Neurological syndromes are often divided into those caused by focal lesions versus those caused by diffuse lesions, based on the distribution of dysfunctional tissue in the nervous system. The word diffuse in this context refers to widespread dysfunction of both sides of a few types of tissue, such as the brain, the nerves, or the skeletal muscles. Even though muscle is not part of the nervous system, syndromes involving weakness from diffuse dysfunction of skeletal muscle are usually covered with neurological syndromes, because diffuse weakness is often from nervous system dysfunction instead. The string-like nerves of the peripheral nervous system are involved in the lower neural functions, so that dysfunction of nerves may cause abnormalities of sensory, motor, or autonomic functions. These abnormalities may be widespread on both sides if there is diffuse dysfunction of the peripheral nervous system, or these abnormalities may just involve a body part on one side with a focal lesion, such as dysfunction of just one nerve.
Axons in nerves connect the tissues of the limbs and torso to the spinal cord, for the most part, although some nerves connect directly to the brain, mainly from tissues of the head and neck. These nerves form right and left pairs that are attached to parts of the brain or spinal cord. The nerves that pass through the skull, or cranium, are called cranial nerves, which are mostly attached to the brain. The nerves that pass through the spine are called spinal nerves, and each pair of these attaches to one of the many levels of the spinal cord.
The spinal cord is a long thin structure in the spine which is about as thick as a thumb. Axons travel up and down in the spinal cord to connect the brain above to most of the peripheral axons that travel in nerves to the limbs and torso. A complete lesion of one level of the spinal cord may cause abnormalities of sensory, motor, or autonomic functions on both sides for the parts of the limbs or torso that are connected to that level of the spinal cord, and also for all the levels below the lesion, if the lesion is severe enough to disconnect the lower levels from the brain above.
The brain is divided into the cerebrum, which is on top, then the brainstem, which is below the cerebrum and above the spinal cord, and then the cerebellum, which is behind the brainstem. The brainstem is above the spinal cord, but it is short and little thicker. Axons that connect to and pass through the brainstem are involved in the lower neural functions of the entire body, so that a brainstem lesion may cause sensory, motor, or autonomic abnormalities of many parts of the body. Areas of the brainstem also connect to the cerebrum to contribute to the higher neural functions, so that a brainstem lesion may also cause abnormalities of cognition, emotion, or consciousness. The cerebellum is a rounded structure that coordinates movements and performs some other functions. Cerebellar lesions often cause incoordination.
The cerebrum is a larger rounded structure divided into the right and left cerebral hemispheres. It plays a role in almost all the neural functions, particularly the outermost layer called the cerebral cortex. Focal lesions of the cerebrum may cause many syndromes. Focal lesions of the cerebrum often cause sensory or motor abnormalities of the other side of the body, because many of the pathways that carry this information cross from one side to the other inside the central nervous system. Diffuse dysfunction of the cerebrum, particularly the cerebral cortex, may cause abnormalities of higher neural functions with or without also involving the lower neural functions, depending on which areas are affected.