 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, and 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 schwan cells, the glial cells that are supporting the axon, producing the myelin insulating myelinated axons in nerves of the peripheral nervous system. We'll start by going through the cranial nerves, and these are numbered in order 1 through 12. So we'll 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. 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. Sending out the motor commands that excite the contraction of most of the eye muscles, and 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, 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. 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. The glossopharyngeal nerve, the name tells us it's carrying information from the tongue, the glosso, and the pharynx, the throat, the pharyngeal. So glossopharyngeal is both a sensory and a motor nerve. Sensory information includes taste as well as there are receptors and 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. 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 the muscles of the tongue. For example, the genioglossus, 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. So 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 phalamus. 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. 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. 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. 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 stylonastoid foramen to reach the face where it can excite the muscles of facial expression. 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. Here we can see the glossopharyngeal cranial nerve number nine, the vagus cranial nerve number ten and the accessory nerve cranial nerve number eleven 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. And then the accessory nerve cranial nerve number eleven is a motor nerve and it excites contraction of the trapezius and deltoids. 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. Those neuronal cell bodies are surrounded by neuroglial cells called satellite cells that help to regulate the chemical environment surrounding the neuron. Here we see a micrograph of the layers of a spinal nerve where we can see the perinarium that is surrounding a fascicle. There are numerous fibers inside the fascicle, each one surrounded by a layer of endonurium. 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 epinerium. The three layers of connective tissue are endonurium deep inside around the fibers perinerium surrounding the fascicles and epinerium 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 are only seven cervical vertebrae and there are eight cervical spinal nerves. 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. 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. And 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 exciting 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 in 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 branch just to form a deep and superficial branch of the fibular nerve. 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. Now the brachial plexus can be broken down into the roots which are the ventral ramii that come together and then when the ventral ramii come together they form what we call trunks. And so there are superior middle and inferior trunks. And then arising from the trunks are what we call divisions. And so there are three posterior divisions that come off of the superior middle and inferior trunks and come together to form what's known as the posterior cord. And then there are three anterior divisions and the anterior divisions from the superior and middle trunks come together to form what's known as the lateral cord. Whereas the anterior division of the inferior trunk forms what's known as the medial cord. Then extending off of the cords we have branches which are the major spinal nerves traveling through the arms. The ulnar nerve forms as a branch of the medial cord and the ulnar nerve will then travel down through the medial side on the anterior of the forearm and it will excite the flexors of the wrist and fingers that are found there along the medial side. For example the flexor carbide ulnaris and flexor digitorum profundus. The median nerve forms from branches of the lateral and medial cords and travels down the anterior of the forearm just lateral to the ulnar nerve and excites most of the flexors of the wrist and fingers that are found on the anterior of the forearm. For example flexor carbide radialis, flexor digitorum superficialis are both good examples. The musculocutaneous nerve forms as a branch from the lateral cord and will travel into the brachial region to excite the flexors of the elbow joint like the biceps brachii and brachialis. The radial nerve forms as a branch from the posterior cord. The radial nerve will travel down the posterior of the arm and through the posterior and lateral regions of the forearm, so as the name suggests the radial nerve does travel along the lateral side of the forearm and it will excite the muscles on the posterior of the forearm like the extensors of the wrist and fingers for example extensor carbide radialis or extensor digitorum. The radial nerve also excites the extensors of the elbow joint like the triceps brachii and the axillary nerve forms from a branch of the posterior cord that travels into the shoulder in order to excite the deltoid muscles as well as the 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 carbide 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 interior of the forearm including flexor carbide radialis and flexor digitorum superficialis. 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 and also excites the extensors of the elbow joint and so extensors of the elbow joint like triceps brachii are excited by the radial nerve and the extensors of the wrist like extensor carbide 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 entrebrachial region where it excites the extensor carbide radialis and extensor digitorum. Here we see an illustration of the lumbar plexus. The lumbar plexus arises from L1 through L4 and the major nerves coming off of the lumbar plexus include the femoral nerve arising from L2 through L4 and the obturator nerve also arising from L2 through L4. The femoral nerve will run down the anterior of the thigh the anterior femoral region it will excite the muscles that extend the knee joint the quadriceps femoris. It will also excite the muscles that flex the hip joint so the psoas major and iliacus are excited by the femoral nerve and then the femoral nerve also carries sensory information from the anterior femoral region. Now the obturator nerve is a smaller branch coming off of the lumbar plexus which travels down the medial side of the femoral region where it excites the adductors of the leg like adductor magnus adductor longus and will carry sensory information from that medial region of the groin. 