 In order to do neurological localization and figure out where in your patient the difficulty is, in addition to the cranial nerves, it's important to know three tracks or pathways. A track is a collection of axons coming from a similar place and going to a similar location. Now, this track has junctions or connections, or we call them synapses in between. But the two sensory tracks come from the sensory dorsal roots. Here I'm showing you the catechinia because they've been torn off up here. And a sand, either connecting in the cord or in the medulla, relay in deep structures called the thalamus and eventually come to perception in the cerebral cortex. So one pathway, one sensory pathway, is the spinal from the cord to the thalamus, the spinal thalamic tract. The other, and that carries pain and temperature and some touch. The second important pathway is the pathway that we call the dorsal column because some of the information is traveling in these dorsal columns, this area of white matter between the forceps here, on the dorsal aspect or the posterior columns of the spinal cord, relaying up in the medulla, and again onto the thalamus and finally the cerebral cortex. That pathway is called the dorsal column medial lmniscus pathway and that carries proprioception, vibration, joint position, knowing where your body and muscles are in space, as well as fine and discrete touch. So we have two sensory, somatic sensory pathways coming from the body and there are comparable pathways just like it coming in from the trigeminal nerve. But we're just going to talk about the spinal cord contribution today. The spinal thalamic or interlateral pathway has three neurons. It is always the second neuron that decosates or crosses to the opposite side to ascend to the thalamus. The first neuron has its cell body in the dorsal root ganglion and synapses almost immediately in the spinal cord. The second neuron crosses to the opposite side and ascends to the brainstem, to the ventral posterior lateral nucleus of the thalamus. The third neuron, located in that nucleus of the thalamus, ascends through the internal capsule to terminate in the post-central gyrus, which is primary somatic sensory cortex. The second pathway for sensation from the body, the dorsal column medialumniscus pathway, has a very similar pattern and trajectory except for one very important difference. The first neuron, the one with the cell body in the dorsal root ganglion, does not cross immediately but ascends on the same side of the spinal cord to the posterior part of the medulla. There it meets neuron number two and crosses to ascend to the thalamus on the opposite side. The decosation then is in the caudal medulla. It too ascends to the same thalamic nucleus, ventral posterior lateral, and different neurons in that nucleus then send their axons, these are the third neurons, through the internal capsule to the post-central gyrus primary somatic sensory cortex. The third pathway is a motor pathway that comes from the cortex and descends through the spinal cord or down as far as the brainstem cranial nerves to end in the cord or brainstem on motor neurons that go out then to muscles. Those would be muscles, striated muscles, voluntary willed movement, controlling my little finger or my big thumb or pinching, all of that are willed, not reflexes, these are willed responses. This is the corticospinal tract or the motor pathway. The important motor pathway for voluntary control over your striated muscles is called the corticospinal pathway. There are axons traveling with this pathway, coming from the face area of the cortex, pre-central gyrus, also terminating in the brainstem on cranial nerve motor nuclei. This diagram shows you only one going to the spinal cord, descending through the internal capsule, through the brainstem, and crossing, important crossing or decussation at the junction between the medulla and the spinal cord. At this point, left brain controls right body. This neuron, this motor control unit is called the upper motor neuron because it comes from upstairs in the cortex. It descends and terminates in the cord. Therefore, we call the next or second neuron the lower motor neuron. The lower motor neuron is nothing more than the anterior horn cell, also called the ventral horn cell, or just plain motor neuron. This neuron sends its axon through the ventral root to terminate as the motor implant on striated muscle. So, three pathways we're going to look at. Corticospinal motor, spinal salamic pain and temperature, dorsal column, medial amniscus, fine touch, vibration and proprioception. The critical thing is to be able to visualize where they are close to cranial nerves and where they are far apart from them at different levels of the brainstem. So, we're going to come back and have to look at our levels of the brainstem. We're going to look at medulla, we're going to look at pons, and we're going to look at the midbrain as we ascend. We're also going to have to bring this information up to the cerebral cortex. I have a atrophied brain here which makes the gyri stand out remarkably because there's been so much loss of brain tissue that these sulci are deep and these gyri are narrow. And remember, to find the central sulcus you find two parallel gyri that come from the interhemispheric fissure all the way down to the lateral fissure. So, this is the central sulcus. And keep in mind that behind the central sulcus is the postcentral gyris and this is sensory cortex. And in front of the central sulcus is the precentral gyris and that is what we call motor cortex. So, the willed voluntary movement is going to start with neurons in the precentral gyris and the perception of somatic sensation is going to end in the postcentral gyris which is somatosensory cortex.