Autonomic Nervous System

Autonomic nervous system organization

u Sympathetic Nervous System u division of the autonomic nervous system that arouses the body, mobilizing its energy in stressful situations

u Parasympathetic Nervous System u division of the autonomic nervous system that calms the body, conserving its energy Autonomic nervous system organization

u SOMATIC u AUTONOMIC u Voluntary u Involuntary u Skeletal muscle u Smooth , cardiac muscle ; glands • u Single efferent (motor) neuron Multiple efferent neurons u u Axon terminals release Axon terminals release acetylcholine acetylcholine or norepinephrine u u Always excitatory Can be excitatory or inhibitory u u Controlled by the cerebrum Controlled by the homeostatic centers in the brain – pons, hypothalamus, medulla oblongata u Both ANS divisions share the same general structure. u Autonomic pathways always consist of 2 neurons in series. u They synapse in an autonomic ganglion u The 1st neuron in the autonomic pathway is the preganglionic neuron, u cell body in CNS u myelinated u projects to the autonomic ganglion u The 2nd neuron is the postganglionic neuron u Cell body in autonomic ganglion u unmyelinated u projects to the effector Comparison of autonomic and somatic motor system Neurotransmitters and receptors of autonomic nervous system Paulsen and Waschke: Atlas of Human Anatomy , 15 th edition, ElsevierGmb 2015 Preganglionic neurons u Preganglionic neurons u Thoracolumbar part The sympathetic part of the ANS is also called a thoracolumbar part u the somas of the preganglionic neuros are located in uthe lateral horn of the twelve thoracic segments, uthe first two or three lumbar segments of the spinal cord. Preganglionic neurons u Craniosacral part u the parasympathetic part of the ANS is referred to u a craniosacral part because the somas of the parasympathetic preganglionic neurons are located in u the nuclei of four cranial nerves in the area of the brain stem u in the lateral horns of the second to fourth sacral segment of the spinal cord. Cranial nervs u The PNS includes 12 pairs of cranial nerves u Cranial nerves carry both voluntary and involuntary impulses u III Oculomotor Nerve: • eye movements, • pupillary constriction and accommodation , • eyelid muscles u VII Facial Nerve: u controls the muscles of facial expression, u taste from anterior tongue, u lacrimal (tears) and salivary glands Cranial nervs u IX Glossopharyngeal Nerve: u sensation from pharynx, u taste from posterior tongue, u carotid baroceptors u X Vagus Nerve : u increases secretion of HCL (stomach) u stimulates release of bile and pancreas excretion u stimulates peristalsis of intestines u slows heart rate u sensation from pharynx, u muscles of vocal cords, swallowing Paulsen and Waschke:Atas of Human Anatomy , 15 th edition, ElsevierGmb 2015

Autonomic ganglia u Autonomic ganglia are divided into three groups: uSympathetic chain ganglia uSympathetic prevertebral ganglia uParasympathetic ganglia u Sympathetic ganglia are located where the synapses between preganglionic and postganglionic sympathetic neurons interact. Parasympathetic Ganglia u Preganglionic axons of the parasympathetic nervous system u form synapses with postganglionic neurons u in terminal or intramural ganglia . u For the most part, u the ganglia are located near to or inside the wall of an organ . u Axons of preganglionic parasympathetic neurons are u usually longer than most of the axons of preganglionic sympathetic neurons since they reach from the CNS u to an intramural ganglion of the innervated organ. Paulsen and Waschke:Atlas of Human Anatomy , 15 th edition, ElsevierGmb 2015 Autonomic plexuses u Axons, sympathetic and parasympathetic neurons form networks that are called autonomic plexuses . u They run along large arteries and can be found in u thorax, u abdomen u pelvis . u The large cardiac plexus in the thorax is in charge of the innervation of the heart and the pulmonary plexus for the bronchial tree. u The largest autonomic plexus is the coeliac (solar) plexus , which passes on to the liver, gall bladder, stomach, pancreas, spleen, kidneys, adrenal cortex, testicles and ovaries. Neurotransmitter and Receptors of the VNS u Neurotransmitters are assigned to receptors – integral membrane proteins that are located in the plasma membrane of the postsynaptic neuron or a cell of the target tissue.

