Nervous System II

Understand the organisation of the Nervous System

The nervous systems is divided into two main divisions: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain, spinal cord, and cranial nerves I and II (olfactory and optic, respectively), whereas the peripheral nervous system consists of everything else. Both parts of the nervous system consist of nerves as well as other supporting cells.

Another term that you might hear is "autonomic nervous system." The autonomic system refers to the bits of the nervous system that take care of most functions that we don't generally think about but keep us alive- stuff like breathing and pumping blood. The autonomic nervous system encompasses parts of the central and peripheral nervous system, but mostly the peripheral nervous system.

Nerves come in many shapes, but have a few main components. Firstly, they have dendrites, which receive information from surrounding cells. Dendrites are connected to a cell body, where most of the organelles are. The cell body then leads into one or more axons, which carry action potentials away from the cell body. Some nerves might be myelinated- that is, they have a fatty sheath wrapping around the axon, allowing for faster conduction. (You can read about that here.)

Recognise the components of a simple reflex

Pretty much every control pathway in our body has three main components: something to pick up a signal from the environment, somewhere to process the information, and something to cause a change. Simple reflexes are no different. For instance, in the knee jerk reflex, various proprioceptors in the muscle and muscle tendon respond to the sudden lengthening of the muscle by sending a signal to the spinal cord. Information is processed in the spinal cord and output to the muscle, which responds by shortening the muscle.

Be able to explain the spinothalamic, proprioceptive, and corticospinal pathways

Spinothalamic / Anterolateral

The spinothalamic or anterolateral pathway is the main pathway for detecting pain, temperature, and touch. It is a slow pathway that consists of mostly thin, unmyelinated neurons that give poorly-defined sensations. There are three nerves involved in the spinothalamic pathway: one to go from the area of the sensation to the spinal cord, another to go from the spinal cord to the thalamus for integration, and a third to go from the thalamus to the sensory cortex. In the spinal cord, the second nerve crosses over from one side of the spinal cord to the other, so any kind of pain or touch felt on the left side of the body is processed by the sensory cortex in the right side of the brain.

The CNS has a similar system to the spinothalamic system: the trigemino-thalamic system that picks up sensations from the trigeminal nerve. It is also a three-neuron relay that crosses over and sends its output to the thalamus, and then the sensory cortex. (I *think* this pathway also sends proprioceptive input a la the next system that we're going to discuss, just to make things extra confusing.)

Proprioceptive / Dorsal Column-Medial Lemniscal (DC-ML)

The proprioceptive or DCML pathway is the main pathway for detecting proprioception and joint position. In contrast to the spinothalamic pathway, the DCML pathway consists mainly of large, myelinated neurons that are fast and give precise sensations. Once again, there are three nerves involved, but this time the nerves don't cross over until they get to the brainstem.

Corticospinal

The corticospinal pathway is the main pathway for motor output. In contrast to the previous two pathways, which had three-neuron relays, the corticospinal pathway is a two-neuron relay: an upper motor nerve in the motor cortex synapses with a lower-motor nerve located either in the spinal cord or brainstem. The corticospinal pathway crosses over in the base of the brainstem.