The autonomic nervous system is considered to be the involuntary branch of the peripheral efferent division. Skeletal muscles are innervated by the somatic nervous system while cardiac muscle; smooth muscle most exocrine glands and some endocrine glands are innervated by the autonomic nervous system.
Two neuro transmitters; acetylcholine and nor epinephrine are released at the neuronal terminals and are responsible for bringing about all the changes effected by the autonomic nervous system for example bladder contraction and salivary secretion. The involuntary effects of autonomic innervations contrast with the voluntary control of skeletal muscles through the somatic neurons.
In this unit we shall examine the structures and pathways of the autonomic nervous system, the differences in the autonomic and somatic systems as well as describe the structure and general functions of the sympathetic and parasympathetic divisions of the autonomic system.
Structures and Pathways of the Autonomic System Compared to Somatic System
The autonomic nervous is the involuntary branch of the peripheral nervous system. It is also known as the visceral efferent motor system because it is concerned with internal organs or viscera. The autonomic nervous system is exclusively a motor system, involved with influencing (innervating) the activity of cardiac muscle, smooth muscle and glands of the body.
The autonomic nervous system consists of two divisions: the sympathetic and the parasympathetic nervous system. Sympathetic nerve fibres originate in the thoracic and lumber regions of the spinal cord. Each autonomic nerve pathway extending from the CNS to an innervated organ consists of a two-neuron chain.
The cell body of the first neuron in the series is located in the CNS. Its axon, the pre-ganglionic fibre synapses with the cell body of the second neuron, which the lies within a ganglion outside the CNS. The axon of the second neuron, the post-ganglionic fibre, innervates the effector organ. The involuntary effects of autonomic innervation contrast with the voluntary control of skeletal muscles by way of somatic motor neurons.
Neural Control of Involuntary Effectors
Neurons of the peripheral nervous system conduct impulses away from the central nervous. There are two major categories of motor neurons. Somatic motor neurons have their cell bodies within the CNS and send axons to skeletal muscles which are usually under voluntary control.
Unlike somatic motor neurons, which conduct impulses along a single axon from the spinal cord to the neuromuscular junction, autonomic motor control involves two neurons in the grey matter of the brain or spinal cord. The axon of this neuron does not directly reach the effector organ but synapses with a second neuron within an autonomic ganglion. The first neuron is called the preganglionic neuron while the second in this pathway is called the post-ganglionic neuron and has an axon that extends from the autonomic ganglion and synapses with the cells of an effector organ.
Preganglionic fibres originate in the midbrain, hindbrain and in the upper thoracic to the 4th sacral levels of the spinal cord. Autonomic ganglia are located in the head, neck and abdomen. Chains of autonomic ganglia also parallel both sides of the spinal cord. The origin of the preganglionic fibres and the location of the autonomic ganglia help to differentiate the sympathetic and parasympathetic divisions of the autonomic system.
Visceral Effector Organs
Since autonomic nervous system helps to regulate the activities of glands, smooth muscle and cardiac muscle, autonomic control is an integral aspect of the functioning of most body systems. Autonomic regulation therefore partly explains the functioning of the systems/organs of the body like endocrine regulation, functions of the heart and circulation etc.
Unlike skeletal muscles which enter a state of flaccid paralysis and atrophy when their motor nerves are cut, the involuntary effectors are independent of their innervation. Infact damage to an autonomic nerve makes its target organs more sensitive than normal to stimulating agents. Such compensatory mechanism may explain why the ability of stomach mucosa to secrete acid may be restored after vagotomy.
Smooth muscle and cardiac muscle have intrinsic muscle tone. In addition they can contract rhythmically even in the absence of nerve stimulation. This is in response to electrical waves of depolarization initiated by the muscles themselves. Autonomic innervation simply increases or decreases this intrinsic activity. Autonomic nerves also maintain a resting tone in the sense that they maintain a baseline firing rate that can either increase or decrease.
The release of the neurotransmitter, acetylcholine from somatic motor neuron always stimulates the effector organ (skeletal muscle). In contrast some autonomic nerves release transmitters that inhibit the activity of their effectors.
Divisions of the Autonomic Nervous System
The sympathetic and parasympathetic divisions of the autonomic system have some structural features in common. Both consist of pre-ganglionic neurons, which originate in the CNS, and post ganglionic fibres that originate outside the CNS in ganglia. The specific origin of the prreganglionic fibres and their location are however different in the two divisions.
