Brain Anatomy

A basic understanding of brain anatomy is required because stroke is a brain injury and the  effects of stroke depend on the region of brain damaged by the stroke. It helps to have a clear idea about the parts of the brain and their functions.

The main components of brain anatomy are:

• the cerebrum with left and right cerebral hemispheres
• the cerebellum
• the brain stem.

The two cerebral hemispheres are separated by the medial longitudinal fissure and these constitute the largest part of the brain. The cerebellum is a part of the brain located behind the brain stem which is the lowest part of the brain and is structurally continuous with the spinal cord.

The key parts of brain anatomy are identified in the diagram below (from www.howstuffworks.com).

It is also useful to understand the cognitive domains, which explain the brain's functioning.

Brain Anatomy: Cerebrum

The cerebrum is the largest part of brain anatomy, and lies in the most superior and anterior region of the central nervous system. It makes up a major part of the brain's weight. The cerebrum directs the motor functions of the body and is the thinking part of the brain.

The cerebrum is divided into two hemispheres - the right and left hemispheres which are connected by the corpus callosum. The right side of the cerebrum is responsible for controlling imagination and 3-D forms and the left side analyses and controls numbering skills, posture, and reasoning. The cerebral hemispheres also include parts such as the lobes.

Most people have a basic understanding of brain anatomy and are familiar with the concept of the the right cerebral hemisphere controlling the left side of the body and vice versa.

Each cerebral hemisphere is further divided into the frontal, parietal, temporal, and the occipital lobes. These hemispheres also consist of an inner core called the white matter and a wrinkly outer layer called the cortex.

Frontal Lobe

The frontal lobe is responsible for higher mental functions such as reasoning, problem solving, decision making, judgment, impulse control and higher emotions such as empathy and altruism. It is also involved in motor control and non-task-based longer term memories. Impulse control is an important behavioural aspect as this governs how we initiate and inhibit behaviours.

Someone with damage to the frontal lobe may be less inhibited in social situations than others. In normal life, we often don't say everything that we think - someone with less inhibition may do so, and get themselves into some difficult situations.

The motor cortex is part of the frontal lobe and this is borders the parietal lobe. The motor cortex controls movement on the opposite side of the body. When this part of the brain is damaged, the likely outcome is weakness and a loss of coordination.

The central sulcus separates it from the parietal lobe. The lateral fissure divides it from the temporal lobe. Broca’s area is found in the hemisphere that is dominant for language. It is one of the main language areas in the cerebral cortex because it controls the motor aspects of speech. 

The precentral gyrus or the primary motor area lies immediately anterior to the central sulcus. It is known to control the voluntary movements of skeletal muscles. The premotor area lies immediately anterior to the primary motor area, and is responsible for the programming for motor movements.

The frontal lobe is one of the most complex parts of brain anatomy.

Parietal Lobe

The parietal lobe is directly behind the frontal lobe.

The primary sensory cortex is involved in processing pain and touch sensations. If this area is damaged, it often results in a lack of feeling in the opposite side of the body.

An important function of the parietal lobe is the responsibility for body self-awareness. When this is damaged, particularly in the case of the right parietal lobe, the person can have trouble recognizing parts of their own body and they may have spatial difficulties. Practically, this can be dangerous as the person might walk into things as their judgement of the distance between things may not be correct.

Stroke patients with right side damage particularly, may demonstrate what is known as "neglect" of their left side of the body. This can be as extreme as totally ignoring the left side of the body or as simple as not being aware of a shirt hanging out on that side.

The part of parietal lobe behind the sensory cortex is associated with cognition, movement, orientation, recognition and speech. Functionally, this translates into the ability to make logical connections, mathematical ability, time, sequencing and memories of familiar activities.

Temporal Lobe

Long term memories are stored in both temporal lobes. Damage to one side doesn't usually cause a severe loss of memory, however if both sides are damaged, the impact can be significant. This lobe is also involved in emotion, memory and speech.

The temporal lobe is involved in auditory sensation. Wernicke's Area is specifically involved in language comprehension. If Wernicke's Area is damaged, the result is often fluent aphasia, which is where the patient speaks fluently but the words are meaningless. Further they are unable to understand either written or spoken words.

