Divisions of the Brain: Forebrain, Midbrain, Hindbrain

Forebrain (Prosencephalon)

The forebrain is by far the largest brain division. It includes the cerebrum, which accounts for about two-thirds of the brain's mass and covers most other brain structures. The forebrain consists of two subdivisions called the telencephalon and diencephalon. The olfactory and optic cranial nerves are found in the forebrain, as well as the lateral and third cerebral ventricles.

Telencephalon

A major component of the telencephalon is the cerebral cortex, which is further divided into four lobes. These lobes include the frontal lobes, parietal lobes, occipital lobes, and temporal lobes. The cerebral cortex contains folded bulges called gyri that create indentations in the brain. Functions of the cerebral cortex include processing sensory information, controlling motor functions, and performing higher-order functions such as reasoning and problem-solving.

• Frontal Lobes: The prefrontal cortex, premotor area, and motor area of the brain. These lobes function in voluntary muscle movement, memory, thinking, decision-making, and planning.
• Parietal Lobes: Responsible for receiving and processing sensory information. These lobes also contain the somatosensory cortex, which is essential for processing touch sensations.
• Occipital Lobes: Responsible for receiving and processing visual information from the retina.
• Temporal Lobes: Home of the limbic system structures, including the amygdala and hippocampus. These lobes organize sensory input and aid in auditory perception, memory formation, and language and speech production.

Diencephalon

The diencephalon is the region of the brain that relays sensory information and connects components of the endocrine system with the nervous system. The diencephalon regulates a number of functions including autonomic, endocrine, and motor functions. It also plays a major role in sensory perception. Components of the diencephalon include:

• Thalamus: A limbic system structure that connects areas of the cerebral cortex that are involved in sensory perception and movement with other parts of the brain and spinal cord. The thalamus also plays a role in the control of sleep and wake cycles.
• Hypothalamus: Acts as the control center for many autonomic functions including respiration, blood pressure, and body temperature regulation. This endocrine structure secretes hormones that act on the pituitary gland to regulate biological processes including metabolism, growth, and the development of reproductive system organs. As a component of the limbic system, the hypothalamus influences various emotional responses through its influence on the pituitary gland, skeletal muscular system, and autonomic nervous system.
• Pineal Gland: This small endocrine gland produces the hormone melatonin. Production of this hormone is vital to the regulation of sleep-wake cycles and also influences sexual development. The pineal gland converts nerve signals from the sympathetic component of the peripheral nervous system into hormone signals, thereby linking the nervous and endocrine systems.

Midbrain (Mesencephalon)

The midbrain is the area of the brain that connects the forebrain to the hindbrain. The midbrain and hindbrain together compose the brainstem. The brainstem connects the spinal cord with the cerebrum. The midbrain regulates movement and aids in the processing of auditory and visual information. The oculomotor and trochlear cranial nerves are located in the midbrain. These nerves control eye and eyelid movement. The cerebral aqueduct, a canal that connects the third and fourth cerebral ventricles, is also located in the midbrain. Other components of the midbrain include:

• Tectum: The dorsal portion of the midbrain that is composed of the superior and inferior colliculi. These colliculi are rounded bulges that are involved in visual and auditory reflexes. The superior colliculus processes visual signals and relays them to the occipital lobes. The inferior colliculus processes auditory signals and relays them to the auditory cortex in the temporal lobe.
• Cerebral peduncle: The anterior portion of the midbrain consisting of large bundles of nerve fiber tracts that connect the forebrain to the hindbrain. Structures of the cerebral peduncle include the tegmentum and crus cerebri. The tegmentum forms the base of the midbrain and includes the reticular formation and the red nucleus. The reticular formation is a cluster of nerves within the brainstem that relay sensory and motor signals to and from the spinal cord and the brain. It aids in the control of autonomic and endocrine functions, as well as muscle reflexes and sleep and awake states. The red nucleus is a mass of cells that aids in motor function.
• Substantia nigra: This large mass of brain matter with pigmented nerve cells produces the neurotransmitter dopamine. The substantia nigra helps control voluntary movement and regulates mood.

Hindbrain (Rhombencephalon)

The hindbrain is composed of two subregions called the metencephalon and myelencephalon. Several cranial nerves are located in this brain region. The trigeminal, abducent, facial, and vestibulocochlear nerves are found in the metencephalon. The glossopharyngeal, vagus, accessory, and hypoglossal nerves are located in the myelencephalon. The fourth cerebral ventricle also extends through this region of the brain. The hindbrain assists in the regulation of autonomic functions, maintaining balance and equilibrium, movement coordination, and the relay of sensory information.

Metencephalon

The metencephalon is the upper region of the hindbrain and contains the pons and cerebellum. The pons is a component of the brainstem, which acts as a bridge connecting the cerebrum with the medulla oblongata and cerebellum. The pons assists in the control of autonomic functions, as well as states of sleep and arousal.

The cerebellum relays information between muscles and areas of the cerebral cortex that are involved in motor control. This hindbrain structure aids in fine movement coordination, balance and equilibrium maintenance, and muscle tone.

Myelencephalon

The myelencephalon is the lower region of the hindbrain located below the metencephalon and above the spinal cord. It consists of the medulla oblongata. This brain structure relays motor and sensory signals between the spinal cord and higher brain regions. It also assists in the regulation of autonomic functions such as breathing, heart rate, and reflex actions including swallowing and sneezing.