Brain Stem: Definition, Structure, And Function

Brain Stem Definition

The brainstem is part of the brain that connects the cerebrum with the spinal cord. It is located at the base of the brain and is responsible for carrying signals that control body functions. The brainstem is composed of three sections: midbrain, pons, and medulla oblongata.

Background         

Before looking into each of the sub-structures and their respective functions within the brain stem, let’s first look at the brain stem’s relation to the nervous system.

The Nervous System, Neurons, and Brain

The nervous system is responsible for sending messages back and forth from the brain, through the spinal cord, to the nerves throughout the body. It uses tiny cells called neurons to create a communication network.

Billions of neurons work together to send information from sensory neurons in the eyes, ears, nose, tongue, and skin to the brain. Motor neurons carry messages away from the brain to allow muscles to move.

All neurons relay information to each other through a complex electrochemical process that affects how we think, learn, move, and behave.

The brain controls all of the body’s functions and has three main sections: the forebrain, midbrain, and hindbrain. The cerebrum is part of the forebrain and is responsible for intelligence, learning, and memory.

As we grow and learn new things, messages travel from one neuron to another over and over again creating connections or pathways in our brains. One major objective of modern neuroscience is therefore to unravel the patterns of connections within the nervous system—in a word, to map the brain.

The nervous system works together with other parts of our body such as cells, tissues, organ systems in maintaining homeostasis. The cell is the basic unit of a living organism. A group of cells forms a tissue; a group of tissues form an organ; a group of organs working together form a system.

The Brain Stem

The brain stem is the lower part of the brain that connects to the spinal cord and is responsible for regulating most of the body’s automatic functions that are essential for life, such as breathing and heartbeat.

It also plays a role in maintaining consciousness and regulating the sleep cycle. The brainstem contains many control centers for vital body functions, such as swallowing, breathing, and vasomotor control.

The brainstem is composed entirely of neural tissue and gets bathed in cerebrospinal fluid. It contains the cranial nerve nuclei for cranial nerves III-XII; therefore, it provides a motor and sensory function to structures of the head and neck. The three components of the brainstem are the medulla oblongata, midbrain, and pons.

Occasionally, a person’s limbs or torso may move even after brain stem death has been diagnosed. These spinal reflex movements are generated by the spinal cord and don’t involve the brain at all. Therefore, they won’t affect the diagnosis of brain death.

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The Autonomic Nervous System

The autonomic nervous system is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal.

It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric. The sympathetic and parasympathetic nervous systems are antagonistic sets of nerves that control and regulate internal organs without any conscious recognition or effort by the organism.

The autonomic nervous system is responsible for handling unconscious tasks like heartbeat and breathing. It controls the automatic functions of the body that are necessary for survival.

The autonomic nervous system comprises nerve fibers that secrete acetylcholine (cholinergic fibers) and norepinephrine (adrenergic fibers). Acetylcholine has parasympathetic (inhibiting) effects while norepinephrine has sympathetic (stimulating) effects.

Dysfunction of the autonomic nervous system can lead to conditions such as orthostatic hypotension, postural tachycardia syndrome, and neurogenic bladder. Taking care of the body, especially the nervous system is important to avoid conditions that can cause autonomic nerve damage.

Brain Stem Structures and Functions

The brainstem is composed of three sections in descending order: the midbrain, pons, and medulla oblongata. It connects the cerebrum of the brain to the spinal cord and cerebellum.

The brainstem houses many control centers for vital body functions such as breathing, consciousness, blood pressure, heart rate, swallowing, vasomotor control, and sleep.

It contains critical collections of white and grey matter. The midbrain is responsible for visual and auditory reflexes while the pons helps regulate breathing and facial movements. The medulla oblongata controls vital functions such as heart rate, blood pressure, and respiration.

Midbrain Overview

The midbrain is a small region located at the topmost part of the brainstem, connecting the cerebrum of the brain to the spinal cord and cerebellum. It is composed of three sections in descending order: the midbrain, pons, and medulla oblongata.

The midbrain serves important functions in motor movement, particularly movements of the eye, and in auditory and visual processing. It regulates hearing, vision, movement, pain, sleep, breathing, consciousness, blood pressure, heart rate, and sleep.

The midbrain is formed by three main structures: the cerebral peduncle (peduncle meaning ‘foot’ or ‘base’ of the cerebrum), the corpora quadrigemina (which contains two pairs of sensory nuclei called colliculi), and the tegmentum (which contains several nuclei involved in autonomic control).

The superior colliculus receives input from the retina and visual cortex and participates in a variety of visual reflexes. The inferior colliculus does work on auditory signals before those are passed through the thalamus to the main auditory processing center in the cortex.

Injuries or lesions to this area can cause various symptoms depending on which part is affected. For example, damage to one side of the midbrain can cause paralysis on one side of your body while damage to both sides can cause coma or death. Parkinson’s disease is also associated with the degeneration of neurons that produce dopamine in this area.

Midbrain Regions and Functions

The midbrain is a small region located at the topmost part of the brainstem, between the forebrain and hindbrain. It is composed of three main parts: the colliculi, the tegmentum, and the cerebral peduncles.

The colliculi are responsible for processing visual and auditory information. The superior colliculus receives input from the retina and visual cortex and participates in various visual reflexes, while the inferior colliculus processes auditory signals before passing them through the thalamus to the main auditory processing center in the cortex.

