Lecture 5 (Neurophysiology IV) PDF

Neurophysiology IV: Autonomic Nervous System ANSC 3080 G. Bedecarrats Learning Objectives  Be able to differentiate the sympathetic and parasympathetic divisions of the autonomic system on the basis of structure and general function.  Review the...

Neurophysiology IV: Autonomic Nervous System ANSC 3080 G. Bedecarrats Learning Objectives  Be able to differentiate the sympathetic and parasympathetic divisions of the autonomic system on the basis of structure and general function.  Review the neurotransmitters of the autonomic nervous system and their post-synaptic receptors. Autonomous Nervous System (ANS)  Autonomic = controls body functions without conscious thought (automatically, involuntary)  Key in maintenance of homeostasis  Accomplished by reflexes = autonomic reflexes  Controls smooth muscle, cardiac muscle, and glands  Respiration, heart rate, blood pressure, digestion, rumination, sleep, arousal, body temperature  Reflex = sensory cells, sensory nerve fiber, integration center, motor nerve fiber, effectors 1  Mainly controlled by negative feed back  Sensory cells: depend on the system considered (baroreceptors, cold and warm receptors, etc…)  Sensory nerve fibers  No major difference from somatic arc reflexes  Control centers  Hypothalamus – master command  Brain stem – special control function (respiratory center)  Spinal cord – local integration in thoracic and sacral segments  Receive continuous information from sensory nerves (frequency of impulses)  Compare information to set reference  if different, alter frequency on the motor fiber  Motor nerve fibers: Differ from somatic motor system  Target organ (Smooth muscle, cardiac muscle, glands)  Number of neurons in its peripheral circuit  Two peripheral nerves (vs. 1 in somatic arc reflex)  Preganglionic neuron – cell body in the CNS with axon that innervates a second neuron, the postganglionic neuron  Cell body of the postganglionic neuron is in a ganglion Somatic Autonomic Preganglionic fiber GANGLION Postganglionic fiber Effector: Skeletal muscle Cardiac or smooth muscle, viscera, gland 2  Major Division of the ANS  Sympathetic (thoraco-lumbar)  Activated during stress (demanding activities/alarm)  Preganglionic neurons originate in thoracic or lumbar spinal area and send axons to sympathetic ganglia which form a chain parallel to the spinal cord  Parasympathetic (cranio-sacral)  Most active during rest  Preganglionic neurons originate in the brain or sacral part of the spinal cord and send axons to ganglia near or within effector organs  Enteric nervous system: associated with the parasympathetic Sympathetic System Outflow from thoracolumbar spinal cord preganglionic axons enter the paravertebral sympathetic ganglion chain Postganglionic fibers are very long Innervate hollow organs, blood vessels, etc. Some preganglionic fibers extend to the adrenal medulla (adrenal glands) Stimulate the secretion of epinephrine (adrenaline) and norepinephrine (noradrenaline) into blood circulation 3  Important point: Information is shared between ganglia along the chain! Parasympathetic System Very long preganglionic fibers  Cranial nerves III, VII, IX and X supply parasympathetic fibers to body’s gland and organs  X = vagus nerve  major supplier to the body  Several sacral spinal nerves for urinary bladder and genitals Ganglia  Located in the wall or adjacent to target organs  No interconnection between ganglia (no chain) Postganglionic fibers = short axons  Direct connection with specific organs (no branching off) with no enhancement by hormones = selective action Communication Within ANS (for reference and comparison) 4 Receptors to ANS Neurotransmitters Receptors to acetylcholine: Nicotinic Preganglionic synapses (Neuromuscular junctions = skeletal muscle not ANS) Ionotropic = part of an ion channel (here: Na+) Muscarinic In target organs of parasympathetic Separate from ion channel  requires intracellular second messengers to activate target cell Depending on messenger, effect can be stimulatory or inhibitory Slower and more prolonged effect than nicotinic Receptors to (nor)adrenalin (adrenergic):  Separate receptors associated to G-proteins  activate intracellular second messenger  -adrenergic  1 =  Ca2+  2 =  cAMP  Usually constriction of smooth muscles (vascular)  -adrenergic  1 and 2 = cAMP  Main form in the heart   contraction strength & rate  Relaxation of smooth muscles (GI tract, uterus, bronchioles) The specific mode of action of these receptors will be covered later Pharmacological Significance Use of agonists and antagonists to selectively activate or block receptors Hypertension -adrenergic blocker (Propranolol) =  heart rate (1) Asthma 2-adrenergic agonist (salbutamol) = bronchodilation Atropine Muscarinic receptor blocker Drop in eye = dilation of pupils (eye exam) Inhibits contraction of lower GI tract Reduces mucus production in respiratory tract 5  Reciprocal Effects Effectors Sympathetic Parasympathetic Eye Pupil dilation Pupil constriction Salivary gland Stimulation Inhibition Heart rate Increased Decreased Blood vessels Constriction (most) Dilation (few) Lungs (bronchiole) Dilation Constriction Intestinal mobility Inhibition Stimulation Sphincters Stimulate closing Inhibit closing Urinary bladder Muscle tone relaxed Contraction Penis Ejaculation Erection 6