 This video will cover the anatomy of the peripheral nervous system. Most of this material comes from the open-stacks anatomy and physiology textbook, chapters 13 through 16, but some additional content comes from Gray's Anatomy and from Wikipedia. As we go, we will cover the following study objectives, describe the structure of the peripheral nervous system, and functions of major structures. We will identify the layers of connective tissue in a nerve, list the cranial nerves in order, for each describe the major functions, central and peripheral connections, and pathway through the cranium. We will list the major spinal nerves for each describe which nerve plexus each nerve supplies, and major functions of each of the spinal nerves. We will contrast the structure and function of the autonomic and somatic efferent pathways of the peripheral nervous system, and we will contrast the structure and function of the sympathetic and parasympathetic divisions of the autonomic nervous system. A nerve is organized with three layers of connective tissue, an outermost layer of fibrous connective tissue called the epineurium surrounds the entire nerve. Then inside of the nerve are bundles of axons called fascicles, and each of those fascicles is surrounded by a layer of fibrous connective tissue known as the perineurium. Then inside of the fascicle each nerve fiber or axon is surrounded by a layer of loose areolar connective tissue called the endonurium. The endonurium surrounds the axons and also the Schwann cells, the glial cells that are supporting the axon producing the myelin insulating myelinated axons in nerves of the peripheral nervous system. We will start by going through the cranial nerves and these are numbered in order 1 through 12. So we will start with the olfactory nerve, cranial nerve number one that carries the sense of smell from the olfactory epithelium of the superior nasal cavity into the olfactory bulb, and cranial nerve number two is the optic nerve which also carries sensory information. The optic nerve carries the sense of vision from the eye, from the retina in the eye, and it carries that information into the optic tract that will continue to the thalamus. Cranial nerve number three is the ocular motor nerve. It's sending out the motor commands that excite the contraction of most of the eye muscles. So it extends out from the midbrain and to the extraocular eye muscles and those would be somatic pathways to regulate the skeletal muscles that move the eyes. There are also parasympathetic motor pathways that travel through the ocular motor nerve in order to regulate the smooth muscles inside of the eye in the iris and ciliary body. Cranial nerve number four is the trochlear nerve, a motor nerve that excites contraction of the superior oblique eye muscle, a muscle that rotates the eye in its socket. So it extends out from the midbrain to the superior oblique eye muscle. Cranial nerve number five is the trigeminal nerve. There are three large branches of the trigeminal nerve and we'll go through and study each of those and see their functions. There's an ophthalmic maxillary and mandibular branch of trigeminal nerve. But the trigeminal nerve carries both sensory and motor impulses. Sensory information coming from the face and the motor commands include the commands to excite the muscles of mastication, the muscles required to chew. For example, the masseter and the temporalis are muscles excited by the trigeminal nerve. Cranial nerve number six is the abducens nerve. This nerve starts from the pons and extends out to excite the lateral rectus eye muscle, a muscle that pulls the eye to direct your gaze away from the midline. And so the name abducens, remember abduction is to move away from the midline. Abducens nerve causes abduction, that is to move your gaze away from the midline. Cranial nerve number seven is the facial nerve. And so the cranial nerve number seven attaches to the pons and then extends out to the face. There's a sensory pathway as well as a motor pathway. The sensory fibers, the afferent fibers of facial nerve carry the sense of taste from the tongue. Then the efferent pathway, the motor fibers excite the muscles of facial expression. For example, the depressor labii, inferioris, or depressor anguli oris, the orbicularis oris, and orbicularis oculi, the zygomaticus major and minor. These are all muscles that are excited by efferent fibers of the facial nerve. Cranial nerve number eight is the vestibular cochlear nerve. This carries sensory information from the inner ear and the destination where the vestibular cochlear nerve attaches to the brain is at the medulla oblongata. Cranial nerve number nine is the glossopharyngeal nerve. So the glossopharyngeal nerve, the name tells us it's carrying information from the tongue, glosso, and the pharynx, the throat, pharyngeal. So glossopharyngeal is both a sensory and a motor nerve. Sensory information includes taste as well as there are receptors in the large artery traveling towards the