PNS Physiology PDF

Cranial nerves & Periphral Nervous system Introduction Neurons in the PNS transmit signals between the CNS and receptors and effectors in all other parts of the body. The axons are grouped into bundles called nerves. The PNS has 43 pairs of nerves: 12 pairs of cranial nerves and 31...

Cranial nerves & Periphral Nervous system Introduction Neurons in the PNS transmit signals between the CNS and receptors and effectors in all other parts of the body. The axons are grouped into bundles called nerves. The PNS has 43 pairs of nerves: 12 pairs of cranial nerves and 31 pairs of spinal nerves that connect with the spinal cord. The 12 pairs of cranial nerves transmit motor or sensory messages, or both, primarily between the brain or brain stem and the head and neck. All cranial nerves, except for the olfactory and optic nerves, originate from the midbrain, pons, or medulla oblongata.  Cranial nerves 1. Olfactory (I) — Sensory: smell 2. Optic (II) — Sensory: vision 3. Oculomotor (III) — Motor: extraocular eye movement (superior, medial, and inferior lateral), pupillary constriction, and upper eyelid elevation 4. Trochlear (IV) — Motor: extraocular eye movement (inferior medial) 5. Trigeminal (V) — (BOTH) Sensory: transmitting stimuli from face and head, corneal reflex. Motor: chewing, biting, and lateral jaw movements 6. Abducent (VI) — Motor: extraocular eye movement (lateral) Cranial nerves 7. Facial (VII) — (Both) Sensory: taste receptors (anterior two-thirds of tongue); Motor: Facial muscle movement, including muscles of expression (those in the forehead and around the eyes and mouth) 8. Vestibulocochlear (VIII) — Sensory: hearing, sense of balance 9. Glossopharyngeal (IX) — BOTH: swallowing movements; Sensory: sensations of throat; taste receptors (posterior one-third of tongue) 10. Vagus (X). (BOTH) Motor — movement of palate, swallowing, gag reflex; activity of the thoracic and abdominal viscera, such as heart rate and peristalsis. Sensory: sensations of throat, larynx, and thoracic and abdominal viscera (heart, lungs, bronchi, and GI tract) 11. Accessory (XI) — Motor: shoulder movement, head rotation 12. Hypoglossal (CN XII) — Motor: tongue movement. Somatic nervous system The somatic portion of the efferent division of the PNS is made up of all the nerve fibers going from the CNS to skeletal muscle cells. Their large diameter, myelinated axons leave the CNS and pass without any synapses to skeletal muscle cells. The neurotransmitter these neurons release is acetylcholine. Because activity in the somatic neurons leads to contraction of the innervated skeletal muscle cells, these neurons are called motor neurons. Excitation of motor neurons leads only to the contraction of skeletal muscle cells; there are no somatic neurons that inhibit skeletal muscles. Muscle relaxation involves the inhibition of the motor neurons in the spinal cord. Autonomic Nervous System The autonomic nervous system (ANS) innervates all internal organs. Sometimes known as the visceral efferent nerves, autonomic nerves carry messages to the viscera from the brain stem and neuroendocrine system. In contrast to the somatic nervous system, the autonomic nervous system is made up of two neurons in series that connect the CNS and the effector cells. The first neuron has its cell body in the CNS. The synapse between the two neurons is outside the CNS in a cell cluster called an autonomic ganglion. The neurons passing between the CNS and the ganglia are called preganglionic neurons; those passing between the ganglia and the effector cells are postganglionic neurons. Autonomic Nervous System he neurons of the sympathetic and parasympathetic divisions leave the CNS at different levels: 1. The sympathetic fibers from the thoracic (chest) and lumbar regions of the spinal cord 2. The parasympathetic fibers from the brainstem and the sacral portion of the spinal cord. Therefore, the sympathetic division is also called the thoracolumbar division, and the parasympathetic division is called the craniosacral division. Somatic nervous system Autonomic nervous system Voluntary. Involuntary Tissues innervated: skeletal muscle. Tissues innervated: cardiac muscle, Action: always excitatory (cause muscle smooth muscle (e.g. viscera, blood vessels), contraction). glands (most, but not all) CNS control: voluntary (but has Action: may be excitatory or inhibitory involuntary components; e.g. reflexes). CNS control: involuntary in general. No peripheral efferent ganglia or peripheral efferent ganglion and synapses. synapse. Effect of denervation: complete loss of Effect of denervation: generally function, paralysis also atrophy. continues to function, still subject to local and circulating influences, but loss of CNS coordination Divisions: sympathetic nervous system parasympathetic nervous system. Enteric. Sympathetic nervous system These preganglionic neurons enter small relay stations (ganglia) near the cord. The ganglia form a chain that transmit the impulse to postganglionic neurons, forming a sympathetic chain of ganglia that parallels the spinal cord on each lateral side. Then, reach many organs and glands, and can produce widespread, generalized responses.  The physiologic effects of sympathetic activity include: 1. Pupillary dilation and ciliary muscle relaxation. 2. Blood vessels vasoconstriction. 3. Increased heart rate and contractility. 4. Elevated blood pressure. 5. Enhanced blood flow to skeletal muscles. 