The tegmentum stretches down the length of the brainstem but a portion of it forms part of the midbrain. It contains two areas named after specific colors: The iron-rich red nucleus (which actually looks pink) is involved in coordinating movements, while the black-colored substantia nigra produces dopamine that helps regulate movement.

The midbrain also contains clusters of serotonin-producing neurons called raphe nuclei that send serotonin throughout the central nervous system.

The midbrain regulates sensory and motor information such as movement of eyes, upper limbs, vision, hearing, pain perception, sleep/wake cycle regulation, arousal (alertness), and temperature regulation. It acts as a relay station for auditory and visual information.

Midbrain lesions can cause various symptoms depending on their location within this region. For example, damage to one side of the substantia nigra can lead to Parkinson’s disease characterized by tremors or rigidity on one side of the body.

Pons Overview and Functions

The pons is a part of the hindbrain that is situated in front of the cerebellum and continuous behind and below the medulla oblongata. It gets its name from the Latin word for “bridge” because it connects the rest of the brainstem to the cerebral cortex.

The pons serves as a coordination center for signals and communications that flow between the two brain hemispheres and help regulate facial movements, hearing, balance, breathing, heart rhythms, blood pressure, swallowing, sleep-wake cycles, alertness, and attentiveness to surroundings.

The brainstem is composed of three sections in descending order: midbrain (or mesencephalon), pons (part of metencephalon), and medulla oblongata (myelencephalon).

The midbrain contains an important nucleus called substantia nigra that is rich in dopamine neurons and considered part of the basal ganglia. In Parkinson’s disease, neurodegeneration occurs in the substantia nigra causing movement dysfunction.

The medulla oblongata lies between the pons rostrally and the spinal cord caudally. It is continuous with inferior cerebellar peduncle and pyramids.

Medulla Oblongata Overview

The medulla oblongata is a part of the brainstem that connects the brain to the spinal cord. It is located at the bottom-most part of the brain and is continuous posteriorly with the spinal cord, with which it merges at the opening (foramen magnum) at the base of the skull.

The medulla oblongata plays a critical role in transmitting signals between the spinal cord and the higher parts of the brain. It is also a key conduit for nerve signals to and from your body, making it an essential connection point in your nervous system.

The medulla oblongata controls many vital processes such as heartbeat, breathing, blood pressure, swallowing, coughing, vomiting, and sneezing. It also manages other important functions of your body such as taste sensation and muscle movements in your tongue.

The medulla oblongata consists of both myelinated (white matter) and unmyelinated (gray matter) nerve fibers. The white matter of the medulla is intermingled with gray matter giving rise to part of the reticular formation which plays a role in regulating sleep-wake cycles.

Damage to or disorders affecting the medulla oblongata can cause serious health problems. For example, damage to this area can lead to respiratory failure or even death due to its control over breathing.

Disorders affecting this area can also cause difficulty swallowing or speaking, dizziness or vertigo, nausea or vomiting, and loss of coordination or balance, among others.

Medulla Oblongata Regions and Functions

The medulla oblongata is the lowest part of the brainstem and is continuous posteriorly with the spinal cord. It is divided into two main parts: the ventral medulla (the frontal portion) and the dorsal medulla (the rear portion).

The dorsal medulla is also known as the gray matter on the inside and is the site of origin for the last seven cranial nerves, most of which exit the medulla ventrally.

The medulla consists of both myelinated (white matter) and unmyelinated (gray matter) nerve fibers, and, similar to other structures in the brainstem, the white matter of the medulla, rather than lying beneath the gray matter, is intermingled with it.

The medulla oblongata carries multiple important functional centers that control vital processes like heartbeat, breathing, blood pressure, swallowing, vomiting, coughing, and sneezing.

It houses nuclei such as cranial nerve nuclei (IX-XII), an inferior salivatory nucleus, a spinal nucleus of the trigeminal nerve, a solitary nucleus, a nucleus ambiguus, a dorsal nucleus of the vagus nerve, a nucleus of the accessory nerve and a nucleus of the hypoglossal nerve.

The paramedian bulbar arteries arise from vertebral arteries and supply medial aspects of the medulla. At its most caudal part lies paramedian bulbar arteries.

Cranial Nerves of the Brain Stem

The brainstem contains the cranial nerve nuclei for cranial nerves III-XII, which are responsible for the innervation of muscles controlled by these nerves. The optic nerve (CN 2) is associated with the thalamus, while the oculomotor (CN 3) and trochlear (CN 4) nerves are associated with the midbrain.

The trigeminal nerve (CN 5) is associated with the pons, and the abducent (CN 6), facial (CN 7), vestibulocochlear (CN 8), glossopharyngeal (CN 9), vagus (CN 10), accessory (CN 11), and hypoglossal (CN 12) nerves are associated with different parts of the medulla oblongata.

The cranial nerves can be described as being sensory, motor, or both. They can transmit seven types of information; three are unique to cranial nerves: special somatic sensory, special visceral sensory, and special visceral motor.

Cranial nerves I, II, and VIII pertain to organs of special sense. The others that are largely efferent are III, IV, VI, XI, and XII. The ones that contain both afferent and efferent fibers are V, VII, IX, and X.