The Encephalic Trunk: anatomy, structures and function

The Encephalic Trunk: anatomy, structures and function

The brainstem or brainstem It is one of the most important regions of the human brain and one of the most vital regions for the survival of our body. It forms the connection between the brain and the spinal cord, maintains vital control of the heart and lungs and coordinates many important reflexes.


  • 1 Anatomy
  • 2 The cranial nerves
  • 3 Reticular formation
  • 4 midbrain nuclei
  • 5 Bulb Cores
  • 6 descending tracks
  • 7 Functions of the brainstem


It is a tubular mass of nerve tissue of little more than 8 cm in length. It is located at the base of the brain, superior to the spinal cord and inferior to the brain.

The outside of the brain stem is composed of white substance, which conducts nerve signals inside the brain stem and into the spinal cord and other regions of the brain. The masses of gray matter, known as nuclei, play the role of brain stem processing. The reticular formation, a mixed network of gray and white matterIt extends throughout the interior of the brainstem and plays an important role in stimulating the brain and body muscles.

Three main regions make up the brainstem: medulla oblongata (medulla), bulge and midbrain.

  • The medulla is the lowest region of the brainstem that connects the brain with the spinal cord. It is a tube structurally similar to the spinal cord, but is wider and contains several masses of gray matter internally.
  • Above the medulla is the boss, which is larger and structurally more complex than the medulla.
  • Finally the midbrain It forms the highest and most complex region of the brainstem.

The ventral or anterior surface of the brain stem has the following components:

The dorsal or posterior surface of the brain stem is largely covered by the cerebral hemispheres and by the cerebellum.

When the hemispheres and the cerebellum are removed, some of the characteristics of the dorsal surface of the brain stem are revealed. These features are as follows:

The dorsal white columns, which are part of the Goll and Burdach fascicle, are distinguished in the medulla.

The bump is under the IV ventricle.

The midbrain is characterized by the presence of four small lumps, two on each side of the midline, the upper and lower collicles, or quadruplet tubers. The collicles are relay stations for the auditory (lower) and visual (upper) pathways.

Cranial nerves

There are no cranial nerves around the brain stem, which are the peripheral nerves of the brain.

The cranial nerves are part of the nervous system and consist of efferent motor fibers that arise from nuclei in the brainstem and afferent sensory fibers that originate in the peripheral ganglia. The motor nuclei of the cranial nerves receive impulses from the cerebral cortex through the corticonuclear tracts.

There are twelve pairs of cranial nerves:

  • Ten of the twelve cranial nerves leave the trunk.
  • One of the cranial nerves, the olfactory nerve, leaves the olfactory bulb (telencephalon).
  • One of the cranial nerves, the optic nerve, leaves the optic chiasm (diencephalon).

Ventral view of the brain in which cranial nerves stand out.

Unlike spinal nerves that are mixed, motor and sensory, cranial nerves can be exclusively sensory, motor, or mixed (they combine motor and sensory functions).

The cranial nerves originate or end in a series of nuclei that are inside the brainstem. Can be distinguished nuclei of the motor cranial nerves and nuclei of the sensory cranial nerves.

Cranial nerve sensory nuclei

They are the areas where the fibers of the sensory branches of the cranial nerves end. Sensory neurons that carry information outside the CNS synapt in these nuclei and have their sum outside the trunk, in nodes located on both sides of the trunk.

In the bulb are many of the cranial nerve nuclei, some of these with vital functions. Thus, Bulb injury is very dangerous, as it can cause the death of the individual.

Cranial nerve motor nuclei

They are the places where the fibers that form the motor branches of the cranial nerves originate.

The dorsal nucleus (X) is the origin of the most important fibers (with vital functions) that innervate the vagus nerve. Thus, cranial nerve injuries range from those that cause subtle deficits to injuries that cause death

Reticular formation

The reticular formation is a network of neurons that extends from the spinal cord to the thalamus, with connections to the medulla, the midbrain, the bump and the diencephalon. These neurons, along with their axons and dendrites, are sandwiched between the cranial nerve nuclei and the tracts found in the brain stem. It is important to note that although the nuclei of the reticular formation are not as well defined as those of the cranial nerves, regions that contain gray matter do appear.

Location and structure

The reticular formation, such as the cranial nerve nuclei, is distributed longitudinally throughout the brain stem.

The reticular formation is divided into three columns: rafe nuclei (median), gigantocellular reticular nuclei (medial area) and parvocellular reticular nuclei (lateral zone).

