 In this video I will compare and contrast somatic reflexes with autonomic reflexes, contrast the roles of the parasympathetic and sympathetic divisions of the autonomic nervous system, define colonergic and adrenergic fibers. Somatic reflexes have skeletal muscles as the effector organs, and the efferent pathway of a somatic reflex consists of a single axon of a motor neuron extending from the central nervous system to form a synapse with a skeletal muscle fiber. The example of a somatic reflex shown here is the patellar deep tendon stretch reflex, a simple monosynaptic spinal reflex. The receptor is the muscle spindle stretch receptor inside of the extensor muscle known as the quadriceps femoris. Then the efferent pathway is the axons of the sensory neurons that are relaying information from the muscle spindle stretch receptor in through the femoral nerve and spinal nerve roots L2 through L4 to travel in to the spinal cord. The integration center is in the spinal cord where the efferent neurons synapse directly with efferent neurons. The efferent pathway is the axons of the somatic efferent neurons that extend out through spinal nerve roots L2 through L4 into the femoral nerve down through the femoral nerve to the effector organ which is the skeletal muscle, the quadriceps femoris muscle. The effector, the quadriceps femoris will contract producing extension of the leg at the knee. This is an ipsilateral reflex because the effector muscle, the quadriceps femoris, is on the same side of the body, the same left or right side as the receptor, the muscle spindle, because the muscle spindle is located within the very same muscle, the quadriceps femoris, that is also the effector in this example. In contrast to somatic reflexes, autonomic reflexes have smooth muscles, the heart or glands as the effector organs. And the efferent pathways for autonomic reflexes involve preganglionic fibers which are the axons of preganglionic motor neurons extending from the central nervous system to a ganglia of the peripheral nervous system where they form synapses with postganglionic neurons. Then the postganglionic fibers are axons extending from the ganglia of the peripheral nervous system to form synapses with the effector organs. The example of an autonomic reflex shown in the illustration here is the pupillary light reflex. The receptors are photoreceptors in the retina of the eye that detect light and then relay information through the efferent pathway of the optic nerve, creating nerve number two that relays information into the brain. The brain is the control center that will process this sensory information and if the light levels are too high, the brain will activate the efferent pathway which is a pathway through the autonomic efferent fibers of the parasympathetic division of the autonomic nervous system. The preganglionic fibers travel in the oculomotor nerve cranial nerve number three to reach the postganglionic neurons found in the ciliary ganglia. So the postganglionic fibers extend from the ciliary ganglia through ciliary nerves into the iris where there are smooth muscles in the iris that are the effector cells for this reflex. The pupillary constrictor muscles of the iris are stimulated by the postganglionic fibers and will contract to decrease the diameter of the pupil so the pupil will constrict in response to that reflex to highlight level. The opposite side of this reflex if the brain process sensory information and determine that the light level is too low then the efferent pathway will involve activation of the sympathetic motor neurons where the preganglionic neuron is located in the spinal cord in the cervical region of the spinal cord and the preganglionic fiber will travel to a ganglia known as the cervical ganglia of the sympathetic chain. Then the postganglionic fiber extends from that cervical ganglion all the way into the smooth muscle of the iris to stimulate contraction of the radially arranged pupillary dilator muscles, the smooth muscles that will cause the pupil to open and dilate allowing more light to enter the eye. So this idea that a single organ in this case the iris in the eye, a single tissue or a single organ in the eye is regulated by both the sympathetic and the parasympathetic divisions of the autonomic nervous system is known as dual innervation. So the sympathetic nervous system causes dilation of the pupils whereas the parasympathetic nervous system causes constriction of the pupils and we'll see this is in part the result of distinct neurotransmitters that are released by the postganglionic fibers. The postganglionic parasympathetic fibers release the neurotransmitter acetylcholine to stimulate the pupillary constrictor muscles whereas the postganglionic sympathetic fibers release the neurotransmitter norepinephrine to stimulate the pupillary dilator smooth muscles. The autonomic efferent pathways of the peripheral nervous system are regulated by regions within the brain. The hypothalamus, a region of the diencephalon, receives sensory information and other processing that's relayed from structures of the limbic system and then sends commands out through the autonomic and endocrine systems. For example, if you're scared or angry, that emotional processing by the limbic system will stimulate nuclei in the hypothalamus that activate the sympathetic division of the autonomic nervous system to help the body respond to the stress. Similarly, there are control centers in the brainstem that are important for regulating the autonomic nervous system. For example, we can see in the midbrain is the oculomotor nucleus, the eddinger Westfall nucleus that sends the parasympathetic commands to cause constriction of the pupils. There's also a dorsal motor nucleus of the vegas that sends the parasympathetic commands leading to stimulation of the digestive organs and decreased heart rate. And so the cardiovascular control centers found in the medulla oblongata are within the brainstem and are regions that are involved in regulating the autonomic nervous system to help maintain blood pressure, regulating the heart rate, the force of heart contraction, as well as regulating smooth muscle blood vessels. We'll also see that there are autonomic control centers involved in regulating the smooth muscle of the respiratory tract to cause dilation of the bronchioles in response to a stressful situation to increase airflow to help increase the amount of oxygen that can be transported into the blood and carbon dioxide that can be transported out of the blood. So the sympathetic division of the autonomic nervous system is commonly referred