The body network that regulates all bodily functions. The nervous system is complex and contains numerous structures and subsystems that are responsible for specific activities or processes. Billions of neurons, or nerve cells, compose these structures and their subsystems. There are three major divisions of the nervous system. Two are structural as well as functional: the central nervous system, which consists of the brain and spinal cord; and the peripheral nervous system, which consists of all other nerve structures. The autonomic nervous system is a functional division of the nervous system that includes peripheral and central components to control many of the bodies involuntary responses. Voluntary and involuntary actions require the concerted function of central and peripheral nervous system components. Parkinson’s disease damages the structures of the central nervous system, thereby affecting voluntary and involuntary functions. In mid to late stages of the disease damage to the autonomic nervous system may become a significant source of disability.
Central Nervous System
The central nervous system consists of the brain and spinal cord and the functions of these structures that regulate the body’s conscious (voluntary) and (involuntary) actions. It is the master control center of the entire nervous system from which other functional divisions emanate. Directly or indirectly, the central nervous system has a role in every body activity from breathing and movement to cognition and memory. Parkinson’s disease affects the structures and functions of the central nervous system in numerous ways.
The brain is the primary organ of the nervous system. It has three physically distinctive structures:
• The cerebrum, which controls conscious activity including intellect, thought, speech, emotions, and memory. The organizational units of the brain are its four paired lobes: frontal, temporal, parietal, and occipital. Voluntary movement begins in the cerebrum, with the conscious (although often unaware) decision to move. This initiates a complex and intricate sequence of nervous system functions. The dopaminE depletion that is the hallmark of Parkinson’s disease directly affects the structures and functions of the cerebrum.
• The cerebellum, which controls coordination of both voluntary and involuntary movement. A switching station of sorts, the cerebellum processes millions of messages that relay information between the cerebrum and the body’s neuromuscular structures per second. Although the direct functions of the cerebellum remain intact in Parkinson’s disease, the chaotic communication among brain neurons that results from dopamine depletion in the basal ganglia disrupts signals before and after they pass through the cerebellum.
• The brainstem, which is the conduit for communication between the brain and the body. Some of its direct functions, which control involuntary functions such as breathing and blood pressure, are part of the autonomic nervous system. Structures of the brainstem also control the function of smooth muscle, such as in the digestive tract, blood vessels, and heart. Some of these structures, most notably the midbrain, contain dopaminergic neurons that also die as Parkinson’s progresses, impairing the functions of the mid-brain and possibly other brainstem structures.
Nestled at the junction of these three structures are the basal ganglia, a cluster of specialized nerve structures that control voluntary movement. The death of dopaminergic neurons in the basal ganglia, primarily in the substantia nigra, initiates the changes that cause the neuromuscular dysfunctions such as tremors and bradykinesia that characterize Parkinson’s disease.
The spinal cord is a ropelike collection of nerve tissues and fibers that extends from the brain stem to immediately above the tail bone and is the “highway” through which nearly all nerves and nerve signals enter and leave the brain. In an adult, the spinal cord is about 18 inches long and the thickness of a thumb. The spinal cord carries motor signals from the brain and sensory signals to the brain. It also controls certain basic involuntary motor responses, for instances, reflexes such as the knee jerk. Its divisions are anatomical rather than functional, correlating to the sections of the body
the spinal cord passes through: cervical (neck), thoracic (chest), lumbar (lower back), and sacral (tail bone). The spinal cord ends in a splay of nerve fibers, called the cauda equina (“horse’s tail”), that extends downward along the final segments of the spine. Parkinson’s disease does not affect the structure of functions of the spinal cord.
Peripheral Nervous System
The nerves that extend from the central nervous system to communicate with other structures of the body form the peripheral nervous system. This includes the cranial nerves, spinal nerve roots, nerve connection points (called ganglia), and peripheral nerves. Although the peripheral nervous system’s structures remain intact in Parkinson’s disease, the progressive degeneration of central nervous system structures and functions affects the functions of the peripheral nervous system. Distorted signals from the basal ganglia cause confused and chaotic responses in peripheral motor neurons, resulting in dysfunctional movement (tremors and dyskinEsias) and impaired motor function.