6. Increase in respiratory rate. 7. Smooth muscle relaxation of the bronchioles. 8. GI tract, and urinary tract sphincter contraction. 9. Reduced pancreatic secretion. 10. Increased sweat gland secretion Parasympathetic nervous system After leaving the CNS, the long preganglionic fiber of each parasympathetic nerve travels to a ganglion near a particular organ or gland, and the short postganglionic fiber enters the organ or gland. Parasympathetic nerves have a specific response involving only one organ or gland.  The physiologic effects of parasympathetic system activity include: 1. Pupillary constriction. 2. Increased pancreatic, salivary, and lacrimal secretions. 3. Reduced heart rate, contractility, and conduction velocity. 4. Bronchial smooth muscle constriction. 5. Increased gi tract tone and peristalsis with sphincter relaxation. 6. Urinary system sphincter relaxation and increased bladder tone. 7. Vasodilation of external genitalia, causing erection. 8. The parasympathetic system has little effect on mental or metabolic activity. Neurotransmitters of the Autonomic Nervous System  The 2 most common neurotransmitters released by neurons of the ANS are acetylcholine and norepinephrine. Neurotransmitters are synthesized in the axon varicosities and stored in vesicles for subsequent release. Nerve fibers that release acetylcholine are referred to as cholinergic fibers.  These include: 1. All preganglionic fibers of the ANS, both sympathetic and parasympathetic systems. 2. All postganglionic fibers of the parasympathetic system; and sympathetic postganglionic fibers innervating sweat glands and some arterioles.  The cells of the adrenal medulla are considered modified sympathetic postganglionic neurons. Instead of a neurotransmitter, these cells release hormones into the blood. Approximately 20% of the hormonal output of the adrenal medulla is norepinephrine. The remaining 80% is epinephrine. Unlike true postganglionic neurons in the sympathetic system, the adrenal medulla contains an enzyme that methylates norepinephrine to form epinephrine. The synthesis of epinephrine, also known as adrenaline, is enhanced under conditions of stress. These 2 hormones released by the adrenal medulla are collectively referred to as the catecholamines. AUTONOMIC ACTIONS General principle: 1. Sympathetic system: responses associated with vigorous, often emotional, activity (“fight or flight”). 2. Parasympathetic system: homeostatic (“vegetative”) activities to maintain body internal environment (e.g. digestion and absorption), (rest and digest). 3. In organs innervated by both the sympathetic and parasympathetic divisions, their effects are usually (but not always) opposite. Note: often both divisions are active and the state of the organ is determined by the balance between the two. AUTONOMIC ACTIONS General principle: 4. Some organs are innervated by only one division (adrenal medulla, sweat gland and kidney innervated by sympathetic division only, also the blood pressure controlled by sympathetic division only) 5. Sympathetic effects are sometimes general throughout the body, parasympathetic effects are mainly localized The Sympathetic is the "Fight or Flight" Branch of the ANS Emergency situations, where the body needs a sudden burst of energy, are handled by the sympathetic system. The sympathetic system increases cardiac output and pulmonary ventilation, routes blood to the muscles, raises blood glucose and slows down digestion, kidney filtration and other functions not needed during emergencies. Whole sympathetic system tends to "go off" together. In a controlled environment the sympathetic system is not required for life, but it is essential for any stressful situation  The Parasympathetic is the "Rest and Digest" Branch of the ANS The parasympathetic system promotes normal maintenance of the body- acquiring building blocks and energy from food and getting rid of the wastes. It promotes secretions and mobility of different parts of the digestive tract. Also involved in urination, defecation. Does not "go off" together; activities initiated when appropriate. The vagus nerve (cranial number 10) is the chief parasympathetic nerve Other cranial parasympathetic nerves are: III (oculomotor), VII (facial) and IX (glossopharyngeal) AUTONOMIC RECEPTORS A. Cholinergic receptors: Normal agonist: Acetylcholine Acetylcholine Receptors; ACh acts on two different types of receptor: 1. Nicotinic receptors; are found at the; A. Neuromuscular junction of skeletal muscles (only) B. Post-ganglionic neurons of the parasympathetic nervous system C. Neurons in the brain. Nicotine is an agonist , Curare is an antagonist (hence its ability to paralyze skeletal muscles) AUTONOMIC RECEPTORS 2. Muscarinic receptors; are found at the; a) neuromuscular junctions of cardiac and smooth muscle. b) Glands. c) post-ganglionic neurons of the sympathetic nervous system in sweat glands and some arterioles. Muscarine (a toxin produced by certain mushrooms) is an agonist. Atropine is an antagonist. AUTONOMIC RECEPTORS B. Adrenergic receptors Normal agonists: epinephrine and norepinephrine. Norepinephrine is the primary neurotransmitter released by postganglionic sympathetic neurons, with the exception of the cholinergic postganglionic sympathetic neurons serving the sweat glands. Note: norepinephrine is the same thing as noradrenaline. The responses of target cells to norepinephrine depend on the specific adrenergic receptor type (i.e., α1, α2, β1, β2, or β3)