brain, the carotid artery. And the sensors in the carotid artery are detecting blood pressure as well as the concentration of gases like oxygen and carbon dioxide dissolved in the blood. The motor pathways through the glossopharyngeal nerve excite the muscles of the pharynx for swallowing. So the glossopharyngeal nerve connects to the brain at the medulla oblongata and travels to the tongue and the pharynx. Cranial nerve number ten, the vegas nerve, has both sensory and motor functions. The sensory information is coming from our visceral organs including a bororeceptor that measures blood pressure in the aorta, the large artery that carries blood out from the left ventricle of the heart. There's also motor pathways through the vegas nerve and so the vegas nerve regulates the activity of visceral organs through parasympathetic efferent fibers that travel in the vegas nerve. For example, they travel to the heart in order to decrease the heart rate. They travel to the digestive organs to stimulate the activity of the digestive organs. The vegas nerve attaches to the brain at the medulla and then the peripheral attachments are in the visceral organs. Cranial nerve number eleven is the accessory nerve. It's a motor nerve and it excites the trapezius and the deltoids. It's attached to the brain at the medulla and extends out from the medulla in order to excite those skeletal muscles of the trapezius and deltoids. Cranial nerve number twelve is the hypoglossal nerve. Hypoglossal nerve is a motor nerve that excites the muscles of the tongue. It attaches to the brain at the medulla and extends out to the tongue where it excites muscles of the tongue. For example, the genealoglossus, styloglossus, hyoglossus are all excited by the hypoglossal nerve. Here we can see cranial nerve number one traveling through the cribriform foramina of the ethmoid bone. The branches of cranial nerve number one travel through the numerous foramina in the cribriform plate and then synapse with neurons inside of the olfactory bulb. Their function is sensory to carry the sense of smell. Here we can see the pathways for cranial nerves two through five that are traveling out towards the eye or in from the eye. A sensory nerve, the optic nerve, cranial nerve number two carries visual information from the retina in through the optic foramen. And its destination will ultimately be in the optic tracts that connect to the foulness. The superior orbital fissure contains the oculomotor nerve, the trochlear nerve, the ophthalmic branch of the trigeminal nerve, and also the abducens nerve. So we can see here, cranial nerve three, the oculomotor nerve that extends through superior orbital fissure from the midbrain to excite the majority of the muscles of the eye. Cranial nerve number four, we can see just lateral to cranial nerve three is extending from the midbrain to the superior oblique muscle. The skeletal muscle that causes the rotation of the eye in its socket. Then the ophthalmic branch of cranial nerve number five is carrying sensory information from the superior regions of the face, including the skin around the orbits. Then we will see that cranial nerve number six is out of view here, but also travels through this superior orbital fissure in order to excite the lateral rectus eye muscles. Here we have a lateral view showing the nerves connecting to the orbit. So we already saw cranial nerve number two that travels through the optic foramen and is carrying a sense of vision from the retina into the thalamus. But cranial nerve three, four, the ophthalmic branch of five, and cranial nerve six all travel through the superior orbital fissure. Cranial nerve three extends from the midbrain out to excite most of the muscles of the eye. That's the oculomotor nerve as the name suggests it moves the eye muscles. Cranial nerve number four, the trochlear nerve also extends from the midbrain and extends out to the superior oblique muscle. The ophthalmic branch of cranial nerve five is carrying sensory information from the superior regions of the face and coming into a destination in the ponds. Then cranial nerve six extends from the ponds out through the superior orbital fissure in order to excite the lateral rectus muscles that direct our gaze away from the midline. Now the maxillary branch of the trigeminal nerve travels through foramen rotundum and carries sensory information from the middle region of the face including the upper jaw. Here we can see the three large branches of the trigeminal nerve. There's the ophthalmic branch that travels through superior orbital fissure carrying sensory information from around the eyes and the upper regions of the face. There's the maxillary branch that travels through foramen rotundum carries sensory information from the middle region of the face including the upper jaw. Then the mandibular branch travels through foramen ovale and carries sensory information from the lower regions of the face including the lower jaw, the mandible, but also excites the contraction of