive at the central nervous system, are processed and integrated and efferent responses exit the system at the brainstem through cranial nerves or at the sacral spinal cord. These responses terminate near or within the various organs of the body.
This division is responsible for the Rest and Digest response.
Function of the Autonomic Nervous system
Homeostasis is the balance between the two divisions of this system.
Target effectors will have dual innervation: synapses for the sympathetic division and synapses for the parasympathetic division.
Stimulation of the synapses creates opposite responses.
Sympathetic stimulation creates increased heart rate
Parasympathetic stimulation creates decreased heart rate.
Atypical synapses
Autonomic Varicosities
The connection between autonomic fibers and target effectors is not the same as the typical synapse, such as the neuromuscular junction. Instead of a synaptic end bulb, a neurotransmitter is released from swellings along the length of a fiber that makes an extended network of connections in the target effector.
Chemical signaling
Chemical signals can be neurotransmitters that are released across a synapse to bind to the effector cell or hormones that are released into the bloodstream to bind to effectors some distance away.
Synapse systems:
cholinergic – primary neurotransmitter is acetylcholine
adrenergic – neurotransmitter is norepinephrine
Both systems have several receptor types that respond differently to various neurotransmitters and chemicals. This becomes important when researchers are developing drugs.
Homeostasis and reflexes
Somatic reflexes are basically input sensory neuron to spinal cord then output neuron to muscle.
Autonomic reflexes are input sensory neuron to spinal cord or cranial nerve and output neuron from spinal cord or cranial nerve to a ganglion then another neuron to effector target.
Visceral reflexes do not need a central component therefore do not result in conscious perception i.e. you do not “feel” high blood pressure or high blood sugar.
However, sometimes you do “feel” visceral sensation.
This is referred pain.
Visceral sensory fibers enter the spinal cord at the same level as the somatosensory fibers and the brain misinterprets the signal.
Referred Pain
Conscious perception of visceral sensations map to specific regions of the body, as shown in this chart. Some sensations are felt locally, whereas others are perceived as affecting areas that are quite distant from the involved organ.
Autonomic reflexes
Short and Long Reflexes
Sensory input can stimulate either a short or a long reflex.
Long reflexes involve a sensory neuron that projects to the CNS then the efferent branch goes to the ganglion and effector.
The short reflex involves the direct stimulation of the ganglion and back out to the effector. E.g. Enteric Nervous System: Stretch of the stomach wall after eating afferent to ganglion, efferent back to stomach to produce contraction and peristalsis.
Balance of the systems
Dual innervation to effector sites by sympathetic and parasympathetic systems.
Target cells have receptors for both adrenergic and cholinergic neurotransmitters.
When sympathetic is stimulated, norepinephrine is released and depolarization occurs.
When parasympathetic is stimulated, acetylcholine is released and hyperpolarization occurs.
Example:
See a lion, sympathetic stimulated, norepinephrine released, heart rate increases.
See a kitten, parasympathetic is stimulated, acetylcholine is released, heart rate slows.
In blood vessels within skeletal muscle and sweat glands, no dual innervation. Sympathetic releases acetylcholine, dilates vessels and produces sweat.
Balance of the systems
Another example: Pupillary Reflex Pathways
The pupil is under competing autonomic control in response to light levels hitting the retina. The sympathetic system will dilate the pupil when the retina is not receiving enough light, and the parasympathetic system will constrict the pupil when too much light hits the retina.
Balance of the systems
Tone – organ systems tend to be under more influence from the sympathetic or the parasympathetic at resting state.
E.g. resting heart rate is under parasympathetic tone to keep the rate from rushing off at high rates.
Stress (from anything) increases sympathetic tone and has been shown to result in the release of inflammatory chemicals putting the body at risk for disease.
Doing things like exercise, yoga and meditating can increase parasympathetic tone and decrease that risk.
Central control
The hypothalamus is the source of most of the central control of autonomic function. It receives sensory input from the body and other parts of the brain. It integrates this information and then determines which division, sympathetic and parasympathetic, is activated.
Central control
The amygdala, part of the limbic lobe, is involved in memory and emotion. It connects to the hypothalamus to integrate sensory information to activate the sympathetic or parasympathetic division.
The medulla is continuous with the spinal cord and processes information from the cranial nerves; mostly to the parasympathetic division.
Effect of drugs on the system
Drugs that are able to bind at the effector sites (norepinephrine, acetylcholine, and epinephrine sites) can override homeostasis.
Sympathomimetic – simulate norepinephrine, increase production and release of norepinephrine, or block the removal or reuptake of norepinephrine. Result is increase depolarization and increase of sympathetic tone. E.g. caffeine
Sympatholytic – block the binding of epinephrine and norepinephrine thus deceasing sympathetic tone. E.g. β blocker heart medications. Block the adrenergic sites on the cardiac muscle and blood vessels resulting in decreased hear rate. Antianxiety drugs are also sympatholytic.
Parasympathomimetic – enhance cholinergic effects. E.g. pilocarpine constricts the iris. Used to treat some forms of glaucoma
Anticholinergic – decrease the effect of acetylcholine. E.g. atropine – dilates pupil. Scopolamine used to prevent motion sickness.
Effect of drugs on the system
Mydriasis
The sympathetic system causes pupillary dilation when norepinephrine binds to an adrenergic receptor in the radial fibers of the iris smooth muscle. Phenylephrine mimics this action by binding to the same receptor when drops are applied onto the surface of the eye in a doctor’s office. (credit: Corey Theiss)
Effect of drugs on the system
Belladonna Plant
The plant from the genus Atropa, which is known as belladonna or deadly nightshade, was used cosmetically to dilate pupils, but can be fatal when ingested. The berries on the plant may seem attractive as a fruit, but they contain the same anticholinergic compounds as the rest of the plant.
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