The rafe cores they are several nuclei that form a gray column located in the midline of the brain stem and from this location derives its name (rafe it is 'suture', in Greek) and they are the place of synthesis of the neurotransmitter serotonin, which plays an important role in the regulation of mood.

The gigantocellular nuclei are involved in motor coordination and parvocellular nuclei They regulate exhalation.

The locus ceruleus It is also considered part of the reticular formation. It is mainly in the field of extrusion. This nucleus owes its name to its bluish appearance in fresh tissue. The axons that leave this nucleus are very long and branched and extend to wide areas of the CNS. Norepinephrine is synthesized primarily in this nucleus of the Brain stem.

Ascending reticular pathways

The reticular formation is a place of convergence of information, which receives afferences from most sensory systems and has efferent connections with all levels of the CNS.

Descending reticular pathways

The reticular formation receives many descending influences from the cerebral cortex that converge in the medial reticular formation, which is the effector zone. From several nuclei of this medial zone originate the descending reticular tracts in the spinal cord. Two descending motor tracts originate in the reticular formation.

The pathways that originate in the nuclei of the raphe that are directed towards the medulla and that are related to the internal pain regulation.

Functions of reticular formation

Thanks to the projections that it has, mainly towards the thalamus, the reticular formation intervenes in the excitability of the cerebral cortex and its correct functioning is essential for the maintenance of a normal state of consciousness.

Its main functions are:

  • Sleep-wake cycle (awareness control and alertness)
  • Motor system of the brain and spinal cord
  • Regulation of the visceral activity

The lesion of the pontic or mesencephalic reticular formation causes coma.

An intact normal brain is unable to function consciously on its own, it needs to receive impulses from the reticular formation of the brain stem in a sustained manner.

Many drugs act on the ascending activating reticular system. Some anesthetics general suppress transmission through the reticular formation. The stimulants (amphetamines, cocaine, caffeine) increase the state of general activation by acting on this system. Instead, the sedatives (Like the barbiturates) have a depressing effect on this system.

Midbrain nuclei

Roof cores

On the roof of the midbrain we find the upper and lower collicles.

The lower coliculus It is related to the processing of the auditory information that arrives through the lateral lemnisk.

The upper colliculus It is part of the visual processing pathway. It allows us to orient the head and eyes towards the stimuli that surround us.

Tegment Cores

The gray periacueductal substance surrounds the cerebral aqueduct. It is an area of ‚Äč‚Äčintegration of neuroendocrine and sensory signals that intervenes with descending fibers in sensory modulation. It is part of an endogenous circuit for pain control.

The red core is of great importance for movement control. It consists of two parts: the parvocellular region, fundamentally related to the cerebellum, and the magnocellular region, where descending fibers in the spinal cord originate.

The black substance is immediately dorsal brain peduncles and extends throughout the entire midbrain. It consists of two parts: the compact area and the reticulata. It has important motor functions, it is connected to the neo-striated through the black-striated pathway. This route is dopaminergic, and its injury causes Parkinson's disease.

The ventral tegmental area is between the black substance and the red nucleus. It is a population of dopaminergic neurons. Its axons end in the hypothalamus, hippocampal formation and other parts of the limbic system. These projections are part of the mesolimbic dopaminergic system that has been widely studied in animals, since their actions are blocked by antipsychotic drugs. These drugs are antagonists of dopamine receptors.

Bulb Cores

In the bulb, we can highlight the lower olive, which sends important projections in the cerebellum. Fibers from this nucleus when they enter the cerebellum are called climbing fibers.

Lateral lemnisco

The lateral lemnisco is the main ascending auditory pathway.

The cochlear nuclei (sensory nucleus of the VII cranial nerve) project crossed and non-crossed fibers directly to the lateral lemnisk. In addition, the projections of the upper olive grove carry information to locate the sound from the two ears.

The fibers of the lateral lemnisk end in the lateral geniculate nucleus of the thalamus, passing first through the inferior collicles.

Medial lemnisco

The medial lemnisco is next to the fascicles of Goll and Burdach that carry information on epicritic touch and conscious proprioception.

The fibers of the Goll and Burdach fascicles (primary efferent fibers) come from the sensory receptors and do not establish their first synapse to the lower bulb, in the gracilis and cuneiform nuclei. After this first synapse, the fibers that originate in the gracilis and cuneiform nuclei (second order fibers) cross the midline at the same level at which they originate and rise contralaterally forming the medial lemnisk. It maintains the somatotopic disposition and goes to the thalamus.

The medial lemnisco receives the fibers that come from the sensory nucleus of the trigeminal and that carry the information of the sensitivity of the face.

