I was going to create post on Midbrain lesion, looking at Benedikt syndrome, Weber syndrome, Claude syndrome, and Parinaud syndrome, but it will not be cool if I do not first explain the midbrain anatomy. So today, we will be doing justice to explaining the parts of the midbrain.
The midbrain, also known as the mesencephalon (you know in medicine, we like thing complex), is a portion of the brain located between the hindbrain and the forebrain. In this post, I will be taking the anatomy of the midbrain from the inferior colliculus, and the level of the superior colliculus. The midbrain has the anterior portion (ventral) and the posterior portion (dorsal). The dorsal portion has the colliculi which depending on the location can be superior collilulus and inferior colliculus.
The inferior colliculus is at the dorsal part of the midbrain, and it projects to the ventral part of the midbrain. The ventral portion of the midbrain is composed of the Crus cerebri where motor movement pathways move through. The crus cerebri is made up of three portion, where the medial portion is made up of the corticopontine fibers (which originates from the frontal lobe to the pons),,,, In the medial portion of the crus cerebri is where the corticospinal fiber and corticonuclei fibers run through.. The lateral portion of the crus cerebri is made up of other corticopontine fibers that originates from the temporal parietal, and occipital lobe..
Remember I said that the inferior colliculus projects to the ventral portion of the midbrain. The area where it projects to the crux cerebri is known as the Tegmentum. The Tegmentum is made up of several components including the Substantia nigra which comes after the Crus serebri. The Substantia nigra produces dopamine, and when damaged can cause parkinson. It is made up of the Pars compacta which is the dark neuron that produces dopamine (dopaminergic neuron), and the lighter part which is the Pars Reticulata which produces GABA (gabergic neuron). ,. Above the Substantia nigra are ascending pathways, and just above the substantia nigra is the medial lemniscus. The medial lemniscus is a continuation of the dorsal column system, and it is a sensory pathways for discriminative and fine touch, vibratory sensations and propeioreception .
Dorsally to the Medial lemniscus is the trigeminal lemniscus which takes proprioceptive, touch, and pressure sensation from the face. Above the Trigeminal lemniscus is the Spinal Lemniscus. The Spinal lemniscus carry ascending fibers of sensation from the anterior and lateral spinalthalamic tract. The Lateral Lemniscus comes after the Spinal lemniscus. It connects the superior olivary nucleus and the inferior colliculus, and it functions with the auditory pathway, from the cochlear part of the vestibulocochlear nerve..
In the middle portion of the Tegmentum are components which are very important and let me go through them. Close to the periaqueductal gray matter are the Locus Coeruleus where most noradrenergic neurons are produced which produces norepinephrine for the inhibition of descending pain pathways.. Anterior to the Locus Coerulus is the Dorsal Raphe Nucleus which is also involved in the descending pain pathway. . Ventral to the Dorsal Raphe Nucleus is the Mesencephalic nucleus of Cranial Nerve 5. This part of the trigeminal nerve is located in the midbrain while the other part extends to the medulla. It picks proprioceptive reception from the Trigeminal nerve.,. In the periaqueductal gray matter is the Cerebral Aqueduct which connects the third and fourth ventricle, with the third located at the superior and the fourth located inferior to it.. Located anteriorly to the periaqueductal gray matter is the Medial longitudinal fasciculus. It connects the vestibular nuclei to the cranial nerve nuclei three, cranial nerve nuclei four, and cranial nerve nuclei six.. Another fiber in the Tegmentum is the Tectospinal fibers which takes information from the tectum to the spinal cord to control the primary muscles of the eyes and the neck with relations to its movement. The Tectum is made up if the superior colliculus ad the inferior colliculus.. A descending fiber in the Tegmentum is the Rubrospinal tract/Rubrospinal fibers which is responsible for connecting the red nucleus where it originates from to the lower motor neurons to help control the flexor muscles and control motor movement.,. In the ventral portion of the Tegmentum is the Decussation of the Superior cerebellar peduncles which consist of fibers from the cerebullum to the Superior cerebellar peduncles, supplying the red nucleus, and the thalamus.. In the Tegmetum, the cranial nerve 4 or Trochlear nerve which carries motor sensation to the Superior oblique muscle which is an extraoccular muscle of the eye.. At the posterior of the Tegmentum is the tectum which the inferior colliculus is a component of but since we are discussing the inferior currently, the tectum at the posterior is the inferior colliculus. (Like I mentioned the tectum is made up of the Inferior Colliculus and Superior Colliculus),
Let'srun through the Superior colliculus component, and no doubt they both have almost the same component. Anterior to the Superior colliculus is the Crus cerebri which is made up of the frontopontine, the corticospinal and corticonuclear fibers, and the Corticopontine from the frontal, parietal, and occipital lobe. Next is the substantia nigra which consist of the Pars Compacta and Reticularis, which releases dopamine and GABA respectively.In the Tegmentum are the medial Lemniscus, as well as the Trigeminal Lemniscus, and the Spinal lemniscus but not the lateral lemniscus, as it does not continue into the superior colliculus. In the middle of the Tegmentum is the Mesencephalic nucleus, the cerebral aqueduct, which is in the periaqueductal gray matter. Also in the middle of the Tegmentum is the medial longitudinal fasciculus. In the Superior Colliculus is the Red nucleus which crosses in the ventral portion of the tegmentum. In the Superior colliculus the Tectospinal fiber moves ventrally into the tegmentum and crosses downwards into the inferior colliculus through dorsal tegmental decussation. In the superior colliculus the Oculomotor neuron is present and it carries somatic motor fibers which controls skeletal muscles. Also in the tegmentum of the Superior colliculus is the Edinger-Westphal Nucleus which carrie parasympathetic fibers which innervates smooth muscles and glands of the eyes. In front of the Tectum is the Pretectal nuclei which recieves resensory information from the optic nerve and send stimulation to the Edinger-Westphal Nucleus which helps to control pupillary movement. At the tectum is where the superior colliculus is located, and it recieves visual information from the retina in the eyes, and the visual cortex.,,,.
I will be explaining each and every component in next post one after the other and then we will be discussing Midbrain lesion, and will be explaining each of the lesions one after the other.
Image Reference
Image 1 || Wikimedia Commons || Midbrainsection