to as the fight-or-flight division that helps enable the body to cope with a stressful situation. So when we're excited or frightened, scared, or if we're working hard during exercise, the sympathetic nervous system is dominant. Some of the effects include dilation of the pupils, increased heart rate and blood pressure, dilation of the bronchioles in the lungs to increase airflow through the lungs, and increased blood flow to skeletal muscles and the brain and decreased blood flow to the digestive organs and the skin. Also, the sympathetic nervous system can stimulate the release of glucose from storage in the liver by activating glycogenalysis. And the sympathetic nervous system will respond to the thermoregulatory control center in the hypothalamus. If body temperature is too high, the sympathetic nervous system sends commands to sweat glands and also sends commands to blood vessels to increase the blood flow through the skin. And in the opposite situation, when body temperature is too low, the sympathetic nervous system sends commands that will lead to an increase in metabolic rate to lead to thermogenesis increased heat production by metabolism of cells throughout the body. So the sympathetic nervous system will also involve the production of a hormone. There are preganglionic sympathetic fibers that innervate the adrenal gland and stimulate the adrenal gland to produce the hormone epinephrine, also known as adrenaline. And because this hormone is circulating through the blood in response to a stressful situation when the sympathetic nervous system becomes activated, the body will have a whole body systemic response to help cope with stress. The parasympathetic division of the nervous system stimulates the rest and digest maintenance functions of the body. When the body is not stressed but is relaxed, the parasympathetic nervous system becomes dominant. Some of the functions of the parasympathetic nervous system include constriction of the pupils in the eye, cranial nerve number 3, the oculomotor nerve, contains the preganglionic parasympathetic fibers that travel to the ciliary ganglion, and then the postganglionic fibers travel from the ciliary ganglion into the eye to regulate contraction of the smooth muscle of the iris. And also, regulate contraction of the ciliary muscle that controls the shape of the lens. Cranial nerve number 7, the facial nerve, carries preganglionic fibers that travel to the pterigopalatine ganglion and submandibular ganglion. And postganglionic fibers travel from the pterigopalatine ganglion to the lacrimal gland and mucus membranes of the nasal cavity and oral cavity, where the parasympathetic nervous system will stimulate the secretion of tears and mucus. Cranial nerve number 9 is the glossopharyngeal nerve that contains preganglionic parasympathetic fibers that travel to the autech ganglion, and then postganglionic fibers travel from the autech ganglion to the salivary glands to stimulate the secretion of saliva. Cranial nerve number 10, the vegas nerve is the majority of the parasympathetic efferent fibers. The preganglionic fibers travel through the vegas nerve to reach intramural ganglia that are in the wall of the heart, and then a postganglionic fiber will stimulate a decrease in the heart rate. The parasympathetic nervous system will also stimulate the activity of digestive organs. There are sacral spinal nerves that contain preganglionic parasympathetic efferent fibers traveling to the urinary and reproductive organs. One of the functions of the parasympathetic nervous system is to stimulate micturition, the process of releasing urine, commonly known as urination. The parasympathetic nervous system will stimulate contraction of smooth muscle in the urinary bladder, known as the detrusor muscle, and relaxation of the smooth muscle known as the internal urethral sphincter, which will allow urine to move from the urinary bladder out through the urethra. So dual innervation is a common theme that we will see with autonomic regulation of our visceral organs. The example here is dual innervation of the heart. The parasympathetic efferent pathways traveling down through the vegas nerve, cranial nerve number 10, will lead to a decrease in the heart rate. Whereas the sympathetic efferent pathways traveling through the cervical spinal nerves that are traveling into the heart are going to stimulate an increase in heart rate, as well as an increase in the force of contraction with each heartbeat. There are two neurotransmitters released by the motor neurons of the autonomic division of the peripheral nervous system. These are acetylcholine and norepinephrine. A neuron that releases the neurotransmitter acetylcholine is a cholinergic neuron, so its axon could be called a cholinergic fiber. A neuron that releases the neurotransmitter norepinephrine is an adrenergic neuron, and so its axon could be called an adrenergic fiber. We'll see this terminology also can be applied to receptors. So the acetylcholine receptors are cholinergic receptors and can be categorized into two groups. Nicotinic acetylcholine receptors, which are ligand gated ion channels, and muscarinic acetylcholine receptors, which are G protein coupled receptors. Whereas the adrenergic receptors can be classified as alpha adrenergic receptors or beta adrenergic receptors, but both alpha and beta adrenergic receptors are G protein coupled receptors. So all preganglionic fibers in the autonomic nervous system are cholinergic fibers. So all of the preganglionic fibers release acetylcholine to stimulate the postganglionic neurons in the autonomic ganglia of the peripheral nervous system. And those postganglionic neurons express the nicotinic acetylcholine receptors so that acetylcholine will bind to the receptor, causing opening of an ion channel, producing graded potentials that can then stimulate action potentials in the postganglionic fibers. All postganglionic fibers in the parasympathetic division of the autonomic nervous system are cholinergic. They all release acetylcholine. And the effector organs that are being regulated by the parasympathetic nervous system express the muscarinic acetylcholine receptors. We will see that there are some examples where postganglionic sympathetic fibers release acetylcholine, for example, to stimulate sweat glands. However, the majority of postganglionic sympathetic fibers are adrenergic fibers. So the sympathetic nervous system releases no epinephrine to regulate effector organs. And epinephrine will bind to alpha or beta adrenergic receptors on the surface of the effector organ in order to regulate its activity.