skeletal muscles required for chewing like the masseter and the temporalis. So here we can see an illustration showing the major regions of the face where sensory information comes from for the three branches of the trigeminal nerve. The ophthalmic branch is shown in the green color and is the superior region of the face. The maxillary branch shown in pink is the middle region of the face including the upper jaw and then the mandibular branch of trigeminal nerve carries sensory information from the lower regions of the face including the lower jaw. It's shown in yellow here. Acranial nerves number seven and eight are the facial and vestibulococlear. Vestibulococlear nerve was previously called the acoustic nerve and so you can see it's labeled the acoustic nerve in the image from Grey's Anatomy. The internal acoustic miatus is the pathway for the facial nerve and vestibulococlear nerve to travel in and out of the cranium. And so the internal acoustic miatus allows the vestibulococlear nerve to travel from the inner ear which is deep inside of the temporal bone out into the cranium to carry information to the medulla oblongata. And the facial nerve which carries both sensory and motor information from the face and tongue travels through internal acoustic miatus and then branches of it will come out through the stylo mastoid foramen to reach the face where it can excite the muscles of facial expression. And the facial nerve also carries the sense of taste from the majority of the tongue. And the facial nerve attaches to the brain at the pons. So here we can see the glossopharyngeal cranial nerve number nine, the vagus cranial nerve number 10 and the accessory nerve cranial nerve number 11 which all travel through the jugular foramen. And so the glossopharyngeal nerve is exciting contraction of pharyngeal muscles important for swallowing. It's also carrying information about the pressure of our blood and the concentration of gases like oxygen and carbon dioxide in the blood from the carotid artery. Then the vagus nerve carries both autonomic motor pathways of the parasympathetic branch of the autonomic nervous system and visceral sensory information coming to and from the organs of the ventral body cavities, the visceral organs. Then the accessory nerve cranial nerve number 11 is a motor nerve and it excites contraction of the trapezius and deltoids. So here we can see the hypoglossal nerve which is extending down through the hypoglossal canal of the occipital bone just lateral to foramen magnum. The hypoglossal nerve extends down into the tongue to excite the contraction of the muscles of the tongue like genioglossus, styloglossus, hyoglossus. Now we're going to move on to study the spinal nerves that connect to the spinal cord. On the posterior or dorsal side of the spinal cord, axons extend in through a dorsal root carrying sensory information from afferent fibers. On the anterior or ventral side of the spinal cord, axons extend out through the ventral root carrying information, motor commands out through efferent fibers. The afferent and efferent fibers join together to form the trunk of the spinal nerve and then branches come off of that trunk. The dorsal ramus is a branch that comes off and innervates the posterior of the body carrying both sensory and motor information to and from the posterior of the body. Then the ventral ramus is a branch that extends off to the lateral and anterior regions of the body carrying both sensory and motor information from those regions. The white ramus is a branch that carries the axons of the autonomic neurons that are found in the lateral gray horn that are traveling towards a ganglion in the peripheral nervous system. The gray ramus is where an axon from an autonomic ganglion can extend back to join the spinal nerve. Here is a micrograph of the dorsal root ganglion. You can see that there are numerous cell bodies of sensory neurons found inside of the dorsal root ganglion and there are axons that are traveling from the dorsal root to the dorsal root ganglion. Nearby we can see axons from the ventral root that are coming together to join with the dorsal root in order to form the trunk of a spinal nerve. Here is a higher magnification view of the dorsal root ganglion where we can see the cell bodies of the pseudo-unipolar sensory neurons and those neuronal cell bodies are surrounded by neuroglial cells called satellite cells that help to regulate the chemical environment surrounding the neuron. Here we can see how the spinal nerves attached to the spinal cord, axons of motor neurons in the lateral and anterior gray horns extend out through the ventral root and then axons of the sensory neurons, the afferent fibers, travel in from the dorsal root ganglion through the dorsal root to form synapses in the posterior gray horn or project up through ascending tracts primarily in the posterior white column although there are also some ascending tracts through the lateral white column and anterior white column. Where the dorsal root