Autonomic nervous system

3:58 AM | Labels: autonomic nervous system

I always like nervous system. It's kind of complicated, relating to all parts of the body, need gallons of memorizing and understanding yet mystery, still I'd love to learn them all. Maybe I should consider to a vet neurologist, don't I? hehe

Note: Before you read the below post, make sure to have a little knowledge regarding nervous system first. Nervous system is quite confusing though.

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Autonomic nervous system

The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions.The ANS affects heart rate, digestion,respiration rate, salivation, perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind.

Division of nervous system

The ANS is predominantly an efferent system transmitting impulses from the Central Nervous System (CNS) to peripheral organ systems. Its effects include control of heart rate and force of contraction, constriction and dilatation of blood vessels, contraction and relaxation of smooth muscle in various organs, visual accommodation, pupillary size and secretions from exocrine and endocrine glands. Autonomic nerves constitute all of the efferent fibres which leave the CNS, except for those which innervate skeletal muscle.

There are some afferent autonomic fibres (i.e. transmit information from the periphery to the CNS) which are concerned with the mediation of visceral sensation and the regulation of vasomotor and respiratory reflexes, for example the baroreceptors and chemoreceptors in the carotid sinus and aortic arch which are important in the control of heart rate, blood pressure and respiratory activity. These afferent fibres are usually carried to the CNS by major autonomic nerves such as the vagus, splanchnic or pelvic nerves, although afferent pain fibres from blood vessels may be carried by somatic nerves.

The ANS is primarily involved in reflex arcs, involving an autonomic or somatic afferent limb, and then autonomic and somatic efferent limbs. For instance, afferent fibres may convey stimuli from pain receptors, or mechanoreceptors and chemoreceptors in the heart, lungs, gastrointestinal tract etc.

There may then be a reflex response to this involving autonomic efferent fibres causing contraction of smooth muscle in certain organs (e.g. blood vessels, eyes, lungs, bladder, gastrointestinal tract) and influencing the function of the heart and glands. The efferent limbs of these reflexes may also involve the somatic nervous system (e.g. coughing and vomiting). Simple reflexes are completed entirely within the organ concerned, whereas more complex reflexes are controlled by the higher autonomic centres in the CNS, principally the hypothalamus.

The ANS is divided into two separate divisions called the Parasympathetic and Sympathetic Systems, on the basis of anatomical and functional differences. Both of these systems consist of myelinated preganglionic fibres which make synaptic connections with unmyelinated postganglionic fibres, and it is these which then innervate the effector organ. These synapses usually occur in clusters called ganglia. Most organs are innervated by fibres from both divisions of the ANS, and the influence is usually opposing (e.g.the vagus slows the heart, whilst the sympathetic nerves increase its rate and contractility), although it may be parallel (e.g. the salivary glands). The responses of major effector organs to autonomic nerve impulses are summarised in table below;

Summary of ANS

Sympathetic Nervous System

The cell bodies of the sympathetic preganglionic fibres are in the lateral horns of the spinal segments T1-L2, the so called thoraco-lumbar outflow. The preganglionic fibres travel a short distance in the mixed spinal nerve, and then branch off as white rami (myelinated) to enter the sympathetic ganglia. These are mainly arranged in two paravertebral chains which lie anterolateral to the vertebral bodies and extend from the cervical to the sacral region. They are called the sympathetic ganglionic chains. The short preganglionic fibres which enter the chain make a synapse with a postsynaptic fibre either at the same dermatomal level, or at a higher or lower level, and then the longer postganglionic fibres usually return to the adjacent spinal nerve via grey rami (unmyelinated) and are conveyed to the effector organ.

The sympathetic system enables the body to be prepared for fear, flight or fight. Sympathetic responses include an increase in heart rate, blood pressure and cardiac output, a diversion of blood flow from the skin and splanchnic vessels to those supplying skeletal muscle, increased pupil size, bronchiolar dilation, contraction of sphincters and metabolic changes such as the mobilisation of fat and glycogen.

Some preganglianic fibres do not synapse in the sympathetic chains but terminate in separate cervical or abdominal ganglia, or travel in the greater splanchnic nerve and directly synapse with chromaffin cells in the adrenal medulla. As discussed above, Ach is the neurotransmitter via a nicotinic receptor at the preganglionic synapse. The adrenal medulla is innervated by preganglionic fibres and therefore adrenaline is released from the gland by stimulation of nicotinic Ach receptors.

At most postganglionic sympathetic endings, the chemical transmitter is noradrenaline, which is present in the presynaptic terminal as well as in the adrenal medulla. In sweat glands, however, postganglionic sympathetic fibres release Ach and this transmission is nicotinic.

Parasympathetic nervous system

The preganglionic outflow of the parasympathetic nervous system arises from the cell bodies of the motor nuclei of the cranial nerves III, VII, IX and X in the brain stem and from the second, third and fourth sacral segments of the spinal cord. It is therefore also known as the cranio-sacral outflow.

Preganglionic fibres run almost to the organ which is innervated, and synapse in ganglia close to or within that organ, giving rise to postganglionic fibres which then innervate the relevant tissue. The ganglion cells may be either well organised (e.g. myenteric plexus of the intestine) or diffuse (e.g. bladder, blood vessels).The cranial nerves III, VII and IX affect the pupil and salivary gland secretion, whilst the vagus nerve (X) carries fibres to the heart, lungs, stomach, upper intestine and ureter.

The sacral fibres form pelvic plexuses which innervate the distal colon, rectum, bladder and reproductive organs.In physiological terms, the parasympathetic system is concerned with conservation and restoration of energy, as it causes a reduction in heart rate and blood pressure, and facilitates digestion and absorption of nutrients, and consequently the excretion of waste products.

The chemical transmitter at both pre and postganglionic synapses in the parasympathetic system is Acetylcholine (Ach). Ach is also the neurotransmitter at sympathetic preganglionic synapses, some sympathetic postganglionic synapses, the neuromuscular junction (somatic nervous system), and at some sites in the CNS. Nerve fibres that release Ach from their endings are described as cholinergic fibres.

The synthesis of Ach occurs in the cytoplasm of nerve endings and is stored in vesicles in the presynaptic terminal. The arrival of a presynaptic action potential causes an influx of calcium ions and the release of the contents of several hundred vesicles into the synaptic cleft. The Ach then binds to specific receptors on the postsynaptic membrane and increases the membrane permeability to sodium, potassium and calcium ions, which results in an excitatory post-synaptic potential. The action of Ach is terminated by hydrolysis with the enzyme Acetyl Cholinesterase.