Class Notes (806,720)
Canada (492,424)
Physiology (885)
PHGY 209 (394)
Erik Cook (23)

6. Autonomic Nervous System.pdf

5 Pages
Unlock Document

McGill University
PHGY 209
Erik Cook

Naveen Sooknanan McGill Fall 2011 Autonomic Nervous System: The autonomic nervous system (ANS) is a sensory and motor system which innervates visceral tissue and organs of the body  The ANS is responsible for many involuntary responses to stimulus input  The ANS is intimately connected to the idea of homeostasis we discussed earlier o This system allows the maintenance of a relatively constant milieu interieur which is isolated from the external environment  Factors like blood pressure, temperature control, and many others are all controlled by the ANS The output of the ANS can be subdivided into two main subcategories and is related to another system called the enteric system  The first subcategory is called the sympathetic system and the second is the parasympathetic system o The sympathetic system is in charge of “fight or flight” mechanisms such as panic or excitement  For example, raising of heart rate when running away in fear o The parasympathetic system is in charge of “rest and digest” functions  For example, lowering of the heart rate when resting o These two systems contrast each other and have opposing effects on target tissues o Both systems are always active; they work together to make fine adjustments to the system o These systems innervate cardiac, smooth muscle, and glandular tissue  The enteric system is a separate system which controls the contraction of gut muscles o This allows food to move through the intestines and through the digestive tract Unlike the somatic motor pathway (the one we discussed before) then autonomic motor system’s neurons, called post-ganglionic neurons, are located outside the spinal cord in groups called autonomic ganglia  A ganglia is defined as a group of cell bodies, dendrites and synapses in the peripheral nervous system  These autonomic ganglia are activated by pre-ganglionic neurons which are located either in the spinal cord of brain stem o These synapses for white dots on either side of the spinal cord; these are the axons of the autonomic ganglia  In contract to the somatic motor unit which directly connects the motor neuron to the skeletal muscle, the autonomic motor unit consists of 2 neurons, 1 from the brain of spinal cord to the autonomic ganglia, and the other from there to the tissue o The synapse between the pre and post ganglionic neurons is excitatory In more detail, the sympathetic system motor unit differ from the motor units of the parasympathetic system 1Naveen Sooknanan McGill Fall 2011  In the sympathetic motor unit, the preganglionic neuron is very short and makes a synapse right outside the spinal cord. The postganglionic neuron is very long and travels all the way to the desired tissue o The preganglionic neuron released ACh (an excitatory synapse) to the postganglionic neuron like a neuromuscular junction o The post ganglionic axon synapses with either an α or β adrenergic receptor on the target tissue. This is also an excitatory synapse  There is a release of norepinephrine (NE) at this synapse which activate metabotropic receptors on the target tissue o This can activate sympathetic responses, such as an increased heart rate, or relaxation of bronchial tubes to stop intestinal contraction  You don’t want to waste energy digesting during an emergency o A shot of epinephrine does in effect the same thing as a sympathetic response releasing norepinephrine  In the parasympathetic system, preganglionic neurons are very long and postganglionic neurons are very short (in contrast to above) o The preganglionic/postganglionic synapse is the same as before, release ACh at an excitatory synapse o The postganglionic neuron’s synapse with the target tissue is different than the one above, however. This synapse releases ACh, not NE, to the target tissue  This activates metabotropic receptors, different from the NE receptors, which cause generally opposing effects to the sympathetic system  These receptors are called muscarinic ACh receptors (mAChR) o The activation of these receptors cause a biochemical change in the target tissue  Such events can lower the heart rate during relaxation or increase digestive tract activity The different effects of the sympathetic and parasympathetic systems on the same target tissue can be seen on the heart, known as cardiovascular reflexes  Normally, the two systems work in balance to produce a stable milieu interieur. Sometimes, however, one system can take dominance in special cases  The dominance of the sympathetic system causes a substantial, sudden increase in the heart rate and the strength of each pulse  Dominance of the parasympathetic system causes a sudden decrease in the frequency and strength of the heart beatings The autonomic nervous system generates responses to stimuli received from various sensory input sources all over the body  For example, the stimulation of pain sensing neurons in the skin activate sympathetic neurons which cause local vasoconstriction  The sensory inputs first travel to the brain for processing o They are received by nuclei within the brain 2Naveen Sooknanan McGill Fall 2011  A nucleus in neurology is a group of cell somas, dendrites and synapses within the central nervous system (CNS) o The brain mediates autonomic reflexes through projection of information to higher brain areas such as the hypothalamus and the cerebral cortex to produce more complex responses  Cortexes are very thin sheets of nervous tissue within the brain Input from visceral organs first travels through the nucleus of the solitary tract where they are integrated with automatic output. These signals are then relayed to higher brain centers (such as the hypothalamus) where more complex processing can occur  This entire process is called an autonomic reflex. This type of reflex is internal (within the body) and unconscious (we don’t know it’s happening)  The job of the nucleus in the solitary tract is essentially to pair an input with its appropriate output  The hypothalamus is a crucial part of the brain involved with maintaining homeostasis  The amygdala is also involved i
More Less

Related notes for PHGY 209

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.