brain, spinal cord, and nerve disorders
Weakness
Introduction
Weakness refers to loss of muscle strength. That is, people cannot move a muscle normally despite trying as hard as they can. However, the term is often misused. Many people with normal muscle strength say they feel weak when the problem is Fatigue or when their movement is limited because of pain or joint stiffness. Muscle weakness can be a Introduction to Symptoms of Brain, Spinal Cord, and Nerve Disorders . For a person to intentionally move a muscle (called a voluntary muscle contraction), the brain must generate a signal that travels a pathway from The brain Through nerve cells in the brain stem and spinal cord Through nerves from the spinal cord to the muscles (called peripheral nerves) Across the connection between nerve and muscle (called a neuromuscular junction) Also, the amount of muscle tissue must be normal, and the tissue must be able to contract in response to the signal from the nerves. Therefore, true weakness results only when one or more part of this pathway―brain, spinal cord, nerves, muscles, or the connections between them―is damaged or diseased. Moving a muscle usually involves communication between the muscle and the brain through nerves. The impetus to move a muscle may originate in the brain, as when a person consciously decides to move a muscle—for example, to pick up a book. Or the impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to sense pain or a change in temperature. This sensory information is sent to the brain, and the brain may send a message to the muscle about how to respond. This type of exchange involves two complex nerve pathways: The sensory nerve pathway to the brain The motor nerve pathway to the muscle If sensory receptors in the skin detect pain or a change in temperature, they transmit an impulse (signal), which ultimately reaches the brain. The impulse travels along a sensory nerve to the spinal cord. The impulse crosses a synapse (the junction between two nerve cells) between the sensory nerve and a nerve cell in the spinal cord. The impulse crosses from the nerve cell in the spinal cord to the opposite side of the spinal cord. The impulse is sent up the spinal cord and through the brain stem to the thalamus, which is a processing center for sensory information, located deep in the brain. The impulse crosses a synapse in the thalamus to nerve fibers that carry the impulse to the sensory cortex of the cerebrum (the area that receives and interprets information from sensory receptors). The sensory cortex perceives the impulse. A person may then decide to initiate movement, which triggers the motor cortex (the area that plans, controls, and executes voluntary movements) to generate an impulse. The nerve carrying the impulse crosses to the opposite side at the base of the brain. The impulse is sent down the spinal cord. The impulse crosses a synapse between the nerve fibers in the spinal cord and a motor nerve, which is located in the spinal cord. The impulse travels out of the spinal cord along the length of the motor nerve. At the neuromuscular junction (where nerves connect to muscles), the impulse crosses from the motor nerve to receptors on the motor end plate of the muscle, where the impulse stimulates the muscle to move. If the sensation occurs suddenly and is severe (as when stepping on a sharp rock or picking up a cup of very hot coffee), the impulse may travel to the spinal cord and directly back to the motor nerve, bypassing the brain. The result is a quick response of a muscle—by immediately withdrawing from whatever is causing the pain. This response is called a spinal reflex. Weakness may develop suddenly or gradually. Weakness may affect all of the muscles in the body (called generalized weakness) or only one part of the body. For example, depending on where the spinal cord is damaged, spinal cord disorders may cause weakness only of the legs. Symptoms depend on which muscles are affected. For example, when weakness affects muscles of the chest, people may have difficulty breathing. When weakness affects muscles that control the eyes, people may have double vision. Complete muscle weakness causes paralysis. People may have other symptoms depending on what is causing the weakness. Weakness is often accompanied by abnormalities in sensation, such as tingling, a pins-and-needles sensation, and numbness. 