ganglion meets with the ventral root, the axon of the sensory neuron continues to receptors in the periphery and the axons of motor neurons continue out to excite skeletal muscles. These axons travel together through the spinal nerve and so the spinal nerve is mixed with both afferent and efferent fibers. The spinal nerves that travel off to the anterior and lateral regions of the body form a larger branch known as the ventral ramus of the spinal nerve and the smaller branch coming off of the trunk of the spinal nerve is the dorsal ramus which travels to the posterior region of the body. Here we see a micrograph of the layers of a spinal nerve where we can see the perinurium that is surrounding a fascicle and then there are numerous fibers inside of the fascicle each one surrounded by a layer of endonurium and then out of view in the magnified image but you can see in the top right the low magnification image an outermost layer of fibrous connective tissue surrounding the entire nerve that is the epinurium. And so the three layers of connective tissue are endonurium deep inside around the fibers perinurium surrounding the fascicles and epinurium surrounding the entire nerve. There are 31 pairs of spinal nerve trunks pairs being one on the left and one on the right that are carrying information in and out of the spinal cord. The spinal nerves are numbered starting with C1 which exits above the first cervical vertebrae. So C1 exits above atlas. C2 exits above axis. C7 exits above vertebra prominence. However, there's only seven cervical vertebrae and there are eight cervical spinal nerves. And so spinal nerves C8 exit above the first thoracic vertebrae. And then the numbering system switches so that all of the other vertebrae are numbered according to the vertebral level they exit below. And so the first thoracic nerves exit below the first thoracic vertebrae. The first lumbar nerves exit below the first lumbar vertebrae. So L5 will exit in between the fifth lumbar vertebrae and the sacrum. Then there's five sacral nerves. The first sacral nerves exit through the most superior of the sacral foramina. Sacral nerves four exit through the most inferior of the sacral foramina. Sacral nerve five exits just inferior to the sacrum. And then there are also coxigial nerve that extend out through the coccyx. The ventral ramie of the spinal nerves in the cervical region and the first thoracic form nerve plexus. The cervical plexus forms from C1 through C5. So a nerve plexus is a network of interlacing nerves. The cervical plexus is formed from the ventral ramie of C1 through C5. And most of the branches coming off of the cervical plexus are carrying sensory information from the skin of the neck and from the posterior regions of the head and from the shoulders. And they also have some motor fibers that excite the contraction of muscles in the neck and shoulder region. There's one major nerve that we will study in more detail called the phrenic nerve. The phrenic nerve arises from C3 through C5 in the cervical plexus and extends down to excite contraction of the diaphragm, the skeletal muscle that forms the floor of the thoracic cavity. And when it's contracted causes expansion of the thoracic cavity in order to draw air into the lungs. The brachial plexus forms from C5 through T1. And so the illustration here shows just C6 through T1, but classically we describe it as C5 through T1. So I'll show you on the Gray's Anatomy and illustration showing it start from C5. The brachial plexus connects to the branches that travel through the arms. And so the major nerves that we will study that branch off of the brachial plexus are the musculocutaneous, median, axillary, ulnar, and radial nerves. And these are exetting the skeletal muscles that move the muscles of the arms and also carrying sensory information from the skin and sensory receptors throughout the arm. Then in the lumbar region there is another nerve plexus known as the lumbar plexus. The lumbar plexus arises from L1 through L4. And the nerves travel into the thighs and the inferior portions of the abdominal wall and the psoas muscle. The major nerves that we will study coming from the lumbar plexus are the femoral nerve and the obturator nerve. Then inferior to the lumbar plexus we see here the sacral plexus which arises from L4 through S4. And carries information to and from the legs. The major nerve that we will study is the sciatic nerve which is the largest nerve in the body. And then we will study divisions of the sciatic nerve. There is a tibial nerve and fibular nerve. And the common fibular nerve we will see branches to form a deep and superficial branch of the fibular nerve. So here we can see an illustration showing the cervical plexus which arises from C1 through C5. And carries information to and from the neck and shoulders and the posterior regions of the face. But this major nerve that extends out from the cervical plexus from the lower 3 ventral ramii from C3 through C5 is the phrenic nerve. And the phrenic nerve