The midbrain is a portion of the brainstem, positioned above the pons, at the very top of the brainstem, directly underneath the cerebellum. This is one of the most important components of the central nervous system (CNS), as all neuronal transmissions that pass through the body, throughout the peripheral nervous system (PNS) to the CNS are relayed must at some point – to and/or from the brain – pass through the midbrain.
The midbrain is associated with many functions and systems of the nervous system, some of those functions being the management of auditory and visual sensory information via the inferior and superior colliculi, respectively.
Because these structures are a part of the brainstem, they specifically assist in motor reflexes associated with visual and auditory stimuli (audio-/visual-spinal reflex).
Anatomy and Functions of the Midbrain
The midbrain is the most superior portion of the brainstem, connecting the brainstem to the cerebrum by the cerebral peduncles (not to be confused with the cerebellar peduncles which connect the brainstem to the cerebellum). Just the two cerebral hemispheres, there are also a right and a left cerebral peduncle.
One of the most notable portions of the midbrain is the cerebral aqueduct, a part of the ventricular system that allows cerebrospinal fluid (CSF) to flow down from the third to the fourth ventricle. The cerebral aqueduct is located centrally to the cerebral peduncles.
The midbrain can be further divided into two main regions which lie superior to the crus cerebri and substantia nigra: the tegmentum and tectum. Other regions of this section of the brainstem are the cerebral peduncles and the colliculi.
The tegmentum is an area of gray matter surrounding the cerebral aqueduct and is one of the most superior regions of the midbrain, positioned anteriorly to the tectum. This structure actually extends down the entire length of the brainstem, but a portion of it forms the midbrain.
The portion of the tegmentum that constitutes this specific region of the midbrain is segmented into two areas, named by color: the red nucleus and the periaqueductal gray.
The red nucleus is concerned with the coordination of movement and the periaqueductal gray is a region of gray matter involved in the suppression of pain.
There are also many connections within the tegmentum that play a part in maintaining alertness.
(According to some, the tegmentum is considered to include the substantia nigra as well. Either way, the red nucleus, and periaqueductal gray are widely accepted as regions of the tegmentum, and the substantia nigra is considered to be either directly adjacent to the tegmentum or one of three regions.)
This is one of the brainstem nuclei, and part of the extrapyramidal system, a portion of the motor system specifically dedicated to the modulation and regulation of movement. (This system’s name is derived from its distinction from the fibers that run through the pyramids – the corticospinal and corticobulbar tracts – of the medulla oblongata.)
The red nucleus, as you’ve just read, is in the tegmentum of the brainstem and receives afferent fibers from many locations within the diencephalon: the dentate nuclei, superior colliculi, inner pallidum, and cerebral cortex.
It also sends axons to the rubro-olivary and reticulo-olivary fibers and spinal cord. Together, these two control muscle tone, body position, and gait.
The dentate nuclei are clusters of neurons located in the white matter of the cerebellum and control the planning and execution of voluntary movements. The superior colliculi is a structure that has multiple layers, some of which have individual functions.
For instance, the upper layer of the superior colliculi controls the reception of visual sensory information from the retina, while the lower layers take in other neuronal information to pass to the brain. The superior colliculi also take part in the control of the orientation of the eyes and the head.
The periaqueductal gray – boy, that’s a mouthful, we’re just going to call it the PG – is another structure located within the tegmentum of the midbrain. The PG is a mass of gray matter that surrounds the cerebral aqueduct.
Though it doesn’t have any immediately identifiable anatomical regions, researchers have designated four divisions of the PG: the dorsomedial, dorsolateral, lateral, and ventrolateral columns. These are separated based on connections to the brain and other parts of the nervous system and function.
The PG as a whole is recognized for its role in pain inhibition, as it stifles the sending of neuronal transmissions from pain receptors – not necessarily inhibiting the body’s ability to detect pain. This is accomplished by the suppression of activity from neurons in the spinal cord.
The PG is also involved in many other functions including the regulation of heart rate, blood pressure, autonomic functions like smooth muscle contraction, the production of vocalizations, and the fight-or-flight response.
This crescent-shaped mass of nerve cells in the midbrain has quite a dynamic role. Although it is the smallest portion of the midbrain, it is involved in the regulation of gathering auditory and visual sensory information, motor control (primarily informed by the production and distribution of dopamine), reward-based learning patterns, and the Circadian rhythm.
The substantia nigra is a part of the basal ganglia (which explains its involvement in so many different types of nervous system functions). The basal ganglia is composed of four nuclei in total, the substantia nigra being the largest of the collection.
Like many other parts of the brain, the substantia nigra is situated in a pair, one per hemisphere, and is further divided into subregions: the pars reticulata and pars compacta.
The pars reticulata is responsible for the relaying of thoroughly-processed neuronal signals from the basal ganglia to the thalamus and superior colliculi and is particularly involved in rapid eye movement.
The pars compacta, on the other hand, is also involved in motor control, but indirectly, as its stimulation does not directly influence movement. It is especially involved in finer motor control and has a greater effect on the human nervous system when absent (Parkinson’s disease, for example, is attributed to the absence of the pars compacta).
The tectum is made up entirely of the superior and inferior colliculi at the dorsal end of the midbrain. With these two structures, the tectum controls the “master coordinate system” for other sensory afferent nerves including auditory and somatosensory. This is possible because of how closely the superior colliculi work with the retina.
This allows the nervous system to create a rendering, if you will, of the body’s sensory environment. Interestingly, in non-human animals, the tectum is integral to predatory and escape behaviors due to its role in the perception and recognition of physical forms, conditioning to visual stimuli, and the “search image” an animal uses to forage for or hunt down its prey. The optic tectum is also thought to be involved in visual memory.
The crura cerebri (plural of crus cerebri) are the anterior parts of the cerebral peduncle that contain key efferent nerve tracts, the corticospinal and corticobulbar nerve tracts.
The cerebral peduncles connect the brainstem to the thalami, separated by the interpeduncular cistern (a small space in the brain into which CSF flows. It also contains nerve tracts that connect to the cerebrum as well.
Because the cerebral peduncles are located within the crus cerebri, the two terms are often used interchangeably. The term “basis pedunculi” or “crusta” is a collective name referencing the crus cerebri and substantia nigra altogether.
Summary of Midbrain
The midbrain, also known as the mesencephalon, is one of the primary divisions of the brainstem. Its functions extend to many different parts of the central and peripheral nervous systems, from motor to sensory and cognitive abilities.
It houses two of the twelve cranial nerves, the oculomotor and trochlear nerves. Through these nerve connections, the midbrain directly controls the movement of the eye and pupil construction.
That said, damage to the midbrain may result in damage not only to vision (due to the interference with the ability to control pupil constriction, thereby eliminating the reflex that controls the allowance of light onto the retina) but control of the eyes as well, absent of specific impairment of vision.
Problems with the midbrain can manifest in many ways. This makes sense, given its extensive role in both voluntary and autonomic functions in the nervous system, some functions even expanding outside of simple (simple… ha!) motor control and into memory, consciousness, and awareness.
Not only this but because it is a major component of the brainstem – the point of connection between the central and peripheral nervous systems.
Lastly, the midbrain is supplied by the basilar and vertebral arteries (the same arteries that run on either side of the neck parallel to the jugular veins).
The red nucleus, periaqueductal gray, and substantia nigra are all major structures of the midbrain that allow it to control the movement of the body, along with with the modulation of said movement, visual and auditory sensory information, inhibition of pain, regulation of the Circadian rhythm, and reward-based learning patterns.
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