Wernicke's Area (inthe temporal lobe) and Broca's Area (in the frontal lobe) are the key areas related to speech. These occur only in the dominant hemisphere, which is usually the left one. As a result, those who have a stroke on the left side of their brain often have speech and/or communication problems.

The temporal lobe is also where our appreciation of music is processed. This makes sense as it is part of the auditory experience.

Occipital Lobe

The occipital lobe is at the back of the brain, behind the parietal lobe. This lobe controls visual sensation, perception and processing. This includes the mechanics of how we read, as reading is a visual perception.

If this lobe is damaged, the result can be loss of visual perception and partial or complete blindness.

Brain Anatomy: Cerebellum

The cerebellum is the second largest area of  brain anatomy. It consists of two hemispheres, or halves, as well as a middle portion. The cerebellum is connected to the brain stem, which is located directly below it.

The cerebellum is the part of the brain where the high level functions take place.  The cerebellum is involved in the coordination of voluntary motor movement, balance and equilibrium and muscle tone. 

Cerebellar injury results in movements that are slow and uncoordinated. Individuals with cerebellar lesions tend to sway and stagger when walking.

Brain Anatomy: Brain Stem

The brain stem is the lower-most extension of the brain's anatomy, connecting the cerebrum with the spinal cord. Most of the cranial nerves come from the brain stem.

Critical functions, such as breathing and wakefulness, are controlled by the brain stem. Whilst this part of the brain does not process information, it is fundamental to the operation of the brain. Brain stem damage can cause other parts of the brain to shut down. Extreme damage can cause coma or death. In less extreme cases, brain stem damage can result in severe weakness, loss of sensation and difficulty in swallowing.

The brain stem plays a key role in regulating basic attention, arousal, and consciousness. All information to and from our body passes through the brain stem on the way to or from the brain. Like the frontal and temporal lobes, the brain stem is located in an area near bony protrusions making it vulnerable to damage during trauma.

Whilst the brain stem is relatively small, a stroke in this area can have significantly greater symptoms than a larger stroke in other areas.

The midbrain, pons, medulla oblongata and reticular formation are all part of the brain stem.

Midbrain

The midbrain is the smallest part of the brain's anatomy and contains auditory and visual reflex centers. It controls important functions such as eye movements.

Pons

The pons is a bridge-like structure which links different parts of the brain and serves as a relay station from the medulla to the higher cortical structures of the brain. It contains the respiratory center. It also controls sleep and arousal.

The recticular formation is centered roughly in the pons and signals the cerebral cortex to attend to new stimulations and to remain alert even during sleep. It is an important regulator in the autonomic nervous system.

Medulla oblongata

The medulla oblongata is the lowest part of the brain stem. It functions primarily as a relay station for the crossing of motor tracts between the spinal cord and the brain. It regulates the reflex responses that control breathing, heart beat blood pressure and other essential involuntary actions. 

Reticular formation

The reticular formation signals the cerebral cortex to attend to new stimulation and to remain alert even during sleep.

Brain Anatomy: Limbic system

The limbic system is located oin top of the brain stem. The key components are the thalamus, the hypothalamus, the hippocampus and the amygdala.

The limbic system is primarily involved in our emotions and has a lot to do with the formation of memories. The limbic system is also involved in feelings of pleasure such as those experienced from eating and sex.

Amygdala and Hippocampus

The amygdala and hippocampus play important roles in memory.

The amygdala is involved in emotional responses, hormonal secretions and memory. It determines what memories are stored and where in the brain they are stored. This determination is thought to be based on the magnitude of the emotional response during the event. The hippocampus is particularly involved with sending out memory for long term storage and with the retrieval of such memories.

Damage to this area of the brain may result in an inability to form new memories.

Thalamus and Hypothalamus

The diencephalon is also a part of the limbic system. It is located beneath the cerebral hemispheres and contains the thalamus and hypothalamus. It relays information received from the diverse brain anatomy to the cerebral cortex. 

The thalamus is involved in sensory perception and regulation of motor functions. It connects areas of the cerebral cortex that are involved in sensory perception and movement with other parts of the brain and spinal cord that also have a role in sensation and movement.

The hypothalamus is a very small but important part of the brain's anatomy. It is one of the busiest parts of the brain, and is mainly concerned with homeostatis. This is the body's internal stability. It has regulatory functions around thirst, hunger, body temperature, water balance and blook pressure. There is also a link between the nervous system and the endocrine system.