The autonomic nervous system

Diagram of arrangement of the ANS compared to the SNS. (on desktop)

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Recap

1. Sensory / Visual stimuli is received 2. By the afferent division of the PNS 3. Forwarded on to the CNS -> brain and spinal cord. 4. Output is generated by the CNS, and sent to the PNS. 4b. The efferent division of the PNS. 4c. From here, the response may be due to the Somatic / Autonomic nervous system. 5. IF somatic -> motor neurons -> skeletal muscles (effector organs) 6. IF autonomic -> can be sympathetic, parasympathetic or Enteric NS. 7a. If sympathetic -> Smooth muscle, cardiac muscle, exocrine glands, and some endocrine glands. (all effector organs) 7b. If parasympathetic Smooth muscle, cardiac muscle, exocrine glands, and some endocrine glands. (all effector organs) 7c. If enteric, the stimuli was received from the digestive tract, and the response only affects the digestive organs.

Central nervous control of autonomic activity

Afferent fibres in the internal organs respond to mechanical and sensory information. This information is relayed in the somatic and visceral afferent fibres to spinal cord. These fibres synapse to second order fibres at the segmental level, and information ascends the spinal cord. Information is processed in the thalamus and hypothalamus.

Autonomic nervous systems

Autonomic nerves pass first to autonomic ganglia located outside of the CNS before reaching their target organs. The fibres that project from the CNS to the autonomic ganglion are called Preganglionic fibres. The fibres that connect the ganglia to their target organs are called postganglionic fibres.

Arrangement of pre- and post-ganglionic neurons in the autonomic

Axons leave the spinal cord via the ventral root together with the somatic motor fibers. Dorsal and ventral roots join together and shortly after this point, the preganglion cells leave the spinal cord and travel to sympathetic ganglia. Here they synapse on post ganglionic sympathetic fibres that project to the target organs.

Objectives

Describe the organisation of the autonomic nervous system • Describe the action of the main neurotransmitters of the autonomic nervous system. • Outline the major roles of the sympathetic and parasympathetic divisions. • Explain central nervous control of autonomic activity.

Homeostatic regulation

Dual innervation Antagonistic action Basal tone

ParaSympathetic nervous system organ breakdown

Eye: Pupillary constriction and accomodation. Lacrimal gland: secretion of tears. Salivary glands: vasodilation, secretion of saliva. Heart: decreased heart rate. Blood vessels: mainly vasodilation in certain exocrine glands and external genitalia. Lungs: Bronchial constriction, secretion of mucus. Liver: None Gall bladder: secretion of bile. Adrenal medulla: no effect, no innervation. GI tract: increased motility and secretion, relaxationof sphincters. Kidneys: No effect Urinary bladder: initiation of micturition. Genitalia: erection Sweat glands: none, no innervation Hair follicles: none, no innervation Metabolism: no effect.

Sympathetic nervous system organ breakdown

Eye: Pupillary dilation Salivary glands: vasoconstriction, secretion of viscous fluid. Heart: increased heart rate and force of contraction. Blood vessels: mainly vasoconstriction but in skeletal muscles vasodilation. Lungs: Bronchial dilation via circulating adrenalin. Liver: Glycogenesis, glucogenesis and the release of glucose into the blood stream -> alpha cells -> Glucagon. Adrenal medulla: secretion of circulating adrenalin and noradrenalin GI tract: decreased motility and secretion, constriction of sphincters, vasoconstriction. Kidneys: Vasoconstriction and decreased urine output. Urinary bladder: inhibition of micturition. Genitalia: ejaculation. Sweat glands: secretion of sweat by eccrine glands. Hair follicles: piloerection. Metabolism: increase.

Summary of Parasympathetic

Function: Conserves energy and replenish energy stores. Maintains homeostatis, 'rest and digest' Location of Preganglionic Neuron cell bodies: Brainstem and lateral grey regions in S2-S4, regions of spinal cord. Location of ganglionic neuron cells bodies: Terminal or intramural ganglion. Divergence of axons: few (1 axon innervates <4 ganglionic cell bodies. Length of preganglionic Axon: Long Length of postganglionic axon: short Location of ganglia: terminal ganglia located close to the target organ: intramural ganglia in wall of target organ. Rami communications: none

Summary of Sympathetic nervous system

Function: prepares body to cope with emergencies and intensive muscle activity. (fight or flight) Location of preganglionic neuron cell bodies: lateral horns in T1-L2, regions of spinal cord (thoracolumbar). Location of Ganglionic neuron cell bodies: sympathetic trunk (paravertebral) ganglion or prevertebral ganglion. Divergence of axons: extensive (1 axon innervates >20 ganglionic cell bodies.) Length of preganglionic axon: short Length of postganglionic axon: long Location of ganglia: sympathetic trunk (paravertebral) ganglia located on either side of vertebral column; prevertebral (collateral) ganglia located anterior to vertebral column and descending aorta. Rami communications: White rami attach to T1-L2 spinal nerves; grey rami attach to all spinal nerves.

