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How does the autonomic nervous system affect the musculature of the body? Is it all just acetylcholine and noradrenaline?
The autonomic nervous system is part of the peripheral nervous system which controls the activities of the body that are not under conscious control. The major processes that it controls are, contraction and relaxation of vascular and visceral smooth muscle, the heart beat, exocrine and endocrine secretions and energy metabolism. There are two main divisions of the autonomic nervous system and these are the sympathetic nervous system and the parasympathetic nervous system. Both these divisions have two major efferent pathways which originate in the central nervous system and they innervate the target tissue by a two synapse pathway. The first pathway involves preganglionic neurons whose cell bodies are located in the central nervous system making a synapse with the postganglionic neurons in the peripheral ganglia. The second pathway involves the axons from the postganglionic neurons making synapses with the target tissues which then lead to their innervations. The sympathetic and parasympathetic nervous systems control visceral activity by acting independently to each other; under stress, anxiety, physical activity the output from sympathetic division increases whereas under conditions of sedentary activity or eating the output from the parasympathetic division increases. In this essay the following things about the autonomic nervous system will be considered; its organization, how it affects the musculature of the body (smooth and cardiac muscle) and what neurotransmitters are released. Organisation of the sympathetic nervous system The cell bodies of the preganglionic sympathetic motor neurons lie in the thoracic and upper lumbar spinal cord which are the spinal segments from T1 to L3. The axons of these neurons lie in the intermediolateral cell column which lies between the dorsal and ventral horn and they exit via the ventral roots along with the axons of the somatic motor neurons. The two types of neurons (sympathetic and somatic) then diverge as the preganglionic neurons enter the white rami communicantes (its white as the axons of the preganglionic neurons are myelinated) which then lead to the axons entering the nearest sympathetic paraverterbal ganglion. The paravertebral ganglia is a column that consists of a chain of sympathetic ganglia that extends from extends from the upper part of the neck to the coccyx and lies adjacent to the spinal cord. After entering the paravertebral ganglia the preganglionic sympathetic motor neurons has an option of three pathways: it can synapse with a postganglionic sympathetic motor neuron within that segmental paravertebral ganglion, travel up or down the paravertebral ganglia and synapse within a neighbouring paravertebral ganglion and the final option is that it can enter the greater of lesser splanchinic nerve and synapse within one of the ganglia of the prevertebral plexus. The cell bodies of post ganglionic sympathetic motor neurons can be found in the paravertebral or prevertebral ganglia and their axons go through the grey rami comunicans (grey as postganglionic axons are unmyelinated) which rejoin the mixed spinal nerves. In the case of sympathetic neurons the
postganglionic axons are relatively long. Each preganglionic sympathetic motor neuron can synapse with as many as 200 postganglionic neurons that are either located in the nearby paravertebral or prevertebral ganglia. Therefore sympathetic output can have widespread effects.
Organisation of the parasympathetic nervous system Unlike in the sympathetic nervous system where the cell bodies are found in the thoracic and lumbar spinal cord, the cell bodies of the parasympathetic are found in the cranial and sacral regions. In the cranial region the cell bodies are found in the medulla, pons and midbraine. These ganglionic neurons form four cranial nerves; oculomotor nerve, facial nerve, glossopharyngeal nerve and vagus nerve. In the sacral region, the cell bodies are found in segments S2 to S4 of the spinal cord and these ganglia distribute to form pelvic splanchnic nerves. Like the sympathetic nervous system, the preganglionic parsympathetic motor neurons make synaptic contact with the postganglionic neurons but these are found in the terminal ganglia, which are located within the wall of the target organs. In comparison to the sympathetic ganglia the terminal ganglia are more peripherally located and are more widely distributed. Another difference is that the postganglionic neurons of the parasympathetic division are short in comparison the sympathetic nervous system. Effects of the autonomic nervous system on visceral targets The innervation of skeletal muscle by the somatic nervous system is always excitatory. Whereas for visceral targets that are innervated by the autonomic nervous system the response can be excitatory or inhibitory. This is because both the sympathetic and parasympathetic nervous system make individual synaptic contacts with the target. For organs that are stimulated during physical activity such as the heart, innervations by the sympathetic division triggers an excitatory response which results in the heart rate increasing whereas, if the parasympathetic division innervated the heart rate it would lead to the heart rate slowing down. However the opposite is true for organs whose activity increases while the body is at rest. For example the parasympathetic division triggers the peristalsis of the gut whereas the sympathetic division inhibits it. During periods of fear and exercise the sympathetic division innervates all the end organs simultaneously and leads to the inhibition of the parasympathetic nervous system. This occurs as it allows the body to prepare for life threatening situations by making it more alert and this response is known as the 'fight or flight response.' This response involves the following things: increase in heart rate, cardiac contractility, blood pressure, ventilation of the lungs, and liberation of glucose into blood. This mass response is essential for survival but it can also be triggered spontaneously during panic attacks.
The parasympathetic nervous system is switched on during periods of rest and unlike the sympathetic nervous system it innervates discrete organs. This allows it to control simple reflexes such as urination in response to bladder distension, salivation in response to sight or smell and contraction of the colon in response to food in the stomach. Both the cardiac and smooth muscle are target tissues that are innervated by the autonomic system. Before discussing how they are innvervated their structure and function will be considered.
Structure and function of the cardiac muscle The cardiac muscle is an involuntary striated muscle which is composed of individual uninucleate cells that are connected to each other by intercalated discs. These discs are made up of three different cell to cell junctions; two of these junctions, adherens junctions (link actin filaments in neighbouring cells) and desmosomes (link intermediate filaments in neighbouring cells) are involved in mechanical coupling whilst the third known as gap junctions are involved in electrochemical coupling. These junctions allow cells to be united as a mechanical and electrical synctium which enables for a rapid spread of action potentials and also allows for synchronised contractions which pumps blood out of the atria and ventricles. Like skeletal muscle, cardiac muscle is also made up of two main structural proteins; actin and filament. However a key difference between the two muscles is that for cardiac muscle, contraction is not initiated by neurons, instead it is caused by the sinoatrial node which is responsible for generating spontaneous and periodic contractions. The sinoatrial node is a group of specialised myocytes that generates action potentials and generates the heart's inherent myogenic rhythm. The main reason why the sinoatrial node creates spontaneous action potentials is because it generates a pacemaker current when the myocytes in the sinoatrial node are undergoing repolarisation. The pacemaker current is formed by inward currents and outward currents. The inward currents are caused by an influx of calcium ions through calcium channels and an influx of sodium ions through the HCN channels, sodium ion leak channels and sodium calcium exchange protein. Whereas the outward current is created by potassium ions. As the inward current is greater than the outward current there is a net inward current and this causes a slow positive increase in the membrane potential of the myocyte from its resting membrane potential to a threshold value which then triggers the opening of voltage gated calcium channels and leads to the formation of the next action potential. The slope generated by the pacemaker current determines the rate of approach to the threshold and controls the overall sinus rhythm. Once the action potential is formed
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