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A neural pathway connects regions within the brain to one another or conveys information from the peripheral nervous system to the brain. Two major classes of neural pathways relay sensation to the brain or carry signals for movement to the body from it. They both consist of long, insulated nerve fibers that communicate electrically or by a chemical neurotransmitter. Spinal reflex pathways are local pathways that provide quick responses to sudden stimuli without feedback from the brain.
The long nerve axons that comprise a neural pathway are called white matter because of their insulating substance, myelin. This insulation improves electrical conduction speed over the distances axons cover in the body. In the brain, local connections can occur between unmyelinated neuronal cell bodies, called gray matter. Some neural pathways are actually distinct fibers that serve different functions, as with the corpus callosum, which connects many regions of the right and left hemispheres of the brain.
Some reflex pathways operate without the brain. In the knee-jerk reflex, a sensory neuron coming from the knee synapses with a motor neuron in the spinal cord, which causes a muscle to contract in one leg and the same muscle to relax in the opposite leg. The process happens more swiftly than it would if mediated by the brain. More complex neural pathways are not truly reflexive, but instead receive feedback from the higher regions of the brain, like the cerebral cortex. This slows down the rate that signals travel through them.
A sensory neural pathway relays sensation from the body to the brain, while motor pathways carry instructions from the brain to the muscles that control voluntary movement. One motor neural pathway is the corticospinal or pyramidal tract. Running from the motor region of the cortex to the spinal cord, the pyramidal tract then crosses into two separate pathways that each control movement on one-half of the body. The corticobulbar tract moderates voluntary movement of the facial muscles, and runs from the cortex to the brain stem nuclei that control the cranial nerves of the face. The arcuate fasciculus pathway connects the neurons that process speech recognition with those needed to imitate sounds vocally.
Within the brain, some pathways work through a specific chemical messenger, called a neurotransmitter. For instance, dopamine is used in many pathways to effect motivation, reward, and fine-motor control, among many other functions. Dopamine is the neurotransmitter of the mesocortical neural pathway that modulates reward behavior from the midbrain to the frontal lobes of the cerebral cortex. Since the dopamine-based nigrostriatal pathway assists fine motions, it is frequently diseased in movement disorders such as Parkinson's disease.
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