Chapter 14 ( The Autonomic Nervous System)

Autonomic Nervous System
– Regulates activity of smooth muscle, cardiac muscle & certain glands
– Stability of internal environment depends largely on it.
Structures involved in the ANS
– general visceral afferent neurons
– general visceral efferent neurons
– integration center within the brain
Basic Anatomy of ANS
– Preganglionic neuron
– Postganglionic neuron
– Autonomic ganglion
Preganglionic neuron
– Cell body resides in the brain or spinal cord
– axon is lightly myelinated and extends to autonomic ganglion
Postganglionic neuron
– cell body lies outside the CNS
– axon is unmyelinated that extends to the visceral effector
Autonomic ganglion
cell body collection of autonomic neurons
Overlap of Automatic and Somatic Nervous Systems
Skeletal muscles working hard during exertion (exercise) they need more oxygen and glucose AUTONOMIC mechanisms speed up heart rate and dilate airways, dilate blood vessels to meet the exercise demands and maintain homeostasis
Divisions of the ANS
1) Parasympathetic
2) Sympathetic
– one speeds up organ
– one slows down organ
– 2 divisions counterbalance one another to keep body systems running smoothly
– Both generally innervate/serve the same organs but cause opposite effects. One stimulates certain smooth muscles to contract of a gland to secrete, the other inhibits that action
– These systems are never 0 but they switch back and forth from which one needs to be more powerful at a certain time
Parasympathetic
– Digestion
– Defecation
– Diuresis (urination)
– Directs general “house-keeping” activities
– promotes maintenance functions and conserves body energy
– Reason it’s important to rest and not exercise after a big meal. So that Sympathetic activity doesn’t interfere with digestion
– Emerge from the brain (cranial nerves) and sacral spinal cord (craniosacral)
Sympathetic
– Exercise
– Excitement
– Emergency
– Embarrassment
– “Fight-or-Flight” system
– mobilizes the body during activity, always is giving you energy
– Originate in the thoracolumbar region of the spinal cord
Cranial nerves that contain parasympathetic fibers
– Oculomotor (pupillary reflect, constrict under light)
– Facial (tear glands, salivary glands)
– Vagus (heart rate, breathing rate, digestive system activity)
– Glossopharyngeal (salivary glands)
Sympathetic Anatomy
Origin of Fibers
– Thoracolumbar region of the spinal cord
Length of Fibers
– Short preganglionic and long postganglionic
Location of Ganglia
– Close to the spinal cord
Parasympathetic Anatomy
Origin of Fibers
– Brain and sacral spinal cord
Length of Fibers
– Long preganglionic and short postganglionic
Location of Ganglia
– In the visceral effector organs
Pathways to the Head
– Dilator muscles of the eyes (iris)
– Inhibit nasal and lacrimal glands
Pathways to the Thorax
Stimulate heart, lungs
Pathways to the Abdomen
Stomach, intestines, liver, spleen, kidneys
Pathways to the Pelvis
Inhibit reproductive and urinary organs
Kidneys
causes body to retain water; vasocontriction
Liver
stimulates release of glucose
Spleen
– plays important roles in regard to red blood cells (also referred to as erythrocytes) and the immune system.
– It removes old red blood cells and holds a reserve of blood, which can be valuable in case of hemorrhagic shock, and also recycles iron.
– As a part of the mononuclear phagocyte system, it metabolizes hemoglobin removed from senescent erythrocytes
Adrenal Medulla (Gland)
– Produces hormones epinephrine and norepinephrine into blood
– Cause skeletal muscles to contract more strongly and quickly. Side effect neural bursts put muscle contractions on a hair trigger are great if you have to make a quick jump or run but can be disabling to a nervous musician or surgeon
ANS Neurotransmitters
– Acetylcholine (ACh)
– Norepinephrine (NE)
– Effects of Ach and NE are not consistently inhibitory or excitatory. It depends on the receptor that it binds
– Classified as either cholinergic or adrenergic neurons based upon the neurotransmitter released
– Cholinergic
– Adrenergic
Acetylcholine (ACh)
– Secreted by somatic motor neurons (neuromuscular junction)
– Secreted by all ANS preganglionic axons
– Secrete by all parasympathetic postganglionic axons at synapses with their effector organs
– Fibers that release ACh are cholinergic
Norepinephrine (NE)
Secreted by sympathetic postganglionic axons
Nicotinic receptors
response to ACh binding is always stimulatory
Muscarinic receptors
response to ACh binding is either stimulatory or inhibitory depending on the subclass of muscarinic receptor on the target organ
Hypothalamus
– regulates balance (tone) between sympathetic and parasympathetic activity levels
– Coordinates heart activity, BP, body temp, water balance, and endocrine activity
– Also regulates our reaction to fear
Sympathetic Responses
– Dominance by the sympathetic system is caused by physical or emotional stress –“E situations”
– emergency, embarrassment, excitement, exercise
– Alarm reaction = flight or fight response
– dilation of pupils
– increase of heart rate, force of contraction & BP
– decrease in blood flow to nonessential organs
– increase in blood flow to skeletal & cardiac muscle
– airways dilate & respiratory rate increases
– blood glucose level increase
Parasympathetic Responses
– Enhance “rest-and-digest” activities
– Mechanisms that help conserve and restore body energy during times of rest
– Normally dominate over sympathetic impulses
– Responses = salivation, lacrimation, urination, digestion & defecation
– 3 “decreases”- decreased HR, diameter of airways and diameter of pupil
Regulation of the ANS
an individual has limited ability to maintain homeostasis and readily succumbs to sudden changes in environmental conditions
Autonomic reflexes
– These reflexes control most of the activity of visceral organs, glands, and blood vessels
– Presence (and necessity) of sensory fibers are often overlooked but necessary
Cerebrum
– remembering a frightening event makes our heart race
– thinking of cupcakes makes our mouths water
Functional Generalizations about the ANS
1. Both divisions produce stimulatory and inhibitory effects
2. Most organs are innervated by both divisions
3. Usually each division produces an opposite effect on a given organ
4. Either division alone or both working together can coordinate the activities of different structures
5. Sympathetic activity generally prepares the body for physical activity
6. Parasympathetic activity is more important for vegetative functions like sitting through an Anatomy and Physiology lecture!
ANS DISORDERS
– Hypertension
– Autonomic neuropathy
– HTN
– Causes of neuropathy in diabetics
Hypertension
– type of ANS disorder
– High BP
– Can be promoted by overactive sympathetic response to chronic stress
– Risk factor for CV disease
– Sometimes treated with adrenergic-receptor blockers (Beta-blockers)
Autonomic neuropathy
– type of ANS disorder
– Common complication with diabetes
– Due to uncontrolled blood glucose
– May cause problems with digestion, bowel/bladder function, sexual dysfunction, heart rate, BP control, respiration, and eyes
– Autonomic neuropathy causes changes in digestion, bowel and bladder function, sexual response, and perspiration.
– It can also affect the nerves that serve the heart and control blood pressure, as well as nerves in the lungs and eyes.
HTN
– type of ANS disorder
– also caused by high sodium diet which promotes water retention which increases BP, diuretics prescribed for HBP
Causes of neuropathy in diabetics
– type of ANS disorder
– metabolic factors, such as high blood glucose, long duration of diabetes, abnormal blood fat levels, and possibly low levels of insulin
– neurovascular factors, leading to damage to the blood vessels that carry oxygen and nutrients to nerves
– autoimmune factors that cause inflammation in nerves
– mechanical injury to nerves, such as carpal tunnel syndrome
– inherited traits that increase susceptibility to nerve disease
– lifestyle factors, such as smoking or alcohol use