I'm going to talk about nerves! A nerve (shown here in yellow, which is common for anatomy, it's just a tradition to show nerves in yellow); a nerve is an enclosed cable-like bundle of nerve fibers, aka axons. It's a bundle of axons in the peripheral nervous system. A nerve transmits electrical impulses and it's the basic unit of the peripheral nervous system. A nerve provides a common pathway for nerve impulses (action potentials) that are transmitted along each of the axons to the peripheral organs, or, in the case of sensory nerves, from the periphery back to the central nervous system. Each axon within the nerve is an extension of an individual neuron, along with other supportive cells, such as some Schwann cells that coat the axons in myelin. We have peripheral nerves that come from the brain and that come from the spinal cord. The 12 pairs of nerves that come from the brain are cranial nerves 1 through 12. We've mapped the foramen that these cranial nerves travel through in the skull, and we'll study these 12 pairs of cranial nerves more in lab. Cranial nerves can be sensory nerves, motor nerves, or both sensory and motor. Spinal nerves are all both. That is, they have sensory and motor axons within them. Here's the structure of a peripheral nerve: they're modular, they're wrapped in connective tissue just like muscle fibers were, and they have many similar names to the connective tissues that wrap around the modular structures of muscles. The very smallest structure is the axon, and the axon would be enclosed by a cell membrane, and then it would be surrounded by neuroglia, Schwann cells in the form of a myelin sheath, and then all of that the axon and potentially the myelin sheath are wrapped in a connective tissue layer called the endoneurium. Remember we had endomysium around individual muscle fibers? Around individual axons is wrapped endoneurium. Then, axons that are wrapped together in a bundle are referred to as a fascicle, same as we saw in the muscles, and that whole fascicle is enclosed in perineurium, and then all of those fascicles that are wrapped together make up one nerve, and that nerve is wrapped in epineurium. So we have epineurium covering the nerve, enclosing the nerve, perineurium enclosing the fascicles, the bundles of axons within the nerve, and the endoneurium which encloses individual axons, individual nerve fibers. On the left is a microscopy image. The hollow spaces are the blood vessels that are traveling within the nerve, the arteries and veins that are carrying blood and traveling within the nerve. For spinal nerves, we see a relatively consistent anatomical structure and pattern of distribution. Spinal nerves emerge from the spinal cord in two different places: on the dorsal side and on the ventral side. The dorsal roots are afferent and they form a dorsal root ganglion, where the cell bodies are. These cell bodies would be pseudo-unipolar, they're sensory. The ventral roots are efferent, they have their cell bodies in the gray matter of the spinal cord, so they don't have a ganglion, and they merge to form the spinal nerve. So we have dorsal root which is sensory, and ventral root which is motor, and they merge to form a spinal nerve. The dorsal ramus is innervating the muscles, joints, and skin of the back, and the ventral ramus is traveling to innervate the structures of the lateral...the anterior trunk and the limbs. Both the dorsal ramus and the ventral ramus have sensory and motor axons traveling in them, so they're a split between fibers that have come from the dorsal root and that come from the ventral root. There can also be communicating rami that are present in the thoracic and the superior lumbar segments, and these contain axons of the sympathetic nervous system. We'll learn more about that toward the end of the nervous system unit. So we have spinal nerves originating from the spinal cord and we can now see the neurons illustrated here. That dorsal root is illustrated in blue, and the circles are meant to illustrate the nuclei of individual cells. So the nuclei of these sensory axons that are present out here in the dorsal root ganglion, they're synapsing on neurons that are present in the gray matter of the spinal cord. For the ventral root, they have their cell bodies within the gray matter of the spinal cord, and then their axons travel out as part of the ventral root. They join together, the dorsal root and the ventral root, to make the spinal nerve and then both sensory and motor axons extend out to the body through the dorsal ramus and through the ventral ramus. A good way to remember afferent versus efferent neurons is that afferent arrives A A and efferent exits E E. Afferent neurons are neurons whose axons travel towards or bring information into a central point, while an efferent neuron is a cell that sends an axon or carries information away from a central point. So these blue neurons, these sensory neurons, would be afferent neurons. They're arriving at the spinal cord, and then these neurons illustrated in red would be efferent, they're exiting the spinal cord, and they're carrying motor commands to effectors. A way to remember this division within the spinal cord is to remember the phrase SAME (write this same that's supposed to be an e) DAVE and if you remember SAME DAVE you'll remember "sensory afferent, motor efferent". Sensory is afferent, motor is efferent. Then DAVE is "dorsal afferent, ventral efferent". The dorsal side of the spinal cord is afferent and sensory ("sensory afferent") and the ventral side is motor and efferent, and that's the general pattern that we'll see throughout the spinal cord. So in the sensory line we have pseudo-unipolar cells with their cell bodies in the dorsal ganglia, that's what the blue cells are. Illustrated in gray are interneurons in the central nervous system, and then for the red neurons, those motor efferent lines, that's a multi-polar cell with its cell body in the central nervous system, controlling skeletal muscle and exiting through that ventral route.