Sympathetic Division (Thoracolumbar)
It is also called the thoracolumbar division because its preganglionic fibres leave the spinal cord from the first thoracic (TI) to the second lumber (L2) levels. Most sympathetic nerve fibres however separate from the somatic motor fibres and synapse with postganglionic neurons within a double row of sympathetic ganglia or para vertical ganglia located on either side of the spinal cord.
The myelinated preganglionic sympathetic axons exit the spinal cord in the ventral root of spinal nerves, but soon diverge from the spinal nerves within white rami communicants. The axons within each ramus enter the sympathetic chain of ganglia where they can travel to ganglia at different levels and synapse with post-ganglionic sympathetic neurons.
The axons of the post-ganglionic sympathetic neurons are unmyelinated and form the grey rami communicates as they return spinal nerves to their effector organ. Since sympathetic axons form a component of spinal nerves, they are widely distributed to the skeletal muscles and skin of the body where they innervate blood vessels and other involuntary effectors.
Many preganglionic fibres that exit the spinal cord in the upper thoracic level travel into the neck, where they synapse in cervical sympathetic ganglia. From here post ganglionic fibres innervate the smooth muscles and glands of the head and neck.
Many preganglionic fibres that exit the spinal cord below the diaphragm pass through the sympathetic chain of ganglia without synapsing. Beyond the sympathetic chain of ganglia they form the splanchnic nerves. These preganglionic fibres in the splanchnic nerves synapse in collateral ganglia. These include the coeliac, superior mesenteric and inferior mesenteric ganglia. Post ganglionic fibres that arise from the collateral ganglia innervate organs of the digestive, urinary and reproductive systems.
The adrenal medulla, the inner portion of the adrenal glands is considered a modified sympathetic ganglion, its cells having been derived from the same embryonic tissue as ganglionic sympathetic neurons. It secretes the hormones epinephrine (80%) and norepinephrine when stimulated by the sympathetic system.
Like a sympathetic ganglion, the preganglionic sympathetic fibre enervates the adrenal medulla and causes it to secrete epinephrine into the blood. The effect of epinephrine becomes comparable to those of the neurotransmitter norepinephrine which is released at post ganglionic sympathetic nerve endings. No other post ganglionic fibre is needed. For this reason, and because the adrenal medulla is stimulated as part of the mass activation of the sympathetic system, the two are grouped together as a single sympathoadrenal system.
Diagram of parasympathetic and sympathetic nerves
Parasympathetic (Cranio Sacral) Division
The preganglionic fibres of this division originate in the brain (specifically midbrain, medulla, oblongata and pons) and in the second through 4th sacral levels of the spinal column. It is therefore also called craniosacral division. These fibres are long in comparison to sympathetic preganglionic fibres because they do not end until they reach the terminal ganglia that lie next to or within the effector organs.
Most parasympathetic fibres do not travel within spinal nerves as do sympathetic fibres. As a result, cutaneous effectors (blood vessels, sweat glands and erector pili muscles) and blood vessels in skeletal muscles receive sympathetic but not parasympathetic innervation.
Four of the twelve pairs of cranial nerves contain preganglionic parasympathetic fibres. These are oculomotor (III) facial (VII) glossopharyngeal (ix) and vagus (x) nerves. Preganglionic fibres from cell bodies located in the midbrain are conveyed by the oculomotor (III) cranial nerve to a synapse in the ciliary ganglion.
The post ganglionic axon terminals from there innervate the constrctor muscles in the iris, as well as the ciliary muscles that change the shape of the lens to focus the eyes. Preganglionic fibres originating in the lower pons leave by way of the facial cranial (vii) nerve to either the pterygopalatine or submandibular ganglia where they synapse.
Post ganglionic fibres innervate the lacrimal glands, which secrete tears, and the nasal, oral and pharyngeal cavities. Preganglionic fibres of the glosso-pharyngeal nerve from nuclei in upper medulla synapse in the otic ganglion which sends post ganglionic fibres to innervate the parotid salivary glands.
Preganglionic fibres that emerge from cell bodies located in the dorsal vagal nucleus of the medulla are conveyed by the very long vagus (x) nerve. They synapse in terminal ganglia located in many regions of the body. The preganglionic fibres travel through the oesophageal opening in the diaphragm into the abdominal cavity.
