- The autonomic nervous system (ANS) controls the bodies internal
environment
- It is important in the process of homeostasis
- Helps control the heart rate, blood pressure, digestion,
respiration, blood pH and other bodily functions
- These controls are done automatically below the conscious level
- The hypothalamus has important role in co-ordinating autonomic
function
- In the ANS there are two nerves between the central nervous system
(CNS) and the end organ
- The nerve cell bodies for the second nerve are organized into
ganglia
- The ANS effects its function via neural transmission as follows:
- Central nervous system
- Preganglionic nerve
- Ganglion
- Postganglionic nerve
- End organ
- At each junction neurotransmitters are released
- The ANS has two divisions that differ in anatomy and function
Sympathetic nervous system
- The sympathetic nerves come from the thoracic and lumbar regions of
the spinal cord
- The preganglionic nerves are short and synapse in paired ganglia
adjacent to the spinal cord
- The adrenal medulla, technically an endocrine gland, is functionally
a part of the sympathetic nervous system
- Acetylcholine is the neurotransmitter released from the
preganglionic neurone
- Noradrenaline is the neurotransmitter released from the
postganglionic neurone
- The sympathetic nervous system is the "fight or flight" branch of
the ANS
- Emergency situations are handled by the sympathetic system
- The sympathetic system increases cardiac output and pulmonary
ventilation, routes blood to the muscles, raises blood glucose and slows
down digestion, kidney filtration and other functions not needed during
emergencies
- The adrenal medulla behaves like a combined autonomic ganglion and
postsynaptic sympathetic nerve
- Releases both adrenaline (80%) and noradrenaline (20%)
Parasympathetic nervous system
- Parasympathetic nerves come from the cranial and sacral regions of
the CNS
- The cranial nerves involved are III, VII, IX, X
- They have long preganglionic nerves which synapse at ganglia near or
on the organ innervated
- Acetylcholine is the neurotransmitter released from both the
pre and postganglionic neurones
- The parasympathetic nervous system is the "rest and digest" branch
of the ANS
- The parasympathetic system promotes normal maintenance of the body
- It promotes secretions and mobility of different parts of the
digestive tract
- Also involved in urination, defecation.
Autonomic receptors
- There are multiple types of receptors in the autonomic nervous
system
- For the sympathetic system the major receptor types are alpha and
beta
- These are subdivided into alpha-, alpha-2, beta-1 and beta-2
- The parasympathetic system has nicotinic and muscarinic receptors
- Receptors subtypes are not evenly distributed throughout the body
- Adrenergic receptors:
- Usually stimulated by noradrenaline or adrenaline
- Alpha-1 type is found in the smooth muscle of most arterioles and
in sphincter muscles of the GI tract and bladder
- Alpha-2 type is found in presynaptic nerves and parts of the GI
tract
- Beta-1 type is the dominant type in the heart
- Beta-2 type is found in the bronchioles of the lung, the wall
muscles of the bladder and other locations
- Cholinergic receptors:
- Usually stimulated by acetylcholine
- Nicotinic types are found in autonomic ganglia
- These receptors are different nicotinic receptor found in
neuromuscular junctions
- Muscarinic types are found on all organs with parasympathetic
nerves
Actions of the ANS
- The sympathetic and parasympathetic systems often have opposing
actions on the same organ
- Some examples of autonomic functions are as follows
| |
Sympathetic |
Parasympathetic |
| Eye |
Iris dilates |
Iris constricts |
| Heart |
Increased heart rate |
Decreased heart rate |
| Bronchioles |
Bronchodilatation |
Bronchoconstriction |
| Bladder |
Sphincter constricts |
Sphincter relaxes |
| |
Detrusor muscle relaxes |
Detrusor muscle contracts |
| Intestine |
Secretions decrease |
Secretions increase |
| |
Motility decreases |
Motility increases |
| Rectum |
Sphincter relaxes |
Sphincter constricts |
| |
Muscle wall contracts |
Muscle wall relaxes |
|