The initiation and regulation of movements require a complex set of events that integrate information from many regions of the brain, brain stem, and spinal cord (Figure 2-3). When an action potential is generated in the brain, it travels along axons and down the spinal cord via the corticospinal tract to the motor neurons at speeds upwards of 100 meters per second, resulting in contraction of a muscle and a movement. However, before it reaches the motor neurons, the information is modulated by neurons found in the basal ganglia, cerebellum, and brain stem. When the signals finally reach the motor neurons, these specialized nerve cells provide the final conduit for the transmission of the signals to muscles throughout the body, stimulating muscles to contract. Thus, an injury or disruption to the motor pathways leading to and from the brain could cause a patient to lose motor function.
The circuitry between the primary motor cortex and the motor neurons of the ventral horn of the spinal cord is very complex. Many regions of the CNS, including the basal ganglia, cerebellum, and brain stem, help regulate movements (Figure 2-3). The degree of cortical control varies depending on the motor function. For example, movement of the fingers requires more integration from the brain than gross movement of the legs, which relies more on circuitry confined to the spinal cord. The majority of the signals from the brain are transmitted along bundles of axons that make up the corticospinal tract, which connects the primary motor cortex in the brain to the motor neurons in the ventral horn of the spinal cord. The motor neurons in turn transmit the information from the ventral horn directly to the muscle. Motor control of most other body parts involves additional circuitry, or connections, between the primary motor cortex and the motor neurons. Signals are transmitted either from the primary motor cortex to intermediate layers within the spinal cord to modulate the tone or reflex gain and to cause direct contraction of the muscles or through intermediate processing stages in the midbrain or pons of the brain stem.
In addition to regulating voluntary movements, neurons in the descending motor tracts traveling from the brain down to the spinal cord are also responsible for regulating the smooth muscles of internal organs. Descending motor tracts also contain neurons associated with the autonomic nervous system, which regulates blood pressure, body temperature, and the body’s response to stress.