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Is the posterior part of the brain, adjoining and structurally continuous with the spinal cord.
It includes the midbrain, the pons, and the medulla oblongata.
Sometimes the diencephalon, the caudal part of the forebrain, is included.
It provides the main motor and sensory innervation to the face and neck via the cranial nerves.
Ten of the 12 pairs of cranial nerves come from the brainstem.
The nerve connections of the motor and sensory systems from the main part of the brain to the rest of the body pass through the brainstem.
This includes the corticospinal tract (motor), the posterior column-medial lemniscus pathway (fine touch, vibration sensation, and proprioception), and the spinothalamic tract (pain, temperature, itch, and crude touch).
The brainstem also plays an important role in the regulation of cardiac and respiratory function, the central nervous system, and is pivotal in maintaining consciousness, awareness, alertness, and regulating the sleep cycle.
The brainstem has many basic functions including heart rate, breathing, sleeping, and eating.
The midbrain is divided into three parts: tectum, tegmentum, and the ventral tegmentum.
The tectum which forms the ceiling of the midbrain, and comprises the paired structure of the superior and inferior colliculi and is the dorsal covering of the cerebral aqueduct.
The inferior colliculus, is the principal midbrain nucleus of the auditory pathway.
The inferior colliculus receives input from several peripheral brainstem nuclei, as well as inputs from the auditory cortex.
The superior colliculus marks the rostral midbrain, and is involved in the special sense of vision and sends its superior brachium to the lateral geniculate body of the diencephalon.
The floor of the midbrain is the tegmentum and is ventral to the cerebral aqueduct.
The tegmentum contains several nuclei, tracts, and the reticular formation.
The ventral tegmentum transmits axons of upper motor neurons of paired cerebral peduncles.
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The midbrain consists of:
Periaqueductal gray area:
Oculomotor nerve nucleus:
Trochlear nerve nucleus:
Red nucleus:
Substantia nigra pars compacta:
Reticular formation:
Central tegmental tract:
Ventral tegmental area:
Rostromedial tegmental nucleus:
The periaqueducral area of gray matter around the cerebral aqueduct contains various neurons involved in the pain desensitization pathway.
The Oculomotor nerve nucleus is the third cranial nerve nucleus.
The Trochlear nerve nucleus is the fourth cranial nerve.
The Red nucleus is a motor nucleus that sends a descending tract to the lower motor neurons.
Substantia nigra pars compacta is a concentration of neurons in the ventral portion of the midbrain that uses dopamine as its neurotransmitter and is involved in both motor function and emotion.
Substantia nigra pars compacta dysfunction is implicated in Parkinson’s disease.
Reticular formation an area in the midbrain and contains lower motor neurons, is involved in the pain desensitization pathway, is involved in the arousal and consciousness systems, and contains the locus coeruleus, which is involved in intensive alertness modulation and in autonomic reflexes.
Central tegmental tract is anterior to the floor of the fourth ventricle, this is a pathway that projects up to the cortex and down to the spinal cord.
Ventral tegmental area is a dopaminergic nucleus located close to the midline on the floor of the midbrain.
Rostromedial tegmental nucleus is a GABAergic nucleus located adjacent to the ventral tegmental area.
The pons lies between the medulla oblongata and the midbrain.
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Rostral to the superior cerebellar peduncle, there is the superior medullary velum and then the two trochlear nerves.
The main supply of blood is provided by the basilar arteries and the vertebral arteries.
Ten of the twelve pairs of cranial nerves either target or originate from the brainstem.
The nuclei of the oculomotor nerve (III) and trochlear nerve (IV) are located in the midbrain.
The nuclei of the trigeminal nerve (V), abducens nerve (VI), facial nerve (VII) and vestibulocochlear nerve (VIII) are located in the pons.
The nuclei of the glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI) and hypoglossal nerve (XII) are located in the medulla.
These cranial nerves exit the brainstem from these nuclei.
The brainstem has three main functions:
The brainstem plays a role in conduction, as all information relayed from the body to the cerebrum and cerebellum and vice versa must traverse the brainstem.
The ascending pathways coming from the body to the brain are the sensory pathways and include the spinothalamic tract for pain and temperature sensation and the dorsal column, fasciculus gracilis, and cuneatus for touch, proprioception, and pressure sensation.
Descending tracts are upper motor neurons that synapse on lower motor neurons in the ventral and posterior horns.
There are upper motor neurons that originate in the vestibular, red, tectal, and reticular nuclei of the brainstem which also descend and synapse in the spinal cord.
Out of the brainstem emerge cranial nerves III-XII, supplying the face, head, and viscera.
The first two pairs of cranial nerves arise from the cerebrum.
Diseases of the brainstem can result in abnormalities in the function of cranial nerves that may lead to visual disturbances, pupil abnormalities, changes in sensation, muscle weakness, hearing problems, vertigo, swallowing and speech difficulty, voice change, and co-ordination problems.
Brainstem injuries can be life threatening.
Localizing neurological lesions in the brainstem may be very precise, although it relies on a clear understanding on the functions of brainstem anatomical structures and how to test them.
Brainstem stroke syndrome can cause a range of impairments including locked-in syndrome.