That which is in red was updated after input from Dr. Winterson...See below
The SNS has preganglionic neurons with cell bodies in the BS or cord that are located mainly in the intermediolateral cell column of the thoracic cord. The NT here is ACh via nicotinic receptors. The axons project along the preganglionic fibers to the autonomic ganglion which are usually located in the paravertebral column. The postgangionic neuron has cell bodies in the ganglion and send axons far out to the effector cells. The exception is the adrenal medulla where the postganglionic neuron is the adrenal medulla itself. The primary NT of the postganglionic neurons is norepinephrine. Another NT is epinephrine which is released directly by the adrenal medulla which also releases some norepi. Most of the norepi is stored in vesicles located along the visceral motor axons as swellings which give it a beaded appearance. There are also some receptors on the postganglionic neurons which respond to ACh – these are hair follicles, pilo-erector muscles and some sweat glands. These receptors are said to be cholinergic muscarinic where as the norepi or epi receptors are adrenergic. Stimulation of the glands will likely produce some secretion but the blood vessels to the glands in general are constricted when the SNS is stimulated so the secretion will diminish to most glands except the sweat glands. There are certain sweat glands that are under adrenergic control thus are differentiated by the term adrenergic sweating. These are the soles of the feet, the axilla, the groin, the palms of the hands. There are several other adrenergic receptors. These are delineated below.
|
Receptors type Adrenergic |
a 1 |
a 2 |
b 1 |
b 2 |
|
Target organs |
Blood vessels –art>>>veins |
Presynaptic auto-receptor |
Heart – myocardium |
Lungs |
|
Neurotransmitter responds to: |
Norepi –primary |
Norepi, epi, dopa |
Epi-primary |
Epi |
|
Physio responses |
vasoconstriction |
Reduce SNS activity |
HR, CO |
Bronchodilation |
|
Other physio responses |
Iris dilation |
Lipolysis |
Vasodilation |
The typical SNS response is a flight or fight one. It can be considered energy mobilizing for the following reasons: heart rate & heart contractility increase which allows for the increased blood flow to the skeletal muscles to increase muscle strength, the liver begins to use up the available glucose and then the glycogen stores and once depleted will then aid in the mobilization of fat stores, increased mental awareness and activity, bronchodilation to allow for increased RR and to allow for increase in skeletal muscle activity, increased rate of blood coagulation and BMR, sweat gland via adrenergic receptors increases secretion which allows for temperature regulation. These all will increase the body’s energy level. To allow these to occur the following will decrease: blood flow to kidneys, ducts of liver, gb, ureter, bladder, bronchi are inhibited, gi motility slows, sphincter tone decreases due to decreased blood flow, glandular secretion decreases.
Sympathetic Nervous System as ‘energy mobilizing’
Peripheral responses to SNS activation
Sensory input (usually fear or pain), mediated via the spinal cord, brainstem and hypothalamus (especially) cause widespread reaction to mobilize resources for intense activity in the ‘fight or flight response’.
Sympathetic nerve fibers secrete neurotransmitter to send a chemical signal to their effector organ receptors and cause an effect. The SNS preganglionic NT is acetylcholine, and the receptor is nicotinic. The postganglionic NT is norepinephrine (and sometimes Ach), and the receptor is nicotinic. Before the NT can cause an effect, it must first bind with highly specific receptors on effector cells on the outside of the cell membrane of the target organ. When the NT binds the receptor, a conformational change occurs in the membrane protein. This change either excites or inhibits the target organ by 1) changing the ion channels and altering the permeability to certain ions either depolarizing or hyperpolarizing the cells, or 2) by initiating the G-protein second messenger system to initiate an action. The adrenergic receptors for norepinephrine are alpha 1 & 2, and beta 1 & 2 specifically.
| The comments in this box are from Dr.
Winterson. They have been integrated into the essay outline above
and highlighted in red. "When you mention the sweat glands you must mention the cholinergic control as well and distinguish from adrenergically controlled sweating. I think it would improve the narrative, if you first listed and explained the organ systems and tissues that (1) actually "mobilize energy" or aid in that process, (2) those organs and tissues that are needing this increase of energy and oxygen and (3) those organs and tissues that are be shut down to conserve energy for these other tissues. Pupil dilation aids in fight or flight by increasing the sensitivity to light. Strictly speaking it is neither energy conserving or mobilizing, it just fits with a general alarm reaction. Sweating of course serves temperature regulation when metabolic rate is increased dramatically. I have been telling students to work in the receptors subtypes when you describe the individual actions in particular tissues (e.g. beta 2 bronchodilation) You should also describe the increased release of epinephrine from the adrenal medulla and mechanisms involved." |
Last Updated 09/06/01 08:53:36 PM
Return To The MNA2001 Homepage