In each region some of these pre-ganglionic fibres branch from the main trunks of the vagus nerves and synapse with postganglionic neurons located within the effector organs. These very long preganglionic vagus fibres provide parasympathetic innervation to the heart, lungs, oesophagus, stomach pancreas, liver, small intestine and the upper half of the large intestine. Post ganglionic parasympathetic fibres arise from terminal ganglia within these organs and synapse with effector cells (smooth muscles and glands).
From the sacral levels of the spinal cord arise preganglionic parasympathetic innervation to the lower half of the large intestine, rectum and to the urinary and reproductive systems. These fibres like those of the vagus synapse with terminal ganglia located within the effector organs.
Functions of the Autonomic Nervous System
The sympathetic and parasympathetic divisions of the autonomic system affect the visceral organs in different ways. Mass activation of the sympathetic system prepares the body for intense physical activity in emergencies or stressful situations, such as a physical threat from the outside environment. This response is typically referred to as the flight or fight response because the sympathetic system readies the body to fight against or flee from the threat.
Think about the body resources needed in such circumstances. The heart beats more rapidly and more forcefully, blood pressure, is elevated because of generalized constriction of the blood vessels; the respiratory airways open wide to permit maximal airflow, glycogen (stored sugar) and fat stores are broken down to release extra fuel in the blood; and blood vessels supplying skeletal muscles dilate.
The effects of parasympathetic stimulation are in many ways opposite to the effects of sympathetic stimulation. They dominate in quiet, relaxed situations. Under such non-threatening circumstances the body can be concerned with its own general activities like digestion and emptying of the urinary bladder. The parasympathetic system promotes these kinds of bodily functions while slowing down those activities enhanced by the sympathetic system.
The parasympathetic system however is not normally activated as a whole. Visceral organs respond differently to sympathetic and parasympathetic nerve activity because the postganglionic fibres of these two divisions release different neurotransmitters.
Neurotransmitters of the Autonomic Nervous System
Sympathetic and parasympathetic preganglionic fibres release the same neurotransmitter, acetylcholine (ACH) but the postganglionic endings of these two systems release different neurotransmitters. Parasympathetic postganglionic fibres release acetylcholine.
Accordingly, they, along with all autonomic preganglionic fibres are called cholinergic fibres. In contrast, most sympathetic post ganglionic fibres are called adrenergic fibres because they release norepinephrine (noradrenaline). There are very few exceptions where some sympathetic fibres release ACH. Examples are in the sympathetic supply to blood vessels in skeletal muscles as well as to sweat glands.
Responses to Adrenergic Stimulation
It has been found that both excitatory and inhibitory effects can be produced in different tissues by the same chemical. For example adrenergic stimulation from sympathetic nerves causes the heart, muscles of the iris and the smooth muscles of many blood vessels to contract. However the same sympathetic stimulation dilates the smooth muscles of the bronchioles and some blood vessels.
The possible explanation lies in the biochemistry of the tissue cells, especially differences in the membrane receptor proteins. For example two major classes of these receptor proteins have been designated alpha and beta adrenergic receptors. There are also two subtypes of each class for example alpha 1 and alpha – 2 and beta 1 and beta 2.
From this, compounds which selectively bind to one or the other of adrenergic receptor have been developed. As a result of their binding capacity to adrenergic receptors, drugs have been developed which either promote or inhibit adrenergic effect. It has also been possible to determine which sub types are present in each organ.
A drug that binds to receptors for a neurotransmitter and promotes the process stimulated by that neurotransmitter is called an agonist of that neurotransmitter. A drug that blocks the action of a neurotransmitter is said to be an antagonist. The use of drugs that selectively stimulate or block a1, a2, ß1 and ß2 receptors has proved extremely useful in medical application. For example, people with hypertension have been treated with a beta-blocking drug known as propranodol. This drug blocks B1 receptors to produce the desired effect of lowering the cardiac rate and blood pressure etc.
Responses to Cholinergic Stimulation
Somatic motor neurons, all preganglionic autonomic neurons and most postganglionic parasympathetic neurons are cholinergic releasing ACH as a neurotransmitter.
