Midbrain

The midbrain is a section of the vertebrate brain that acts as a bridge between the higher functioning forebrain and the more automatic and instinct-oriented hindbrain. The midbrain is a relatively small part of the brainstem, and it is responsible for motor control, eye movement, and visual and auditory processing. Scientists have discovered that damage to some parts of the midbrain has been associated with Parkinson's disease, a progressive brain disorder that affects a person's movement. The midbrain is also known as the mesencephalon, a word that comes from the Greek words mesos (middle) and enkephalon (inside the head).

Background

The human brain is a vital organ that acts as the control center for the body's nervous system, which is made up of the brain, spinal cord, and peripheral nerves. The average brain weighs about three pounds and makes up about 2 percent of a person's total weight. The brain is in charge of both voluntary functions—speech, movement, and coordination—and involuntary functions—regulating blood pressure, heart rate, and breathing.

The brain is divided into three main sections: the forebrain, midbrain, and hindbrain. The forebrain comprises the cerebrum, thalamus, and hypothalamus. The cerebrum is the topmost and largest section of the brain. It makes up about 85 percent of the brain's weight. The cerebrum is responsible for higher brain functions such as speech, learning, emotional control, reasoning, fine motor skills, and interpreting touch, sights, and sounds. The thalamus acts as a relay station, shuttling information to and from the outer layer of the cerebrum, called the cerebral cortex. The thalamus helps the body sense pain and plays a role in alertness and the sleep cycle. The hypothalamus acts as a master control for several systems that regulate body temperature, blood pressure, hunger, and emotions.

The hindbrain is located at the base of the brain near the spinal cord. On an evolutionary scale, the hindbrain is the oldest area of the brain, and it is responsible for some automatic functions and instinctual responses. It consists of the cerebellum, pons, and medulla oblongata. The cerebellum coordinates sensory input with muscular movements, helping maintain posture and balance. The pons acts as a bridge connecting the cerebrum with the cerebellum and medulla oblongata. It also assists in regulating breathing and sleep. This area helps regulate some body functions and passes motor and sensory signals from the spinal cord to the higher parts of the brain.

Overview

The pons, medulla oblongata, and the midbrain are collectively referred to as the brainstem. As the name suggests, the midbrain is located near the midsection of the brain, just above the pons and below the forebrain. It acts as a nerve conduit that connects the cerebrum with the cerebellum and other parts of the hindbrain. The midbrain is divided into three main parts called the tectum, cerebral peduncle, and substantia nigra.

The tectum, from the Latin word for "roof," is the area near the top of the midbrain. The tectum houses four structures known collectively as the corpora quadrigemina. This consists of the two superior colliculi, which are located above the two inferior colliculi. The colliculi act as messengers, taking sensory information from the eyes or ears and relaying it to the thalamus, which sends it to the appropriate destination. The superior colliculi handles visual information from the eyes, and the inferior colliculi are responsible for auditory information from the ears.

The cerebral peduncle consists of large ropelike bundles of nerve fibers that connect the forebrain to the hindbrain. This section contains a structure called the tegmentum, located near the base of the midbrain. A part of the tegmentum known as the reticular formation relays motor and sensory information to and from the spinal cord to the cerebrum. It helps regulate the release of hormones, affects muscle reflexes, and influences the body's sleeping and waking states. Near the center of the tegmentum is a large mass of cells known as the red nucleus. This structure works to refine the body's motor skills to make walking and other movements easier. Two important sets of nerves also originate within the cerebral peduncle. These nerves are responsible for muscle movement in the eyelids and eye sockets, allowing a person's eyes to rotate.

The substantia nigra, named for the dark pigment in its cells, is a mass of cells that produces a substance called dopamine. Dopamine is a neurotransmitter, a chemical messenger released by the brain that passes information from one brain cell to another. Dopamine plays a key role in regulating the body's reward and pleasure centers. It activates receptors in the brain that let the body know an experience is pleasurable, and it motivates a person to seek out that experience as a reward. Dopamine also affects a person's emotions, attention span, and ability to learn, and it is responsible for regulating physical movement. Some scientists believe lower dopamine levels may lead to a lack of desire to exercise and a less healthy lifestyle.

The loss of nerve cells in the substantia nigra is believed to be a primary cause of a neurological disorder known as Parkinson's disease. The death of nerve cells in the substantia nigra could lead to a substantial drop in dopamine levels. Medical research has linked this condition to Parkinson's, a progressive illness that causes tremors, loss of motor and speech skills, and difficultly moving. Doctors are unsure what causes the changes within the substantia nigra that could lead to Parkinson's disease, though they suspect genetic and environmental factors may be responsible. The disease has no cure, but it can be treated with medication and surgery.

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