The 12 paired cranial nerves originate in the brain. One pair, the olfactory, originate in the cerebrum, a second pair, the optic, originate in the thalamus; they have solely sensory functions. The remaining pairs originate in the structures of the brainstem and have mixed sensory and motor functions. The pair known collectively as the vagus nerve is a key structure of the autonomic nervous system that controls such functions as breathing and heart rate. Parkinson’s disease affects some of the functions for which cranial nerves are responsible, for example, by creating disturbances of vision and smell, but scientists are not certain whether this effect involves damage to the cranial nerves or to the areas within the brain that interpret sensory signals. Parkinson’s symptoms such as difficulty in moving the eyes or swallowing relate to neuromuscular disruptions that take place in the basal ganglia and affect neuron communication to the muscles, rather than to damage that involves the cranial nerves that also have functions related to the muscles that move the eyes, tongue, and face. The route of transit remains intact although the signals traveling it are distorted.
The spinal nerves branch from the spinal cord in 31 pairs named for their corresponding locations along the spine: eight cervical pairs, 12 thoracic pairs, five lumbar pairs, five sacral pairs, and one coccygeal pair. Each spinal nerve attaches to the spinal cord with two roots, a dorsal root containing afferent neurons that carry signals to thalamus and brainstem via the spinal cord and a ventral root containing the oxons (arms of the neuron that serve as communication cables) of efferent neurons that carry signals away from the spinal cord. The spinal nerves quickly branch as they leave the spinal cord, forming an extensive network of nerves that eventually reaches to all parts of the body. Parkinson’s disease does not directly damage the spinal nerves, which continue carrying signals to and from the brain. However, there is no process within the spinal cord, spinal nerves, or other nerve pathways to decode or correct faulty nerve signals; signals from the brain that are incomplete or jumbled travel nerve pathways and reach their destinations just as incomplete or jumbled as when they left the brain.
All of the nerve structures that branch from the cranial and spinal nerves are the peripheral, or outer, nerves. Nerves that serve organ systems are called visceral or enteric; nerves that serve the musculoskeletal system are called somatic. As with the cranial and spinal nerves, the peripheral nerves remain structurally and functionally sound in Parkinson’s disease. The disruptions and dysfunctions that cause the neuromuscular symptoms of Parkinson’s originate in the regions of the brain that control movement.
Autonomic Nervous System
The autonomic nervous system regulates the body’s automatic functions such as heartbeat, blood pressure, breathing, digestion, excretion, temperature regulation, and actions of smooth muscle (such as in the heart, blood vessels, and intestinal tract). Its primary purpose is to sustain life; its structures and mechanisms are the nervous system’s most primitive and most basic. The loss of dopaminErgic neurons and corresponding dopamine depletion that characterizes Parkinson’s disease affects parts of the brainstem, particularly the subthalamic nucleus and the midbrain, the are involved in functions of the autonomic nervous system such as smooth muscle control.
The autonomic nervous system has two components that have counterbalancing functions, the sympathetic and parasympathetic nervous systems. Structurally these systems exit the central nervous system with certain cranial and spinal nerves. The sympathetic pathways then involve a connection with a neuron in a ganglia chain that parallels the spinal cord, this neuron then forms the connection with the end organ (usually the smooth muscle of viscera, the heart, or glands). Parasympathetic axons travel all the way to the wall of the target organ before connecting to the neuron that actually connects to the end organ. The sympathetic and parasympathetic nerve systems are important for sexual arousal and function, including erection and ejaculation in men. Many anti-parkinson’s medications, particularly anticholinergics and dopamine agonists, affect the functions of the autonomic nervous system.
Sympathetic nervous system
A nerve network that extends from the thoracic and upper two lumbar segments of the spinal cord constitutes the sympathetic nervous system. The main functions of this nerve network to maintain adequate heart rate, blood pressure, and breathing rate and to prepare the body for “fight or flight.” The ganglia chains terminate in various organ systems; the adrenergic neurons at the ends of the chains release norepinephrine, which causes neurotrans-mitters cause blood vessels to dilate, heart rate to increase, and blood pressure to rise. They also open airways in the lungs and accelerate breathing. At the same time, the sympathetic nervous system slows the function of smooth muscle in the digestive tract. The sympathetic nervous system is also important for ejaculation and sexual climax.
Parasympathetic nervous system
The nerve networks of the parasympathetic nervous system extend into the body from two points, the brainstem and the lower region of the spinal cord. The main functions of the parasympathetic nervous system are to relax the cardiorespiratory system (heart rate, blood pressure, breathing) and stimulate functions of digestion. The parasympathetic nervous system is also important for sexual arousal: erection in men, and probably vaginal lubrication, labial engorgement, and clitoral engorgement in women. The terminal neurons of the parasympathetic ganglia chains are cholinergic; when activated they release acetylcholine, which causes smooth muscle tissues to relax and dilates blood vessels, slows heart rate, and increases blood flow to organs such as the stomach and intestines. See also conditions similar to Parkinson’s;
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