excites contraction of the diaphragm. And so here we can see the phrenic nerve extending down on the right and the left to innervate the diaphragm. Here we see an illustration of the brachial plexus the interlacing network of nerves that connect to the upper limbs. The brachial plexus arises from the ventral ramii of spinal nerve trunks C5 through T1. The ventral ramii of C5 through T1 are also known as the roots of the brachial plexus. And the roots continue on to form the trunks of the brachial plexus. C5 and C6 come together forming the superior trunk. Then there's a middle trunk which is the continuation of C7. And C8 and T1 come together forming an inferior trunk of the brachial plexus. Now each of those trunks give rise to divisions. There are both anterior and posterior divisions so six divisions total. The anterior divisions are shown in yellow and the posterior divisions are shown in the shaded black and yellow. All of the posterior divisions come together forming a cord called the posterior cord of the brachial plexus. The anterior divisions of the superior and middle trunks form the lateral cord and the anterior division of the inferior trunk forms the medial cord. And then branches come off of the cords of the brachial plexus and these include the major nerves that travel through the brachial and anti-brachial regions. The ulnar nerve arises from the ventral rami of spinal nerve trunks C8 through T1. So C8 and T1 travel down the brachial plexus forming the ulnar nerve that branches off of the medial cord. And the ulnar nerve continues along the medial side of the arm and forearm. The ulnar nerve will excite flexors of the wrist and fingers including flexor digitorum profundus and flexor carpi ulnaris. The median nerve arises from spinal nerve trunks C6 through T1 that connects to both the lateral cord and the medial cord. And the median nerve travels down the anterior of the arm and forearm along the midline. So just lateral to the ulnar nerve is the median nerve and the median nerve excites most of the flexors of the wrist and digits that are muscles found here on the anterior region of the forearm. Including flexor carpi radialis and flexor digitorum superficialis. The musculocutaneous nerve arises from C5 through C7 and forms as a branch of the lateral cord that travels down the anterior of the arm in between two muscles that it excites the brachialis and biceps brachii. And as the name suggests cutaneous it also has branches that travel through the skin carrying sensory information from the anterior region of the arm. The radial nerve forms as a branch of the posterior cord and forms from the ventral rami of C5 through T1. So all of the five roots of the brachial plexus extend out connecting to the radial nerve. And the radial nerve travels down the posterior of the arm where it excites triceps brachii and continues down the posterior of the forearm as the name suggests it travels along the lateral side next to the radius. And it excites the extensors of the wrist and digits like extensor carpi radialis and extensor digitorum. The axillary nerve is a smaller branch coming off of the posterior cord. It arises from the ventral rami of C5 and C6 and as the name suggests it travels into the axillary region into the shoulder exciting the deltoids and teres minor. So here we can see major nerves of the brachial plexus as they're traveling down through the arm and forearm. We have the ulnar nerve along the medial side exciting the flexor carpi ulnaris and flexor digitorum profundus. The median nerve just lateral to the ulnar nerve travels along the midline of the arm and forearm and excites most of the muscles on the anterior of the forearm including flexor carpi radialis and flexor digitorum suberfishialis. The musculocutaneous nerve you can see travels down the brachial region in between biceps brachii and the brachialis and excites both of those muscles that flex the elbow joint. Here we can see the radial nerve as it's emerging on the lateral side of the forearm and the radial nerve will excite the extensor muscles on the posterior of the forearm. It also excites the extensors of the elbow joint and so extensors of the elbow joint like triceps brachii are excited by the radial nerve. The extensors of the wrist like extensor carpi radialis and extensors of the extensor digitorum are all muscles excited by the radial nerve. So these nerves are mixed functions so that means that they carry both motor and sensory impulses and so the sensory information from the region surrounding the muscles that are excited are also carried in through these nerves. Here we can see a posterior view of the arm showing the major nerves of the brachial plexus that are traveling through the posterior. We have the axillary nerve that branches off of the posterior cord and travels in to excite the deltoids and teres minor. And then here we see the radial nerve the larger branch that extends from the posterior cord travels down the posterior brachial region to excite the triceps