8- Neurophysiology (ANS)- Pt 2.docx

- **Sympathetic System: General Info** - The sympathetic system is composed of the **thoracolumbar system** which prepares the body for emergency situations, and is known as the "fight or flight" system. - The postganglionic fibers reach effector organs by...

- **Sympathetic System: General Info** - The sympathetic system is composed of the **thoracolumbar system** which prepares the body for emergency situations, and is known as the "fight or flight" system. - The postganglionic fibers reach effector organs by accompanying the vasculature supplying them, and they are widely distributed as blood vessels. - Some effector organs are controlled solely by the sympathetic division, such as sweat glands, piloerector muscles, and most blood vessels. - The **adrenal medulla** is essentially a postganglionic ganglia/neuron of the sympathetic system that secretes neurotransmitters (epinephrine/adrenaline and norepinephrine/noradrenaline) into the blood stream. - The neurotransmitters are produced by the chromaffin cells from the amino acid tyrosine. - These neurotransmitters bind to adrenergic receptors. - **Sympathetic and Parasympathetic Tone** - Normally, the sympathetic and parasympathetic systems are continually active, and the basal rates are known as "**tone**". - Tone allows a single nervous system to both increase and decrease the activity of a stimulated organ. - For example, most blood vessels lack parasympathetic innervation, and their diameter is regulated by sympathetic nervous system input, so that they have a constant state of sympathetic tone. - A decrease in sympathetic stimulation or tone allows vasodilation. - For example, the iris is innervated by parasympathetic and sympathetic fibers receiving basal tone from both. - However, dilation or contraction in response to light is mainly mediated by the increase or decrease of parasympathetic tone. - A decrease in parasympathetic tone allows for pupillary dilation (mydriasis). - **Sympathetic Stress Response: General Info** - Under physical or emotional stress, the sympathetic system is capable of massive, coordinated output with widespread effects, such as a massive discharge of catecholamines by the adrenal medulla. - This action increases the ability of the body to perform vigorous muscle activity, far more than would otherwise be possible. - Sympathetic effects last longer than those from the parasympathetic system. - Insulin stores glucose (right after eating). - Glucagon uses stored glucose- AKA: glycogen (while fasting). - **Systemic Stress Response: Receptor Stimulation** - Stimulation of alfa receptors (**alfa1 and alfa2**) in the arterioles of most visceral organs causes contraction of arteriole smooth muscle, raising blood pressure, and restricting blood flow - Stimulating **alfa1 and alfa2** will also cause increased rate of blood coagulation, increasing platelet aggregation. - Stimulation of **Beta 1** receptors causes: - Increased heart rate and force of contraction with each heartbeat - Increased renin release by the kidneys, increasing blood pressure - Stimulation of **Beta 2** receptors causes: - Vasodilation of skeletal muscle, liver arterioles, and coronary blood vessels so they can respond to the stressor - Relaxed smooth muscle from airways (beta2 receptor), known as bronchodilation - Increased gluconeogenesis - Stimulation of **Beta 2 and Alfa1** receptors causes: increased breakdown of glycogen in live and muscles. - Stimulation of **Alfa 2** receptors causes: inhibited insulin secretion and increased glucagon release, increasing glucose availability. - Stimulation of **Beta 2 and Beta 3** receptors causes: stimulated lipolysis in adipose tissue, increasing fatty acid availability. - Stimulation of **Alfa 1** receptors causes: dilated pupils. - **Parasympathetic System** - The parasympathetic system is known as the "rest and digest" system, and it involves the craniosacral system which conserves and restores energy sources of the body. - The preganglionic fibers are relatively long, projecting to ganglia located in the vicinity of or in the wall of their target tissues. - The postganglionic neurons have short axons. - The parasympathetic system does NOT innervate structures of the body wall and extremities. - The parasympathetic system has a greater degree of independent control as well as more precise control of tissues and organs, compared to the sympathetic division. - **Parasympathetic Tone** - Parasympathetic