Autonomic Nervous System Anatomy and Physiology

The Autonomic

Nervous System

• The Autonomic Nervous System(ANS) along with endocrine system coordinates the regulation amd integeration of body functions.

•The ANS largerly independent (autonomous) in that its activities are not under direct conscious control. It is concenred primarily with visceral functions such as cardiac output, blood flow to various organs and digestions, which is necessary for life.

• Drugs that produce their primary therpeutic effect by mimicking or altering the functions of ANS are called autonomic drugs.

- The autonomic agents act either by stimulating portion of ANS or by blocking the action of autonomic nerves.

Nervous System overview :

• The N.S is divided into 2 anatomical divisions : The Central Nervous System which is composed of brain & spinal cord and the Peripheral Nervous System.

• Peripheral nervous system includes neurons which located outside the brain and spinal cord.

• The peripheral nervous system is subdivided into afferent and efferent divisions. The efferent neurons carry signals away from brain and spinal cord to the peripheral tissues. And the afferent neurons brings information from periphery to the CNS.

• The efferent neurons of the peripheral nervous system is furthur divided into two major functional subdivisions : The Somatic system and the ANS.

• The somatic efferent neurons are involved in volutary control of functions such as contraction of skeletal muscles essential for locomotion. The ANS regulates the everyday requirements of the vital body functions without conscious participation of the mind.

=>• B/c of the involuntary nature of ANS as well as its functions, it is also known as VISCERAL, VEGETATIVE or INVOLUNTARY NERVOUS SYSTEM.

Anatomy Of Autonomic Nervous System :

(1) Efferent neurons :

• The ANS carries nerve impulses from the CNS to the effector organs through two types of efferent neurons: the preganglionic neurons and post-ganglionic neurons.

• The preganglionic neurons emerge from the brain stem or spinal cord and make a synaptic connection in ganglia. Ganglia is an aggregation of nerve cell bodies located in te peripheral nervous system.

• The cell bodies of postganglionic nuerons originates into ganglion. It is generally non-mylineated and terminates on effector organs, such as: visceral smooth muscles, cardiac muscles& the exocrine glands.

(2) Afferent neuron :

• The afferent neurons of ANS are important in reflex regulation of the system. For example : by sensing the pressure in the carotid sinus and aortic arch.

• It also send signals to CNS to influence the efferent branch of system to respond.

(3) Sympathetic neurons :

• The efferent ANS is divided into autonomic system which contain sympathetic , parasympathetic and enteric nervous systems.

• Anatomically sympathetic and parasympathetic neurons originates in the CNS and emerge from two different spinal cord regions.

• The preganglionic neurons of sympathetic nervous system comes from thoracic and lumber regions(T1 to L2) of spinal cord.

• In sympathetic ANS preganglionic neurons are short in comparison to postganglionic ones.

(4) Parasympathetic neurons :

• The parasympathetic preganglionic fibres arise from cranial nerves 3(occulomotor), 7(facial), 9(galssopharyngeal), 10(vagus) as well as from sacral region (S2 to S4) of spinal cord and synapse with ganglia near or on the effector organs.

• In most instance there is one to one connection between the preganglionic and postganglionic neurons, enabling the discrete response of this system.

• NOTE : Vagus nerve accounts for 90% of the preganglionic parasympathetic fibres.

• In parasympathetic N.S, preganglionic fibres are long in comparison to postganglionic fibres.

(5) Enteric neurons:

• It is the collection of nerve fibres that innervate the gastronintestinal tract, pancrease, and gall bladder. And it constitues the “Brain of the Gut”.

• This sytem is independent of CNS and controls motility, exocrine amd endocrine secerations and microcirculation of the GI tract.

• It is modulated by the both Sympathetic and Parasympathetic nervous system.

Physiology of Autonomic nervous system :

(1) Functions of sympathetic Nervous system :

• Sympathetic division is responsible for adjusting in response to stressful situations, such as: trauma, fear, hypoglycemia, cold and excercise.

(i) Effects of stimulation on Sympathetic division:

• The effect of sympathetic stimulation is an :

(a) increase in heart rate and blood pressure

(b) mobilization of energy stores

(c) increase blood flow to skeletal muscles & heart

(d) Diverting blood flow from skin and internal organs.

(e) Dilation of pupil and branchioles

(f) reduce GI motility

(g) also effects the functions of bladder & sexual organs

(ii) Fight or Flight response :

• The changes experienced by the body during the emergencies are referred to as fight or flight response.

• These reactions are triggered both by direct sympathetic activation of effector organs and by stimulation of adrenal medulla to release epinephrine and norepinephrine.

• Hormones released by adrenal medulla directly enters the bloodstream and promotes response in effector organs that contain adrenergic receptors.

(2) Functions of Parasympathetic Nervous System :

• The parasympathetic division is involved in maintaining the homeostasis within the body. It is required for life since it maintains essential body finctions.

• The prasympathetic divisions usually acts to oppose or balance the actions of sympathetic division and generally predominates the sympathetic system in “rest and digest” situations.

• Parasympathetic fibres innervating specific organs such as Gut, heart or eye are activated seperately and the system affects these organs individually.

Role Of CNS in the control of ANS :

• Although the ANS is a motor system , it does not require sensory input from peripheral structures to provide information on current state of body.

• This feedback is provided by streams of afferent impulses, originating in the viscera and autonomically innervate the structures that travel to the CNS such as : hypothalamus, medulla oblangata amd spinal cord.

• These centres respond to stimuli by sending out efferent reflex impulse via the ANS.

• Stimuli that evoles the strong feelings , such as rage, fear and pleasure, can modify the activities of the ANS.

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