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 grey horn extends its efferent fiber out through the ventral root and then down a spinal nerve to reach an effector muscle, a skeleton 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 pre-ganglionic neuron, a neuron that has its cell body in the lateral grey 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 pre-ganglionic neuron which extends its axon from the spinal cord to the ganglion of the peripheral nervous system. Then a post-ganglionic 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 pre-ganglionic neuron and a post-ganglionic neuron. The pre-ganglionic neuron will have a myelinated axon, and so we'll see that myelinated axon traveling through the white ramus, and the post-ganglionic neuron is unmyelinated. We'll see that if that axon returns back to the trunk of a spinal nerve, it will travel through a grey ramus. And then when the post-ganglionic 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 varicocities, post-ganglionic varicocities, which release neurotransmitters onto smooth muscle, cardiac muscle, and glands to regulate their activity. Here we can see the connections between the sympathetic nervous system and the effector organs that are being regulated by the sympathetic division of the autonomic nervous system. And so the sympathetic division can be called the thoracolumbar division. It's commonly referred to as the fight-or-flight division of the autonomic nervous system, because it functions to mobilize energy reserves and help us deal with a stressful situation. The sympathetic nervous system will become activated during exercise and during stress. And it's called the thoracolumbar region because we see the pre-ganglionic neurons are found in the lateral gray horns in the thoracic and lumbar regions of the spinal cord, and they extend out through the thoracic and lumbar spinal nerves. And so the target organs, the effectors that are being regulated include the eye. So the smooth muscles inside of the eye will be stimulated to pull open the iris to allow more light into the eye. So we have dilation of the pupil in response to this sympathetic pathway. And so notice that the ganglion for the sympathetic nervous system is located on either side of the vertebral column. We call this the paravertebral ganglion or the chain ganglion. So it's called the chain ganglion because these ganglia are all interconnected through nerves. They're forming an interlacing network of nerves, so they sometimes also called aplexis. And so the paravertebral ganglion are very close to the spinal cord. And so the pre-ganglionic neuron has a short axon in the sympathetic division, extending from the lateral gray horn out to the paravertebral ganglion. And then the post-ganglionic neuron shown with the dotted line will extend from the paravertebral ganglion out to the effector organs like the eye. Or it can also extend out to the heart in order to stimulate contraction of the heart, increase the heart rate, and increase the force of contraction. They'll extend out to the digestive organs in order to decrease the activity of the digestive system. And they'll extend out to the urinary system in order to decrease urine production and stimulate the external genitalia during sexual intercourse. The sympathetic nervous system will stimulate the orgasm and ejaculation. So here we can see three pathways for the axon from the central sympathetic neuron in the lateral gray horn to travel out to form a synapse. A fiber can project out to a ganglion that's at the same level. And so that's what we see in number one here. At the top, this pre-ganglionic neuron in the lateral gray horn extends its axon out through the ventral root, then down through the white ramus to then synapse with a neuron, a post-ganglionic neuron that has its cell body in the paravertebral ganglion. And then that neuron can extend its axon out through a nerve to reach an effector organ. And so the axon of the pre-ganglionic neuron here in the sympathetic division is short, whereas the post-ganglionic axon is long extending from the paravertebral ganglion out to the effector organ. B here, the second example, shows that a branch can project to a more superior or inferior ganglion in the chain. And so the pre-ganglionic fiber extends out through the ventral root, then through the white ramus and up through the chain ganglion to reach a post-ganglionic neuron cell body that's either further superior or inferior. And then the last pathway C here shows that there are some pre-ganglionic neurons that can project their pre-ganglionic axon out through the ventral root, then through the white ramus, and then down through a spinal nerve like the splactic nerve to another ganglion that's found a little bit further away from the spinal cord. And so this could be a pre-vertebral ganglion found anterior to the spinal cord, closer to the visceral organs, and one example will be an axon extending all the way to the adrenal gland, to the medulla of the adrenal gland where it will stimulate the adrenal gland to produce a hormone called adrenaline or epinephrine. In contrast, this diagram is showing us the connections of the parasympathetic division of the autonomic nervous system. And so the parasympathetic division can also be called the craniosacral division. We see that the pre-ganglionic fibers travel through cranial nerves and sacral nerves. And so it's parasympathetic because the cranial and sacral nerves are on either side of the thoracolumbar region where the sympathetic system is set up. Now the pre-ganglionic fibers are long in the parasympathetic division extending all the way from the brain and spinal cord out to the ganglion found very close to the effector organs. And so we can see the example of the regulation of the eye, the iris muscles in the eye are regulated also by the parasympathetic nervous system, where the pre-ganglionic axon travels through cranial nerve 3 to the ciliary ganglion in the posterior orbit. And then the post-ganglionic fiber travels from the ciliary ganglion a very short distance into the iris where it will excite the contraction of circular muscles in the iris that cause constriction of the pupil in order to decrease the amount of light entering the eye. Another pathway we can look at is traveling down to regulate the activity of the heart for example. So cranial nerve 10, the vagus nerve, contains the pre-ganglionic fibers that extend out from the medulla oblongata all the way down to a ganglion located surrounding the heart. And so the ganglion inside of the heart then has the cell bodies of the post-ganglionic neurons that extend a very short distance through the heart in order to slow down the heart rate. And so the parasympathetic nervous system is also known as the rest and digest system. So the vagus nerve will also stimulate the activity of the digestive organs where there's going to be a long pre-ganglionic fiber extending down from the medulla oblongata to a ganglion located very close to the digestive organs. Then the post-ganglionic axon is very short traveling into the effector organ in order to increase the activity of the digestive organs. It will stimulate the smooth muscles of the intestines in order to increase the motility, the contractions that move the contents through the digestive system. It will stimulate the glands like the pancreas and the liver and bile ducts to produce secretions that help with digestion. It will stimulate the activity of the urinary system to increase urine production. And it will stimulate the external genitalia in order to cause arousal to stimulate erection.