Lateral and anterior spinothalamic fascicles

These fascicles carry information on pain, temperature, prostate touch and pressure.

First-order efferent fibers that carry information on pain, temperature, protophatic touch and pressure end at the spinal cord's dorsal horn.

The second order fibers cross the midline and form the spinothalamic tracts and rise to the thalamus. During their journey through the brain stem, the spinothalamic fibers send numerous collaterals to the reticular formation.

Other fascicles

The spinocerebel·lous fascicles before entering the cerebellum pass through the brain stem. The entrance to the cerebellum is through the cerebellar peduncles.

Descending tracks

Direct and crossed pyramidal fascicles

They are fascicles that originate in the cerebral cortex and go down the cerebral peduncles, the bulge and the pyramids of the bulb.

Most of the pyramidal or corticospinal fibers cross the midline in the bulb, in the decussation of the pyramids, and form the cross (or lateral) pyramidal (or corticospinal) tract.

Fibers that do not cross in the bulb form the direct (or anterior) pyramidal (or corticospinal) tract. Most of these fibers cross the midline in the medulla, through the anterior white commissure, before going to the medullary gray matter.

Most pyramidal fibers do not establish synapses directly in motor neurons.

Extrapyramidal fascicles

Several extrapyramidal treatments pass through the brain stem, such as rubroespinal treatment, spinal raphe.

Functions of the brainstem

There are three main functions carried out by the brainstem:

It plays an essential role in the transmission of information from body to brain and vice versa. The ascending sensory pathways that go from the body to the brain include the sensation of pain, temperature, touch, proprioception and the sensation of. The descending tracts are motor neurons that transmit the movement information to the muscles and other organs.

The cranial nerves III-XII emerge from the brainstem. These cranial nerves irrigate the face, head and viscera.

Also has integrative functions involved in the control of the cardiovascular system, respiratory control, pain sensitivity, alertness and awareness. Therefore, brain stem damage is a very serious and often life-threatening problem.

The brainstem, like the spinal cord, receives somatic afferences from the trunk and extremities, and visceral afferents from internal organs. It also receives sensory information (somatic and visceral) of the cranial structures.

A portion of the information you receive uses locally to control reflex motor acts with some independence from the other levels of the brain. Too controls the motor innervation (somatic and visceral) of the head through the cranial nerves.

In addition, the trunk is an intercom zone between the medulla and the rest of the brain. Through the trunk pass all the paths that carry sensory information from the medulla, and all descending motor orders of the brain hemispheres. It also affects the excitability of most CNS neurons.

Visit here our Visual and interactive brain atlas


Bear, M.F .; Connors, B.W. i Paradiso, M.A. (1998). Neuroscience: exploring the brain. Barcelona: Masson-William & Wilkins Spain.

Bloom, F.E. i Lazerson, A. (1988). Brain, Mind, and Behavior. Nova York: Freeman and Company.

Bradford, H.F. (1988). Fundamentals of neurochemistry. Barcelona: Labor.

Carlson, N.R. (1999). Behavioral physiology. Barcelona: Ariel Psychology.

Carpenter, M.B. (1994). Neuroanatomy Fundamentals Buenos Aires: Panamerican Editorial.

From April, A .; Ambrose, E .; De Blas, M.R .; Caminero, A .; From Pablo, J.M. i Sandoval, E. (eds) (1999). Biological basis of behavior. Madrid: Sanz and Torres.

Diamond, M.C .; Scheibel, A.B. i Elson, L.M. (nineteen ninety six). The human brain Work book. Barcelona: Ariel.

Guyton, A.C. (1994) Anatomy and physiology of the nervous system. Basic Neuroscience Madrid: Pan American Medical Editorial.

Kandel, E.R .; Shwartz, J.H. and Jessell, T.M. (eds) (1997) Neuroscience and Behavior. Madrid: Prentice Hall.

Martin, J.H. (1998) Neuroanatomy. Madrid: Prentice Hall.

Nelson, R.J. (1996) Psychoendocrinology. The hormonal bases of behavior. Barcelona: Ariel.

Netter, F.M. (1987) Nervous System, Anatomy and Physiology. A Ciba Collection of Medical Illustrations (volum 1) Barcelona: Salvat.

Nolte, J. (1994) The human brain: introduction to functional anatomy. Madrid: Mosby-Doyma.

Related tests
  • Depression test
  • Goldberg depression test
  • Self-knowledge test
  • how do others see you?
  • Sensitivity test (PAS)
  • Character test