Main structures of the midbrain and their functions
The midbrain includes several critically important nuclear clusters, each performing specific tasks:
- Substantia nigra (Substantia nigra). It consists of dopaminergic neurons. It is responsible for organizing emotional behavior and regulating fine motor skills (e.g., finger movements). This structure is also involved in ensuring the sequence of complex acts such as chewing and swallowing.
- Periaqueductal gray matter (Periaqueductal gray). It performs a unique receptor function with respect to cerebrospinal fluid (CSF). Its main task is to maintain homeostasis and the constancy of the internal environment of the body.
- Noradrenergic system. It is represented by neurons of the locus coeruleus (Locus coeruleus) and adjacent areas of the ventrolateral reticular formation. Axons of these cells provide wide irradiation to the brainstem, cerebellum, hypothalamus, hippocampus, amygdala, as well as the cingulate and entorhinal cortex.
Red nuclei and muscle tone
Unlike a spinal animal, the so-called "mesencephalic animal" (in which the forebrain is separated from the midbrain by transection at the level of the superior colliculi) is capable of independently maintaining body position in space and retains adequate muscle tone. Such a preparation lacks decerebrate rigidity because the midbrain and its red nuclei (Nucleus ruber) are preserved.
The red nuclei exert a constant inhibitory influence on Deiters' vestibular nuclei located in the medulla oblongata. Deiters' nuclei, in turn, receive excitatory afferentation from muscle proprioceptors and the ear labyrinths via the vestibulospinal tract.
If the red nuclei are destroyed, the inhibition of Deiters' nuclei ceases. As a result, at the hindbrain level, the activity of extensor muscles selectively and sharply increases, leading to decerebrate rigidity. This rigidity is reflex in nature: it is maintained by a constant flow of impulses from proprioceptors and the vestibular apparatus. Proof of this is the fact that transection of the vestibular nerves or the dorsal roots of the spinal cord completely eliminates muscle spasm.
Conduction and reflex functions
Midbrain functions are globally divided into intrinsic (reflex) and transit (conduction) functions.
1. Conduction function. It is carried out through the structures of the cerebral peduncles, where the most important tracts pass:
- Ascending (afferent) pathways: spinothalamic tract and medial lemniscus.
- Descending (efferent) pathways: pyramidal and corticopontine tracts.
2. Reflex function. The midbrain is responsible for posture regulation using a system of tonic reflexes, the classification of which was developed by R. Magnus and A. de Kleijn. These reflexes are divided into:
- Static reflexes: include posture (positional) reflexes and righting reflexes, which allow the restoration of a normal posture when it is disturbed.
- Statokinetic reflexes: provide compensatory reactions when the body moves in space with acceleration.
Briefly: answers to topical questions
Which cranial nerves originate from the midbrain nuclei?
The third and fourth pairs of cranial nerves originate from the midbrain nuclei.
- Oculomotor nerve (n. oculomotorius, cranial nerve III) — its motor and autonomic parasympathetic nuclei are located in the superior colliculi of the midbrain tectum.
- Trochlear nerve (n. trochlearis, cranial nerve IV) — its single motor nucleus is localized in the inferior colliculi of the midbrain tectum.
What functions are performed by the superior and inferior colliculi?
The superior and inferior colliculi act as centers for the primary processing of sensory information and provide unconscious orienting reflexes.
- Superior colliculi — primary visual centers, process visual information.
- Inferior colliculi — primary auditory centers, process auditory information.
Neurons of the corpora quadrigemina implement visual and auditory startle reflexes via the tectospinal tract. Components of these reactions include listening, movement of the eyeballs, regulation of pupil size, and rotation of the pinnae in animals.
What structures form the roof of the midbrain (tectum mesencephali)?
The roof of the midbrain is formed by the lamina quadrigemina, which combines four elevations on the dorsal surface of the brain. Its structure includes:
- Superior colliculi — two large elevations.
- Inferior colliculi — two smaller elevations.
Topographically, the midbrain roof is bounded by an imaginary horizontal plane passing through the cerebral aqueduct. Additionally, the pineal gland is located on the lamina quadrigemina.
"Red brake, black motor": The red nuclei inhibit extensors (preventing rigidity), while the substantia nigra triggers fine motor skills and complex acts.
Class and exam questions
What is the difference between a mesencephalic animal and a spinal animal?
A mesencephalic animal retains the ability to maintain normal muscle tone and body position in space, whereas a spinal animal loses these functions.
What is the mechanism of development of decerebrate rigidity?
It occurs upon the destruction of the red nuclei of the midbrain. Normally, they inhibit Deiters' nuclei; without this inhibition, hyperactivation of the extensor muscles occurs.
How to prove the reflex nature of decerebrate rigidity?
It is sufficient to transect the dorsal roots of the spinal cord or the vestibular nerves — this interrupts afferent impulsation, and rigidity disappears.
What function is performed by the periaqueductal gray matter?
It performs a receptor function with respect to the cerebrospinal fluid and participates in maintaining the constancy of the internal environment (homeostasis).
What else to study on this topic
- Mechanisms of static and statokinetic reflexes of Magnus and de Kleijn
- Connections and functions of dopaminergic neurons of the substantia nigra
- Organization of the vestibulospinal tract and Deiters' nucleus
- Irradiation of locus coeruleus pathways in the cortex and limbic system
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