Lecture The Autonomic Nervous System

The Autonomic Reflex:It is a system that designate both Visceral and Somatic neurons that serve as afferent pathways, making the Autonomic Reflex.

Review:Components of the Autonomic Reflex

Sensory inflow of neurons from the viscera (afferent pathway)

Higher autonomic centres in the cerebral cortex, thalamus, hypothalamus and the medulla of the brain

Lower autonomic centres in the brainstem and spinal cord.

Motor outflow of 2 neurons chain made of preganglionic and postganglionic parts ending in effectors organs (Efferent pathways).

Autonomic Centres

Those centres are in the brain and spinal cord receive sensory inflow from the viscera.

The sensory neurons convey information about events in the viscera (visceral afferent), which are less known with respect to their exact pathway.

Both visceral and somatic afferents serve as afferent pathways for autonomic reflexes.

The effects of sympathetic stimulation:Stress state

The sympathetic is activated during conditions of stress and emergencies: e.g. fear, anxiety, hypoglycemia etc.

These conditions need to be prepared for “fight or flight”.

The effect of sympathetic is to promote mechanisms which increase energy metabolism and the efficiency of body supply systems ( e.g. circulatory and respiratory systems).

The two organs representing the sympathetic effects are:

The eye:

The effects of the sympathetic on the eyeball and vision is seen in the eye structures:

► Iris dilation of the pupil.

► Ciliary muscle relaxation (far vision).

►Lacremal glands none

The heart

The effects of the sympathetic on the heart muscle are:

► S.A. node: Increased Heart Rate (HR)

► Myocardium: Increased strength of contraction.

►Coronary vessels: dilation.

►Blood vessels to skeletal muscles: dilation.

►Skin: Vasoconstriction and sweating

Effects of parasympathetic:Relaxing state

The physiological effects of stimulating parasympathetic branch are related to activities of relaxing actions on the vegetative functions of the body (e.g. feeding, resting and sexual stimulation):

The responses of the digestive system to food intake are due to the parasympathetic, among these:-

Increased secretions of the salivary glands, the stomach, the pancreas, intestines and increased motility and relaxation of sphincters to allow digestion and flow of digesta.

The eye

The effects of the parasympathetic on the eyeball and vision is seen in the eye structures:

Iris constriction. Ciliary muscle contraction. Lacrimal glands Lacrimation.

The heart

The effects of the parasympathetic on the heart muscle are:

► S.A. node decreased HR.

►Myocardium no effect?

►Coronary vessels dilation

Depending on innervation type:SympatheticParasympathetic

Vegetative functions:Organ symp. Parasymp.Salivary ↓ ↑Pancreas ↓ ↑ Secretion &Stomach ↓ ↑ MotilityIntestine ↓ ↑Sphincters constriction relaxationUreters relaxation constrictionBladder contraction relaxationInternal sphincter: Ejaculation ErectionMale organs: secretion of

adrenaline & noneAdrenal medulla: noradrenaline -

Chemical transmitters

The effect of sympathetic and parasympathetic stimulation are produced by release of chemical substances or transmitters which serve two functions:

(1) transmit the nerve impulses (2) combine with receptors in target cells so as to

produce the desired physiological effect. In all ganglia, the transmitter substance is Ach,

therefore all the preganglionic fibres release acetylcholine at the synapses between the pre-and postganglionic neurons.

The adrenal medulla

It is considered as ganglia-homologous with a sympathetic system. The fibres reaching the medulla come from preganglionic neurons and therefore release acetylcholine which synapse on interneurons that secrete dopamine as an inhibitory transmitter.

The function of these dopamine interneurons is to lessen (damp) or inhibit transmission at the ganglia and therefore to limit preganglionic stimulation.

Acetylcholine & Noradrenaline

They are the main transmitters released by the postganglionic fibres.

All parasympathetic postganglionic fibres release acetylcholine (Ach).

Acetylcholine receptors are either nicotinic or muscarinicBESIDES Some postganglionic cholinergic neurons also release

Vasoactive Intestinal Peptide (VIP). Most of the sympathetic postganglionic fibres release

noradrenaline e.g. sweat glands & blood vessels in skeletal muscles are exceptions and release Ach.

In addition:-

NANC

There are some which are Non-adrenergic, Non-cholinergic (NANC) found in the intrinsic nerves of the gut, regulate GIT motility and release of gut hormones:

These are:DopamineEncephalinGABAGastrin -Releasing-Peptide (GRP)Nitric Oxide (NO)Vasoactive Intestinal Peptide (VIP)

Adrenergic receptors

They are either Alpha 1&2-stimulating or Beta 1&2-inhibitory.

Affect vagal tone to the heart rate. Certain drugs have been found to block or inhibit

some of the effects of alpha or beta receptor stimulation.

On the basis of this; such drugs has been classified as alpha-blockers & beta blockers. The organs affected are classified accordingly, e.g. Phenoxybenzamine (Beta receptor blocker), while Dihydroergotamine is B1 and B2 blocker.

Higher control of the ANS

As ANS is a reflexes which their centres in the spinal cord or cranial nerve nuclei in the brainstem, there are several areas of the brain which are concerned with autonomic regulation, e.g. hypothalamus regulate sympathetic functions of the blood pressure and heart rate.

The limbic system (responsible for instinctive behavior and emotions) as it is situated closely to the hypothalamus (responsible of vegetative or visceral functions) and are related to each other. The nuclei of the hypothalamus receive information from various parts of the CNS and gives outputs to the cerebrum , the brainstem and to the anterior thalamus and the limbic cortex in the cerebrum.