tone is generally concerned with restorative aspects of daily living. - For example, parasympathetic tone lowers blood pressure by slowing down the HR (heart rate) and decreasing the force of contraction of the heart (M2). - Parasympathetic tone [enhances digestive activity] by: - Increasing blood flow to the GI tract - Increasing intestinal motility - Stimulating secretion of digestive systems - Relaxing the pyloric sphincter - **Enteric Nervous System** - The enteric nervous system is an intrinsic nervous system located in the walls of the GI system that system mediates the regulation of GI physiology (secretion, motility, nutrient absorption). - The enteric nervous system contains sensory and motor components, and has a complex network that includes the myoenteric plexus and submucosal plexus. - The enteric nervous system has nerve endings in the gut wall and mucosa that convey chemical, mechanical, and stretch sensory information to the intrinsic neural network. - The PSNS and SNS innervation appear to modulate the enteric nervous system in a regulatory manor. - **NANC Fibers** - NANC (non-adrenergic, non-cholinergic) are fiber that do not exhibit the histochemical characteristics of cholinergic or adrenergic fibers. - The enteric nervous system is extensively innervated by NANC fibers, which are very important in the physiological regulation of the GI tract, genitourinary tract, and selected blood vessels. - The GI tract has cholinergic, adrenergic, and NANC fibers. - A good example of NANC fibers are those producing nitric oxide (NO). - **Autonomic Reflexes** - The ANS participates in many homeostatic reflexes such as: blood pressure control and PLR. - **Blood pressure control** - It is very important to maintain sufficient brain blood flow. - Stretch receptors in the internal carotid artery and aorta detect systemic blood pressure. - Mechanoreceptors (like baroreceptors) act as physiological receptors. - **Pupillary Light Reflex (PLR)** - PLR is an autonomic reflex activity controlled by the parasympathetic system. - The PLR is a reflex arc composed by: - Receptors for light (located in the retina) which act as photoreceptors - Afferent neurons such as the optic nerve (CN2) - Brainstem centers for integrations, such as the: - Pretectal nucleus - Parasympathetic nucleus of the oculomotor nerve (CN3) - Efferent neurons such as the oculomotor nerve (CN3) - Effector organ such as the: - Pupillary sphincter muscle contracting (miosis) under bright light - **Direct PLR** leading to miosis in the illuminated eye - **Consensual/Indirect PLR** leading to miosis in the eye that is not illuminated - This response is weaker than direct PLR - The **baroreceptor reflex** is a reflex arc composed by: - Baroreceptors which are receptors for blood pressure - Afferent neurons which carry information to the medulla oblongata - Brain stem centers (medulla) which process information and coordinate an appropriate response - Efferent neurons which have sympathetic and parasympathetic innervation - Effector organs which direct changes in the heart and blood vessels to increase or decrease blood pressure - **Horner's Syndrome** - Horner's syndrome is due to decreased sympathetic stimulation of smooth muscle of the eye and periorbita. - Horner's syndrome can also be caused by a middle ear issue or even otitis. - Horner's syndrome is ultimately caused by damage or dysfunction of the sympathetic pathway as it runs through the neck or chest. - The **common site of injury** for Horner's Syndrome are the: Cervical intumescence (and brachial plexus), and the cervical cranial ganglion (medial to middle ear). - Issues with the following **sympathetic innervations to the eye** can result in Horner's Syndrome: - Damage within the brain and spinal cord is known as **1^st^ order Horner's syndrome (central)**. - 1^st^ order neuron: Hypothalamus to T1-T3 - Damage between the chest cavity and the base of the skull is known as **2^nd^ order Horner's syndrome (preganglionic)** - 2^nd^ order neuron: T1-T3 to the cervical cranial ganglion - This type of damage can be caused by tumors in the thoracic cavity, brachial plexus injury, neck trauma, or can be idiopathic. - Damage between the base of the skull and the eye is known as **3^rd^ order Horner's syndrome (postganglionic)**, and is the most common form within dogs and cats. - 3^rd^ order neuron: Cervical cranial ganglion to the eye. - This type of damage can be caused by middle ear disease, or can be idiopathic.