brachii and then continues down into the anter brachial region where it excites the extensor carpi radialis and extensor digitorum. The lumbar plexus arises from the ventral rami of spinal nerve trunks T12 through L4. The major nerves that arise from the lumbar plexus are the femoral nerve which is the largest nerve branching off of the lumbar plexus which comes from the ventral rami of L2 through L4. There is a more superficial branch of the femoral nerve the lateral femoral cutaneous nerve which carries sensory information from the skin of the femoral region. There are some smaller branches coming off of the femoral nerve to excite the psoas major and iliacus the flexors of the hip. And then the largest portion of the femoral nerve continues deep down through the femoral region on the anterior of the femoral region and will then excite the extensors of the knee the quadriceps femoris group. Another major nerve that arises from the lumbar plexus is the obturator nerve. You can see here the accessory obturator as well as the larger obturator nerve. So just medial to the femoral nerve also arising from the ventral rami of L2 through L4 is the obturator nerve. As the name suggests the obturator nerve travels through the obturator foramen of the coxal bone and then it will travel down the medial side of the femoral region and excite the adductor muscles. Adductors of the hip like adductor magnus and adductor longus. So here we can see the femoral nerve as it's traveling down the anterior femoral region to excite the quadriceps femoris. Here we can see the obturator nerve as it's traveling down the medial femoral region in order to excite adductor longus and adductor magnus. Here's an illustration of the sacral plexus that arises from L4 through S4. The sciatic nerve is the largest nerve in the body extending from spinal nerve roots L4 through S3. So the ventral rami of L4 and S3 come together to form the sciatic nerve and then major divisions of that sciatic nerve we can see the tibial nerve and it says here common peroneal that's a synonym for the common fibular nerve. So here we see the major nerves of the sacral plexus as they're traveling down the posterior of the thigh and leg. Here's the sciatic nerve is the large nerve traveling through the posterior of the thigh and then the sciatic nerve branches to form a common fibular or common peroneal nerve and the tibial nerve. So the tibial nerve travels down through the popliteal space into the serral region where it's going to excite the plantar flexors like the gastrocnemius and soleus and the flexor digitorum longus, flexor helusus longus. Then the common peroneal or common fibular nerve is going to curve around to the anterior of the leg and then branch to form a superficial and deep branch. So here we can see the common fibular nerve as it's wrapping around the lateral side from the popliteal space and branching to form a superficial fibular nerve that runs down superficially on the lateral side in order to excite fibularis longus and fibularis brevis. Then we can see the deep fibular nerve that's traveling down along the tibia and excites the tibialis anterior muscle that performs dorsiflexion. Here we see a comparison of somatic and visceral reflexes. So the somatic reflexes are going to have an efferent pathway where a somatic motor neuron with its cell body in the anterior gray horn extends its efferent fiber out through the ventral root and then down a spinal nerve to reach an effector muscle, a skeletal muscle, and it will form a neuromuscular junction and release acetylcholine to stimulate contraction of the skeletal muscle. And a visceral reflex will have an autonomic efferent pathway where there is a preganglionic neuron, a neuron that has its cell body in the lateral gray horn of the spinal cord which extends its axon out through the ventral root to form a synapse in a ganglion, a cluster of cell bodies outside of the central nervous system. And so that neuron is the preganglionic neuron which extends its axon from the spinal cord to the ganglion of the peripheral nervous system. Then a postganglionic neuron extends its axon from the ganglion out to reach the effector organ like a smooth muscle as is shown here. And so there's two neurons in the efferent pathway for an autonomic efferent pathway, a preganglionic neuron and a postganglionic neuron. The preganglionic neuron will have a myelinated axon and so we'll see that myelinated axon traveling through the white ramus and the postganglionic neuron is unmyelinated. We'll see that if that axon returns back to the trunk of a spinal nerve it will travel through a gray ramus. And then when the postganglionic axon reaches the effector organ the axon doesn't necessarily have to form an axon terminal. Instead it forms bulges along the axon where neurotransmitter are released onto the effector organs which are called varicositis, postganglionic varicositis which release neurotransmitters onto smooth muscle, cardiac muscle and glands to regulate their