u There are cholinergic and adrenergic neurons .

u Cholinergic neurons and receptors u The ANS includes the following cholinergic neurons: u All sympathetic and parasympathetic preganglionic neurons u Sympathetic postganglionic neurons for most of the sweat glands u All postganglionic parasympathetic neurons Cholinergic neurons u release the neurotransmitter acetylcholine (ACh) u stored in synaptic vesicles and liberated by exocytosis u it diffuses through the synaptic cleft and binds to specific cholinergic receptors . u Cholinergic receptors : unicotinergic umuscarinergic receptors Nicotinergic receptors

u Nicotinergic receptors u are embedded in sympathetic and parasympathetic u postganglionic neurons, as well as in the neuromuscular junction. u nicotine simulates the action of ACh after binding to the receptors. u in the plasma membrane of all target tissues u (smooth muscle , myocardium and glands ), Ach is able to activate both types of cholinergic receptors nicotine is not able to activate muscarinergic receptors muscarine is not able to activate nicotinergic receptors. u Acetylcholine is quickly deactivated by the enzyme acetylcholine esterase Adrenergic neurons and receptors u Noradrenaline (NAdr) u is released in the ANS by adrenergic neurons . u A great number of postganglionic sympathetic neurons are adrenergic. The NA is stored in synaptic vesicles, u released by exocytosis, which diffuses through the synaptic cleft u binds to specific adrenergic receptors of the postsynaptic membrane . u The consequence is an excitation or an inhibition of the effector cell. u Noradrenaline as well as adrenaline binds to adrenergic receptors. u The NAdr can be released as u a neurotransmitter by sympathetic postganglionic neurons u a hormone by the adrenal medulla, into the blood. u Adrenaline is solely released as a hormone. the adrenergic receptors u the adrenergic receptors u can be subdivided into two subtypes u which are innervated by most of the postganglionic sympathetic neurons u alpha (α)-receptors u beta (β) -receptors u which are further subdivided u according to their specific answers u and the corresponding binding properties (α1, α2, β1, β2, etc.). u an activation of : u α1- and β1-receptors induces an excitation u α2- and β2-receptors yield an inhibition of the target tissue. Autonomic reflexes u Answers that are triggered by nerve impulses in an autonomic reflex arc are called autonomic reflexes . u The play a key role in the following processes: uBlood pressure (i.e. by adjusting the heart rate) uDigestion (adjustment of motility and muscle tone in the GI tract) uDefecation uUrination (regulating the opening and closing of the sphincter) An autonomic reflex arc u An autonomic reflex arc consists of the following components: u Receptor u The distal end of a sensory neuron is the receptor of an autonomic reflex arc , which reacts to a u Stimulus and triggers a nerve impulse. u Usually, autonomic sensory receptors are associated with interoceptors . u Sensory neurons u The sensory neuron forwards nerve impulses to the CNS. Integration centre u Integration centre u The main integration centres for autonomic reflexes are located in u the hypothalamus u the brain stem. u the spinal cord, which are mostly responsible for urination and defecation u interneurons of the CNS forward signals from the sensory neurons to motor neurons . Motor neurons u Motor neurons u Signals triggered by integration centres leave the CNS, u via motor neurons, towards the target tissue. u Two motor neurons connect the CNS in an autonomic reflex arc with the effector . u The impulse is transduced by u the preganglionic neuron to u an autonomic ganglion from where it is forwarded through the postganglionic neuron to u the target tissue. Target tissue (effector)

u The effectors of the autonomic reflex are u the smooth muscles, u the heart muscle u the glands. The hypothalamus u The main control and integration center of the ANS is the hypothalamus, u which receives sensory information u about visceral functions u Signals of the limbic system, which cohere with emotions u The signals coming from the hypothalamus affect u autonomous centres in u the brain stem u the spinal cord