Test 3 - Autonomic Nervous System

Autonomic Nervous System

• Important for taking care of things we don’t have to think about, like regulating our heartbeat, breathing, digesting, keeping our correct temperature, etc.

• Divided into two branches - the sympathetic and parasympathetic nervous systems

• Different in that there are 2 neurons leaving the spinal cord and leading to the organs

• the first neuron has its cell body within the gray matter of the brain or spinal cord, this is the preganglionic neuron

• the preganglionic neuron then synapses with a second neuron called the postganglionic neuron, which will release the NT to the organ

Sympathetic Nervous System

• Preganglionic Neurons all originate between the 1st thoracic to 2nd lumbar levels (the Thoracolumbar Division)

• Then synapse with another neuron (postganglionic neuron, using Acetylcholine as the NT) which carries the impulse on to the effector organ

• Most of these synapses occur in a double row of sympathetic ganglia running down both sides of the spinal cord called the paravertebral (or sympathetic chain) ganglia

• These ganglia occur at each level of the spinal cord, and form the sympathetic chain of ganglia

Parasympathetic Nervous System

• Preganglionic fibers originate from the midbrain, hindbrain, and 2nd to 4th sacral levels of the spinal cord (Craniosacral division)

• Then synapse with another neuron (postganglionic neuron, using Acetylcholine as the NT) which carries the impulse on to the effector organ

• Ganglia are found within or next to the organ that is being innervated, these are called the terminal ganglia - postganglionic neurons are really short since they don’t have to go far

Function of the Sympathetic Nervous system

• Usually activated as a whole system

• Prepares the body for intense survival ‘fight or flight’ activities

• Also includes the adrenal medulla, which secretes the hormones epinephrine and norepinephrine under control of the sympathetic NS - these hormones are more commonly known as adrenaline, and are what give you an adrenaline rush after something exciting has happened

• Neurotransmitter for the postganglionic neurons in the sympathetic system is norepinephrine (called adrenergic neurons) - helps put things together with the adrenaline from the adrenal gland

• Response to all this is that the heart, iris, and smooth muscles of many blood vessels are all stimulated to contract - causes your pupils to dilate, the heart to pump with more force, and some blood vessels to shut down blood flow to areas like the skin and gastrointestinal tract

• Smooth muscle in your bronchioles, the cardiac blood vessels, are prevented from contracting and dilate, which will help you better breath and get more blood flow to the cardiac muscles

• Preganglionic Neurons: short, use ACh

• Postganglionic Neurons: long, use Norepinephrine

Types of Norepinephrine Receptors

• Different types of receptors allow norepinephrine to have different effects

• There are 2 basic types of receptors:

  • Alpha Adrenergic receptors

  • Beta-adrenergic receptors

Alpha Adrenergic Receptors

• Cause smooth muscle contraction

• Leads to the vasoconstriction of some blood vessels as well as the dilation of the pupils

Beta-Adrenergic Receptors

• Stimulate contraction of cardiac muscles

• Increase heart rate and cause the relaxation of smooth muscle in the digestive tract and bronchioles

• Some hypertensive people receive a beta adrenergic receptor antagonist (beta blocker) like propanolol, which prevents beta stimulation of the heart and reduces the pulse rate, as well as prevents the increase in blood pressure excitement might cause

• People with asthma might be treated with the beta adrenergic agonist epinephrine to cause relaxation of the bronchioles, this is what albuterol is - a chemical that mimics the effects of epinephrine in the lungs

Parasympathetic Nervous System Function

• Uses ACh as its main NT (called cholinergic neurons)

• Have more precise control of individual organs

• not activated as a whole system

• Effects of parasympathetic nervous system are generally the opposite of the sympathetic nervous system

• Has two basic types of receptors:

  • Muscarinic Receptors

  • Nicotinic Receptors

• Preganglionic Neurons: Long, use ACh

• Postganglionic Neurons: short, use ACh

Muscarinic Receptors

• Will respond to the drug from mushrooms muscarine as well as ACh

• Found in the heart, eye, and digestive system

• When activated by either ACh or Muscarine, cause the constriction of pupils, slowing of the heart, and an increase in GI activity

Nicotinic Receptors

• Binds the drug nicotine from tobacco and stimulates the ACh receptors at autonomic ganglia, and at the neuromuscular junction of skeletal muscles

• Blocked by Curare and Atropine

Curare

• Antagonist of Nicotinic receptors at skeletal muscles

• Blocks the effects of nicotinic receptors

• Causes muscle relaxation without effecting the muscarinic receptors

• Surgeons may also use it to prevent muscle contractions during surgery

Atropine

• Blocks the effects of nicotinic receptors

• Derived from the nightshade plant

• Inhibits parasympathetic innervation of the iris, resulting in the dilation of the pupils

• Can also dry mucous membranes in the respirator tract and inhibit contractions of the lower GI tract

Parasympathetic and Sympathetic NS as antagonists

• The Sinoatrial (SA) node of the heart is responsible for controlling the rate of cardiac contractions

• Cells of the SA node are under both sympathetic and parasympathetic control

• Sympathetic stimulation causes the heart to increase the rate of contractions (+chronotropy)

• while parasympathetic stimulation via cranial nerve X (vagus) causes the heart rate to decrease (-chrontropy)

Parasympathetic and Sympathetic NS as Complements

• Parasympathetic NS causes watery saliva to be secreted

• Sympathetic nervous system is capable of causing vasoconstriction and decreased blood flow in the GI tract, resulting in a more viscous saliva with more enzymes in it

Parasympathetic and Sympathetic NS as Cooperatives

• Act cooperatively to control the urinary and reproductive systems

• Parasympathetic nervous system causes vasodilation of veins in the penis, causing an errection

• Sympathetic system controls ejaculation and orgasm

Control of the Autonomic Nervous System

• Cerebral Cortex (conscious) does have some control over our Autonomic NS

• When we are excited, even from a memory, we can undergo the sympathetic response

• Helping with this effect is the Limbic system

• Reticular formation of the brainstem is also involved in helping mediate the eventual responses by the body

Limbic System

• a group of fiber tracts and nuclei around the brain stem

• includes the hypothalamus and the hippocampus

• appears to have a lot to say about the control of the autonomic NS

• involved in things like sex, anger, fear, and hunger

• Elicits many of the physical symptoms associated with emotions (blushing, fainting, cold sweat, fast heartbeat) that are not necessarily physiologically relevant