activity. Here we can see the connections of the sympathetic division of the autonomic nervous system where there are short preganglionic fibers and long postganglionic fibers. The cell bodies of the preganglionic neurons are found in the thoracic and lumbar regions of the spinal cord in the lateral gray horn and extend out through the ventral root to form synapses with ganglia that are very close to the spinal cord. There are the chain ganglia also known as paravertebral ganglia of the sympathetic nervous system which are interconnected ganglia found just lateral to the spinal cord. And then there are prevertebral ganglia which are found anterior to the spinal cord within the ventral body cavities, the celiac ganglion, superior mesenteric ganglion and inferior mesenteric ganglion. Then the postganglionic fibers extend out from the ganglia to the effector organ and are shown in dotted lines here. These postganglionic fibers are much longer than the preganglionic fibers in the sympathetic nervous system. Let's look at a couple examples of effector organs that are regulated by the sympathetic nervous system. The sympathetic nervous system is often known as the fight or flight division of the autonomic nervous system and it will function to mobilize energy. We can see there are nerves extending out from the sympathetic chain ganglion to the heart from the thoracic region of the sympathetic chain ganglion. These postganglionic fibers extend into the heart and will increase heart rate and increase the force of contraction in order to increase the blood flow through the body to help mobilize energy and carry away waste. As we're exercising and dealing with a stressful situation, we want to have increased cardiac output, increased activity of the heart to mobilize energy and carry waste away. The sympathetic nervous system also travels to the respiratory system to smooth muscles of the trachea and bronchi where it will cause them to relax in order to decrease resistance in the airway to help increase airflow. Through the respiratory system to help with gas exchange to maximize the intake of oxygen in the release of carbon dioxide, the sympathetic nervous system regulates smooth muscles in the eye where it will increase the amount of light entering the eye dilating the pupil by stimulating the contraction of radial muscles of the iris and relax the circular muscles of the iris, opening the pupil, dilating the pupil to allow more light into the eye. Sympathetic fibers extend into digestive organs where they will inhibit the activity of the stomach and intestines, decrease motility and decrease the secretions of the digestive organs. The sympathetic nervous system will stimulate the release of nutrients from the liver. For example, glucose has been stored in the liver as a long chain of glucose molecules called glycogen. The fibers that extend from the sympathetic nervous system into the liver will release noradrenaline or norepinephrine as a neurotransmitter that will stimulate the release of glucose from the liver by activating enzymes that break down glycogen. The sympathetic nervous system also stimulates the adrenal gland. The adrenal gland is an endocrine organ found just superior to the kidneys in the abdominal cavity, and the adrenal medulla, the deep inner part of the adrenal gland, produces the hormones norepinephrine or noreadrenaline and epinephrine or norepinephrine. So those are synonyms. The norepinephrine is the same as noreadrenaline, which can be released as a neurotransmitter in the sympathetic nervous system or as a hormone. And then epinephrine is the same as adrenaline, and this is the primary hormone released by the medulla of the adrenal gland. And these hormones will then further coordinate a stress response, a fight-or-flight response throughout the body by releasing glycogen or breaking down glycogen or releasing glucose from the liver and stimulating the heart, increasing blood flow and inhibiting the activity of the digestive organs, decreasing the activity of the kidneys, so decreasing urine production and will prevent urination, so help to constrict the smooth muscle, the inner urinary sphincter muscle that is a smooth muscle that prevents urination. Here we see the sympathetic connections to the chain ganglia, the pre-ganglionic fibers that extend from the thoracic and lumbar regions of the spinal cord are going to travel out through the ventral root and then into the trunk of the spinal nerve and through a branch off of the trunk of the spinal nerve called the white ramus communicans. It's the white ramus because these pre-ganglionic fibers are heavily myelinated and the white ramus communicans connects from the trunk of the spinal nerve to the sympathetic chain ganglia. And then this fiber can form a synapse directly in the ganglion located at the same level as the spinal nerve trunk or it could continue up or down through the trunk of the chain ganglion to synapse with a neuron in another ganglion along the chain or a branch can project