Nervous system

Nervous system   

A. Central Nervous System(CNS)                     b. Peripheral Nervous System(PNS) 

Brain and spinal cord                                            Outside the brain and spinal cord

 

1-Sympathetic	2- Parasympathetic	3- Enteric neurons
(Thoraco-Lumbar outflow) *Preganglionic neurons arise from thoracic and lumbar regions (L1 to L2) of the spinal cord.	(Cranio-Sacral outflow) Preganglionic neurons arise from cranial nerves 24 3rd (Oculomotor) 7th (Facial) 9th (Glossopharyngeal) 10th (Vagus) and from 2nd 3rd and 4th sacral segments.	It is a collection of nerve fibers that innervate the GIT, pancreas, and gallbladder, and it constitutes the "brain of the gut". This system functions independently of the CNS and control the motility, exocrine and endocrine secretions. - It is modulated by both the sympathetic and parasympathetic nervous system.
- Consist of short preganglionic fibers and long postganglionic fibers.	- Consist of long preganglionic fibers and short postganglionic fibers.
Chemical transmitters  - Adrenaline & Nor-adrenaline.  Except: Thermoregulatory (Eccrine) sweat gland & adrenal medulla they release ACh.	Chemical transmitters   - ACh (Acetylcholine).

Most organs in the body are innervated by both divisions of the ANS

(dual innervation)

-Some organs are supplied with one division of ANS:

1.Iris sphincter (pupillary constrictor) muscle (Circular)à Supplied by                       parasympathetic (M3).  

2.Iris dilator (pupillary dilator) muscle (Radial) à Supplied by sympathetic (α1).

3.Arrector pili muscle (pilomotor muscle) Supplied by sympathetic (α 1)àhair erection. The contraction of the muscle is therefore involuntary-stresses  e.g. cold, fear (Goosebumps).

4.Thermoregulatory (Eccrine) sweat gland àSupplied only by sympathetic fibers, but through cholinergic (parasympathetic) receptors (M3).

5.Blood Vessels Vascular smooth muscle is innervated primarily by the sympathetic and indirectly non-innervated by the parasympathetic. 

I.e. Direct acting à innervated sympathetic à α1 receptor (Vasoconstriction).

     Indirect acting Non-innervated parasympathetic M3 receptors (Vasodilatation) via the nitric oxide (NO)/endothelium derived relaxing factor (no direct innervation).

Role of the CNS in the control of autonomic function:

 ANS is provided by streams of afferent impulses, originating in the viscera and other autonomically innervated structures that travel to integrating centers in the CNS, such as the hypothalamus, medulla oblongata and spinal cord. These centers respond to the stimuli by sending out efferent reflex impulses via the ANS.

Reflex arc: A reflex arc is a neural pathway that controls an action reflex. Most of the afferent impulses are involuntarily translated into reflex responses. For example a fall in blood pressure causes pressure- sensitive neurons (baroreceptors in the heart, vena cava, aortic arch, and carotid sinuses) to send fewer impulses to cardiovascular centers in the brain. This prompts a reflex response of increased sympathetic output to the heart and vasculature and decreased parasympathetic output to the heart, which results in a compensatory rise in blood pressure and tachycardia. 

 

 

Chemical transmitters in the ANS:

1.Chemical transmitter for parasympathetic is Acetylcholine (ACh). 

2.Chemical transmitter for sympathetic is mainly Adrenaline (Epinephrine) and Noradrenaline (Nor-epinephrine). 

3.Other transmitters involved: called Non-Adrenergie Non-Cholinergic (NANC) transmitters. Some of the NANC found in peripheral nervous system:

-Nitric Oxide (NO)                                                             -ATP                                 

-Serotonin (5-HT)                                                              -Dopamine                                -Neuropeptide Y (NPY)                                                     -Vasoactive Intestinal Peptide (VIP)

-Gamma-amino-butyric Acid.                   

-Calcitonin gene-related peptide (CGRP) and Substance P. 

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