The autonomic nervous system

Is part of the PNS It is a motor system which regulates the activity of the internal organs (smooth muscle, cardiac muscle and glands) It is not directly affected by voluntary control, but operates automatically (autonomic reflexes and central control). Its major function is homeostatis (maintains internal environment under optimal conditions)

Diagram of effect of Neurotransmitter released at effector in Sympathetic and Parasympathetic

Main difference is only in secretion, as in Parasympathetic, the Postganglionic neuron (short in parasympathetic) - releases Acetylcholine, and the receptor is Muscarinic cholinergic receptor. In Sympathetic, the Preganglionic neuron is short, Postganglionic is long, and the neurotransmitter is norepinephrine, and the receptor: adrenergic receptor.

Chemical transmission

Main transmitters of the ANS are acetylcholine and noradrenaline. The principle transmitter is secreted by the preganglionic fibres within the ganglia, and is acetylcholine. The postganglionic fibres of the parasympathetic nervous system also secrete acetylcholine onto their target tissues. The postganglionic sympathetic fibres secrete noradrenalin (with the exception of those that innervate the sweat glands and errector pili muscles)

Adrenoreceptors

Metabotropic G protein-coupled receptors. There are to subtypes (alpha and beta). Alpha 1- adrenoceptor -> location: smooth muscle of blood vessels, bronchi, GIT, uterus, bladder -> causes smooth muscle contraction, except of the gut, which it relaxes Alpha 2 -> Inhibition of transmitter release. / Smooth muscle contraction occurring in blood vessels. (vasocons) Beta 1-adrenoceptors -> Heart muscle contraction, smooth muscle relaxation, glycogenesis. Beta 2- adrenoceptor ->Smooth muscle of certain blood vessels and bronchi. -> vasodilation and bronchodilation. Beta 3- adrenoceptor ->adipose tissue -> initiate lipolysis to release free fatty acids and glycerol.

Acetylcholine receptors

Nicotinic Acetylcholine receptors -Activated by the alkaloid nicotine. Activation leads to an opening of an ion channel -> ionotropic. Muscarinic Acetylcholine receptors -activated by the alkaloid muscarine. Activation leads to activation of G-protein. (Metabotropic) -> there are 3 types, M1, M2 and M3. M1 -> CNS and peripheral ganglia. Also in gastric parietal cells -> excitatory. M2 -> Heart and nerve terminals of both CNS and PNS -> inhibitory M3 -> secretory glands and smooth muscle -> excitatory

Central nervous control of autonomic activity 2

Other visceral afferents (e.g. arterial baroreceptors) reach the brainstem via the vagus nerve. There are visceral reflexes that can occur at the segmental level or involve processing in the brain. E.g. baroreceptor reflex. (heart and circulation.)

Parasympathetic

Outflow at cranial nerves (CN3, CN7, CN9,CN10) and the sacral portions of the spinal cord (S2-S4). More discrete in its actions. Promotes "restorative" functions. Rest and digest. Innervates the viscera of the body cavities via sequential pathway of preganglionic and postganglionic neurons.

Sympathetic

Outflow begins at the thoracic and lumbar portions of the spinal cord. T1-L2) Prepares the body for activity. Fight or flight. Innervates structures in the body wall and internal viscera via sequential pathway of preganglionic and postganglionic neurons.

Location of acetylcholine receptors

Postganglionic neurons -> Nicotinic receptors. Target tissues -> Muscarinic receptors.

Structure of fibres

The parasympathetic ganglia are usually located close to the target organ or even embedded in it. Preganglionic fibres are myelinated, and postganglionic fibres are unmyelinated. Sympathetic pathways usually consist of one short preganglionic, and one long postganglionic fibre. Parasympathetic pathways consist of one long preganglionic fiber, and one short postganglionic fiber.

Arrangement of pre- and post-ganglionic neurons in the somatic NS

The somatic nervous system has a single neuron running through from the spinal cord, and directly ending in and innervating the skeletal muscle.