The cholinergic effects of somatic motor neurons are always excitatory. The cholinergic effects of postganglionic parasympathetic fibres are also usually excitatory but there are some notable exceptions. For example the parasympathetic (cholinergic) effect on the heart causes slowing of the heart rate instead of excitation.
Just as adrenergic receptors are divided into alpha and beta subtypes, cholinergic receptors are divided into muscarinic and nicotinic receptor subtypes. The drug muscarine stimulates the cholinergic receptors in the heart, digestive system, however does not stimulate the muscarinic subtypes in autonomic ganglia or at the neuromuscular junction of skeletal muscles.
It is rather the drug nicotine that stimulates these cholinergic receptors, therefore the receptors in these places must be nicotinic receptors. The drug used for skeletal muscle relaxation blocks nicotinic receptors but has little effect or muscarinic receptors.
Dual Innervation of Visceral Organs
Most visceral organs are innervated by both sympathetic and parasympathetic nerve fibres. On a general note the two systems exert opposite effects in a particular organ. One system dominates at one time while the other dominates other times depending on circumstances. Usually both systems are partially active at each point in time until the particular circumstances that will cause the activity of one to dominate the other ensue.
These kinds of effect are called Antagonistic effects in conditions of dual innervation. The best example of antagonism in the two systems is their innervation of the pacemaker region of the heart. Here adrenergic stimulation from sympathetic fibres increases the heart rate while cholinergic stimulation from parasympathetic fibres decreases the heart rate.
In a few cases the effects of sympathetic and parasympathetic nerves are complementary or cooperative. Complementary effects occur when stimulation of both divisions produce similar effects. An example is the sympathetic and parasympathetic stimulation of the salivary glands.
The effects are cooperative when sympathetic and parasympathetic stimulation produce two different effects that work together to promote a single action. An example is the parasympathetic effect on the penis causing erection and the sympathetic effect producing ejaculation, they cooperate to promote reproduction.
A few organs in the body however do not have dual innervation. These include the adrenal medulla, erector pili muscles, sweat glands and most blood vessels which receive only sympathetic innervation.
Control of Autonomic Nervous System by Higher Brain Centres
Visceral functions are mostly regulates by autonomic reflexes, sensory input is transmitted to the brain centres that integrate this information and respond by modifying the activity of preganglionic autonomic neurons. The neural centres that directly control the activity of autonomic nerves are influenced by higher brain centres and by sensory input.
The medulla oblongata in the brain stem is the area that most directly controls the activity of the autonomic system. Almost all autonomic responses can be elicited by experimental stimulation of the medulla. The organ contains centres for the control of cardiovascular, pulmonary, urinary, reproductive and digestive systems.
The medulla oblongata itself is responsive to regulation by higher brain areas. One of these is the hypothalamus which is the brain region that contains centres for the control of body temperature, hunger, thirst, regulation of the pituitary gland and together with the limbic system and cerebral cortex also controls various emotional states.
The limbic system which includes the cingulate gyrus of the cerebral cortex, the hypothalamus, hippocampus and amygdaloidal nucleus is involved in basic emotions like anger, fear, sex and hunger. The involvement of the limbic system with the control of autonomic function is responsible for the visceral responses characteristic of these emotional states. Blushing, fainting, racing heart, cold sweats are examples of the many visceral reactions that accompany emotions as a result of autonomic activation.
The somatic nervous system is that part of the motor (efferent) division that supply skeletal muscles. The cell bodies of somatic motor neurons are located within the ventral horn of the spinal cord. Unlike the two neuron chain of the autonomic system, the axon of a somatic motor neuron is continuous from its origin in the spinal cord to its termination on skeletal muscles. Motor neuron axon terminals release acetylcholine which brings about excitation and contraction of the innervated muscles. The effect of motor neuron on skeletal muscles is only stimulation and never inhibition or both. Inhibition of skeletal muscle activity can only be accomplished within the CNS.
Somatic motor neurons are influenced by many converging presynaptic inputs both excitatory and inhibitory but the level of activity in a motor neuron and its subsequent output to the skeletal muscle fibres depend on the balance of EPSPs and IPSPs. The motor neuron is considered to be the final common pathway by which any other parts of the nervous system can influence skeletal muscle activity. The somatic nervous system is considered to be under voluntary control but much of skeletal muscle activity involving posture, balance and stereotypical movements are subconsciously controlled.