Spinal Cord

input from many sources and sends input out to other places, making it crucial for all autonomic functions, postural reflexes, task, sleep, and wakefulness.

There are also the corticospinal tract that starts in the precentral gyrus, travels down to the pons, to the medulla, and finally to the spinal cord for the transfer of movement into action. The corticobulbar tracts are similar but travel to where each motor cranial nerve will originate. Many of these will decussate but again, not all of them do this.

The sensory tracts involve the anterolateral system or the spinothalamic tracts that convey the senses of pain and temperature to the most dorsal aspect of the spinal cord. As you will learn, the dorsal part of the spinal cord is generally sensory, while the ventral part is generally motor in nature. These fibers will also decussate but do this in the spinal cord itself. They travel up to the post-central gyrus of the cortex.

There is a dorsal column-medial lemniscus tract that carries fine touch, two-point discrimination, vibration, and proprioceptive sense upward to the brain. These will decussate at the medulla and later become the medial lemniscus that stays centrally located as it travels up to the thalamus for processing before it reaches the cortex.

The trigeminal lemniscus and the spinotrigeminal tract start at the fifth cranial nerve and carry fibers from the face to the spinal trigeminal nucleus and finally to the spinotrigeminal tract. These fibers will decussate to travel up to the brain via the trigeminal lemniscus.

The lateral lemniscus is what carries auditory information from the cochlear nuclei at the level of the lower pons up to the superior olivary complex and other nuclei, where the auditory information eventually reaches the temporal lobes where they get interpreted and processed.

SPINAL CORD

The spinal cord is housed in the vertebral canal, which is made by central holes in seven cervical, twelve thoracic, five lumbar, and five sacral vertebrae. These connect to make the spinal column that starts in the foramen magnum or output hole at the bottom of the cranium and ends at the coccyx at the base of the spine. There are 31 spinal

segments, including 8 cervical segments, 12 thoracic segments, 5 lumbar segments, 5 sacral segments and a vestigial coccygeal segment.

Out of several sections along the spinal cord, there are spinal nerves that will come off the spinal cord at nearly regular segments. There are sensory nerves that enter the front of the spinal cord and motor nerves that enter the back of the spinal cord. Most cervical nerves emerge from above the named vertebra except for C8 that emerges between C7 and T1. Below this level, they all emerge from the named vertebra above it.

The cone-shaped structure beneath the spinal cord is called the conus medullaris. It has a number of spinal nerves coming off of it called the cauda equina. Even though the spinal column ends at the coccyx, the spinal cord itself ends at L1 to L2.

As mentioned, the motor part of the spinal cord receives anterior motor nerve roots called ventral nerve roots in the front of the spinal cord. There are six reflexes you can identify at any of six different sites in the spinal column. The dorsal roots are the sensory or incoming roots that correspond to different dermatomes. A dermatome is a stretch of skin on one side of the body that will be innervated by a single spinal nerve. Figure 18 shows the different dermatomes of the body:

Figure 18.

The spinal cord is complex, with ascending and descending tracts of fibers that send information from one place to the next up and down the cord. The corticospinal tract is a large ascending motor tract. It is a large myelinated tract starting in the motor cortex and goes to all areas of the body. There are Betz cells in the cortex where these nerve fibers start from in the first place.

There are ascending sensory tracts mainly seen dorsally on the cross-section of the spinal column. There are separate tracts for proprioception, vibratory sense, and discriminatory touch. These are also myelinated tracts that handle different parts of sensation. Figure 19 shows a cross-section of the spinal cord:

Figure 19.