through the white ramus communicans and continue rather than forming a synapse in the chain ganglion it can continue through a splanknic nerve to a pre-vertebral ganglion also known as a collateral ganglion which these are found in the ventral body cavities within the thoracic and abdominal cavities but just anterior to the spinal cord so still located very close to the spinal cord and so the pre-ganglionic neurons have relatively short fibers relatively short axons traveling from the spinal cord to the ganglion and then the post-ganglionic fibers which travel from the ganglia are much longer extending out to the effector organs with there is one exception the pre-ganglionic fibers that travel directly from the lateral grey horn into the adrenal medulla do not form synapses with a post-ganglionic neuron instead they directly synapse with the endocrine cells in the adrenal medulla in order to stimulate the production of the stress hormones adrenaline and a lesser extent noradrenaline also known as epinephrine and norepinephrine here we can see the connections of the parasympathetic division of the autonomic nervous system the pre-ganglionic neurons are located in the brain within the brain stem or they're located within the lateral grey horn of the most inferior regions of the spinal cord that connect to the sacral spinal nerves these pre-ganglionic fibers are much longer in the parasympathetic division whereas the post-ganglionic fibers will be much shorter because the ganglia of the parasympathetic division are located either within the effector organ or very close to the effector organ and so the cranial nerves number 3, 7, 9 and 10 which are ocular motor, facial, glossopharyngeal and vegas all carry the efferent parasympathetic fibers that are the pre-ganglionic fibers of the parasympathetic nervous system we can see the ocular motor nerve travels to the ciliary ganglion which is located very close to the eye then the short post-ganglionic fibers travel into the eye in order to regulate smooth muscles in the iris that will be the circular muscles exciting the circular muscles in order to constrict the pupil to decrease the amount of light entering the eye cranial nerve 7 is extending out to the pteragopalatine ganglion in the facial region where it will stimulate the lacrimal gland to produce tears we can see cranial nerve 9, the glossopharyngeal nerve extends out into the otic ganglion and then the post-ganglionic fibers will travel into salivary glands like the parotid salivary gland the parotid sublingual and submaxillary salivary glands and the parasympathetic nervous system will stimulate the production of saliva the vegas nerve extends out into most of our visceral organs so we can see fibers traveling through the vegas nerve the pre-ganglionic fibers traveling towards the heart will form synapses with parasympathetic ganglia found within the heart that travel very short distances through the heart into regions in the heart called the pacemaker the auto-rhythmic cells in the heart that serve as a pacemaker to regulate the heart rate and the parasympathetic activity will decrease heart rate and will also stimulate the activity of the digestive system increasing the contraction of the smooth muscles in the stomach and intestines increasing the secretions of the glands like the pancreas that will help break down our food and increase blood flow through the digestive organs in order to help with absorption of nutrients and storage of nutrients in the liver we can also see there are the sacral nerves that are traveling down into the large intestine kidney bladder gonads and external genitalia in the urinary system in the kidney the parasympathetic division will stimulate urine production and stimulate urination by relaxing the smooth muscle of the internal sphincter and will also stimulate sexual arousal will stimulate the erection in the gonads or in the external genitalia now the neurotransmitter that's usually released by the post-ganglionic fibers of the parasympathetic division is acetylcholine acetylcholine is really always released by the pre-ganglionic fibers whether it's the sympathetic or parasympathetic division but the post-ganglionic fibers in the parasympathetic division also release acetylcholine whereas most of the post-ganglionic neurons of the sympathetic division release noradrenaline and so the major differences between the two there are short pre-ganglionic fibers and long post-ganglionic fibers in the sympathetic system found in the thoracic and lumbar region and the parasympathetic division found in the cranial and sacral region the craniosacral parasympathetic division has long pre-ganglionic fibers and short post-ganglionic fibers the post-ganglionic fibers release acetylcholine in the parasympathetic division and in the sympathetic division we get a more widespread response with the hormone adrenaline being produced and the neurotransmitters that are usually released by post-ganglionic fibers of the sympathetic division is noradrenaline