The brain is the single most complex machine known to humans. Millions of years of evolution have produced a complex network of neurons and synapses whose coordinated activity controls every function in the human body. Figuring out the organization and structure of the brain is the goal of neuroscience and cognitive science.
The brain has developed different anatomical regions that are specialized for certain functions. Since the brain has evolved over millions of years, it has several distinct layers of organization that reflect different levels of development through evolutionary history. For example, the cerebrum is the highest part of the brain and is the most recent evolutionary addition to the brain’s structure. The cerebrum controls most of the advanced cognitive functions associated with higher mammals, like reasoning, speech, and interpreting sense. The cerebellum is the second layer of the brain and controls more basic functions, like coordinating muscle movement and maintaining posture/balance.
The brain stem is the lowest and “oldest” region of the brain. The brain stem handles the most basic functions required for survival; things like, heart rate, reflexes, breathing, digestion, and regulating sleep. There is a substantial amount of evidence that the brainstem plays an integral role in regulating consciousness and awareness.
The brain stem also serves as an important conduction point that connects the brain to the motor/sensory nerves in the spinal cord. The brain stem gives rise to cranial nerves 3-12 and acts as the relay point that all signals from the peripheral nervous system in the body must pass through to reach the brain. This includes the corticospinal tract (motor functions) the posterior column-medial lemniscus pathway (fine touch, vibration, proprioception), and the spinothalamic tract (heat, pain, pressure). Because the brain stem plays such an integral role in the functioning of the brain, brain stem damage is very serious and often fatal.
Parts And Functions Of The Brain Stem
The brain stem can be divided into three major parts, each of which handles the bulk of some specific set of functions.
The midbrain (also called the mesencephalon) is the highest and first part of the brain stem. The midbrain consists of three general parts. The tectum (Latin: roof) is the highest part of the brain stem and forms the tissue that connects the brain stem to the lowest parts of the cerebellum. The main role of the tectum is to regulate reflex activity in response to visual and auditory stimuli and engage in basic level visuoauditory processing.
The tegmentum is the second part of the midbrain and is much larger than the tectum. The tegmentum is a complex of synaptic neurons that connect to both the cerebellum above and the pons below. The tegmentum primarily plays a role in homeostasis and motor reflex action. Many of the body’s nervous tracts pass through the tegmentum so it serves as a major relay to conduct motor signals to the rest of the brain.
Lastly, the ventral tegmentum is located near the midline of the midbrain. the ventral tegmentum shares connections with the forebrain and is the largest dopamine-producing area in the brain. As such, the ventral tegmentum is heavily involved in the neural reward system.
The pons is the second division of the brain stem and is located between the midbrain and the medulla and anterior to the cerebellum. The pons can be divided into two general regions: the ventral pons, and the dorsal pons. The pons is important as a number of cranial nerve nuclei are present in the pons.
The pons performs many different functions. Most importantly, neural pathways in the pons are responsible for regulating respiration. The pons is the part of the brain that ensures you keep breathing even when you are asleep or not consciously paying attention. The pons also contains a number of cranial nerves and is implicated in swallowing, facial expressions, facial sensation, and eye movement. Damage to the pons can impair any of these functions. For example, Central pontine myelinolysis is a condition caused by damage to the myelin sheath of cells in the pons and is characterized by difficulties in swallowing, speaking, and breathing. The pons also serves as a message center that relays signals between the cerebellum and cerebrum.
Lastly, there is a great deal of evidence that implicates the pons as the brain region that generates REM sleep cycles.
The medulla, also called the medulla oblongata, is the lowest and “oldest” part of the brain stem. Aside from being the thing that makes alligators so ornery, the medulla is in control of the basic functions of breathing, heart rate, and autonomic reflexes like sneezing and vomiting. The medulla has two main parts; the upper open part that connects to the fourth ventricle and the lower closed part that narrow into the obex and connects to the main length of the spinal cord.
The medulla is important for 2 major reasons; First, the medulla controls the most basic and crucial functions needed for life. The medulla controls these things automatically so we do not even have to think of them. Without the medulla, the body could not regulate its own heartbeat, blood pressure or breathing. Autonomic functions are regulated by the medulla using chemoreceptors. For example, the brain stem regulates breathing by chemoreceptors that detect changes in the acidity of the blood. when blood acidity reaches a certain level, the medulla sends electric signals to the intercostal and phrenical muscles to increase the rate of lung contraction.
Second, the medulla forms the region of the brain that all nervous signals (barring some reflex actions) must pass through when going to the brain or to the body. Thus, the medulla connects the higher parts of the brain directly to the spinal cord and serves as the intermediary between the two.
Disorders Involving The Brain Stem
As is often the case in neuroanatomy, one way of figuring out exactly which functions certain parts of the brain stem perform is to see what happens when those parts of the brain are damaged. Sometimes, seeing what happens when a machine malfunctions can give clues regarding its internal organization and structure.
Since the brain stem is an integral part of the brain’s functioning, disorders involving the brain stem are serious and most often fatal. Damage to the brain stem can compromise the most basic functions required for survival, including breathing, circulation, and internal temperature regulation. Because the brain stem handles so many functions, there are a number of different pathologies associated with brain stem injuries.
One particularly extreme and rare condition involving damage to the brain stem is locked-in syndrome. Locked-in syndrome is characterized by complete paralysis of all voluntary muscles in the body except for eye movement and blinking. Patients with locked-in-syndrome are conscious and aware of their environment but are unable to move.
Locked in syndrome is caused by damage to the lower parts of the brain stem. Damage to the brain stem can interrupt the conduction of nerve signals and prevents motor nerve signals from being sent to parts of the body. In very severe cases, patients with locked-in syndrome suffer from respiratory failure and have to be kept alive with a respirator.
Evolution Of The Brain Stem
Since the brain stem is the lowest part of the vertebrate brain, it is considered the “oldest” part of the brain, evolutionarily speaking. Rudimentary versions of the brain stem appear in the evolutionary record somewhere between 600 to 420 million years ago; roughly contemporaneous with the emergence of complex eukaryotic life. Regarding the current function of brain structures, the brain stem generates arousal, reward, and stress; the most basic elements necessary for memory formation and immediate survival.
The evolutionary history of the brain can be gleaned by observing anatomical and functional regions the human brain shares with other species of animals. In a nutshell, by comparing neuroanatomical regions and structures, scientists can work backward to figure out when exactly certain structures of the brain emerged. For example, the hindbrain and associated regions of the midbrain, pons, and medulla are sometimes called the “reptilian brain” to signify neural and structural analogs present in reptilian species. Moving up the evolutionary record, we see the emergence of more complex brain structures in tandem with the evolution of mammals. By the time we reach early hominids, the mammalian cortex has already formed and we begin to see the emergence of the distinctly human pre-frontal lobe.
To sum up, the brain stem is the lowest anatomical region of the brain. The brain stem is divided into three major parts; the midbrain, the pons, and the medulla. The brain stem controls the bulk of autonomic physiological functions, including respiration, heartbeat, reflexes, digestion, and sleep. The brain stem first emerged in the evolutionary record about 600-420 million years ago and is thus the “oldest” part of the human brain.