The adult human brain weighs about 1.4kg. Even though it is approximately 2% of the total body weight, it receives 20% of the blood and energy supplied to the body. Together with the spinal cord, they comprise the Central Nervous System (CNS).
In this post, we will look at the different parts of the telencephalon, the most developed part of the brain.
The brain has three main parts: the cerebrum, cerebellum, and brain stem.
Cerebrum: the largest part of the brain and it consists of two cerebral hemispheres with lobes on each hemisphere. It performs higher functions like interpreting touch, vision, and hearing, as well as speech, reasoning, emotions, learning, and fine control of movement. Each hemisphere is divided into lobes.
Cerebellum: located under the cerebrum. Its function is to coordinate muscle movements, maintain posture, and balance. It also processes information from other areas of the brain and spinal cord, and it has sensory and motor pathways.
Brain stem: acts as a relay center connecting the cerebrum and cerebellum to the spinal cord. It performs many automatic functions such as breathing, heart rate, body temperature, wake and sleep cycles, digestion, sneezing, coughing, vomiting, and swallowing.
Hemispheres
The cerebrum consists of two cerebral hemispheres which are roughly symmetrical in structure and are connected by the corpus callosum. They
📔 The corpus callosum is the region of the brain that connects the left and right cerebral hemispheres (it plays a key role in the coordination between the two hemispheres). It is a thick nerve tract (bundle of commissural fibers) beneath the cerebral cortex. It consists of white matter tracts and myelinated axons, and around 20 million fibers connect the association areas of the cerebral hemispheres. When the corpus callosum and the anterior commissure are destroyed, the two hemispheres work relatively independently.
Hemispheric Specialization
The left and right sides of the brain are specialized to attend to different information, process sensory inputs in different ways, and control different types of behavior. This active collaboration between the two hemispheres is called hemispheric specialization, or brain lateralization.
Each hemisphere is specialized in a series of tasks. In some people, the functions of the left and right brains are reversed.
Left brain: analytical vision and sequencing
Right brain: holistic and synthetic vision
However, this specialization is relative because, with a few exceptions, both hemispheres can process almost all types of information, although they do so in different manners.
Lobes and Functions
📔The lobes of the brain are the major identifiable zones of the cerebral cortex, and they comprise the surface of each hemisphere of the cerebrum.
The two hemispheres have six lobes each. Each lobe of the brain has numerous gyri and sulci that constitute further subzones of the cortex (the lobes are separated by fissures).
Temporal lobe
Frontal lobe
Occipital lobe
Parietal lobe
Limbic lobe
Insula
Brain Development
The brain and the spinal cord arise in early development from the neural tube to form the three primary brain divisions: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These three vesicles further differentiate into five subdivisions: telencephalon, diencephalon (forebrain), mesencephalon (midbrain), metencephalon, and myelencephalon (hindbrain). The mesencephalon, metencephalon, and myelencephalon comprise the brain stem.
The telencephalon is the most developed part of the brain and it has a large number of (superior) functions. It encompasses, among others, the cerebral cortex (outer layer of the brain), the basal ganglia, and the limbic system.
Cerebral Cortex
The cerebral cortex is the outer layer of neural tissue of the cerebrum of the brain. It plays a key role in attention, perception, awareness, thought, memory, language, and consciousness, and it is part of the brain responsible for cognition.
The cerebral cortex is made up of grey matter (comprises cell bodies, axons, dendrites, and neuroglia). It looks grey because nerves in this area lack the insulation (myelin) that makes most other parts of the brain appear to be white.
📔 A gyrus (pl. gyri) is a ridge on the cerebral cortex. It is generally surrounded by one or more sulci (sg. sulcus), which are depressions or furrows. Gyri and sulci create the folded appearance of the brain. 2/3 of the brain is hidden in the furrows.
The three main sulci are:
Longitudinal fissure (or interhemispheric fissure): the deep groove that separates the two cerebral hemispheres.
Central sulcus (or Rolandic fissure): the groove that separates the parietal lobe from the frontal lobe and the primary motor cortex from the primary somatosensory cortex.
Lateral sulcus (or Sylvian fissure) is one of the most prominent features of the human brain. The lateral sulcus is a deep fissure in each hemisphere that separates the frontal and parietal lobes from the temporal lobe.
Histology of the Cerebral Cortex
There are 5 types of neuronal cells in the cerebral cortex.
Pyramidal cells (usually one apical dendrite and multiple basal dendrites with one long axon that leaves the cortex and enters the subcortical white matter). They make up to 75% of the cellular component of the cortex and they are the main output neurons of the cerebral cortex. They send their myelinated axons to other areas of the cortex and the rest of the CNS. They are excitatory neurons and use glutamate as their neurotransmitter.
The rest of the neurons are interneurons, and their axons don’t leave the cortex.
Granular (stellate) cells (usually small, they resemble a star, located all over the cortex).
Fusiform cells (usually placed in the deepest cortical layer).
Horizontal cells of Cajal-Retzius (only seen in the most superficial part of the cortex, very rare, one axon and one dendrite).
Cells of Martinotti (multipolar neurons that are most densely located within the deepest layer of the cortex; their numerous axons and dendrites course towards the surface of the cortex).
Those different kinds of cells are laid out in six horizontal layers parallel to the surface:
Molecular (plexiform) layer - afferent receptor
External granular layer - afferent receptor/efferent
External pyramidal layer - efferent
Internal granular layer - afferent receptor
Internal pyramidal layer - efferent
Multiform (fusiform) layer - Efferent
Types of Fibers in the Cortex
What fibers connect the different parts within, and outside of the brain?
📔 A fiber is a bundle of axons. Fibers can be categorized by their function into association fibers, projection fibers, and commissural fibers.
Association fibers are bundles of axons within the brain that unite different parts of the same cerebral hemisphere. There are two kinds: short-association fibers: (those connecting adjacent gyri) and long-association fibers (those passing between more distant parts).
Commissural fibers are axons that connect an area in one hemisphere of the brain with an area in the opposite hemisphere. Commissural fibers make up tracts that include: the corpus callosum (the largest set of commissural fibers in the brain and is a pathway of crucial importance to speech-language functions), anterior commissure, and the hippocampal commissure.
Projection fibers connect the cortex with other subcortical areas in the CNS (thalamus, brainstem, striatum, etc.). They may be efferent (motor) or afferent tracts (sensory). The most important one is corona radiata (connecting the internal capsule with the telencephalon).
Basal Ganglia
The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, situated at the base of the forebrain and top of the midbrain.
📔 Subcortical structures are a group of diverse neural formations deep within the brain, which include the deep gray and white matter structures (such as the corpus callosum, hippocampus, amygdala, thalamus, and putamen). Basal ganglia, the striatum, the limbic system, and the rhinencephalon (concerned with the sense of smell).
Basal ganglia are strongly interconnected with the cerebral cortex, thalamus, and brainstem, as well as several other brain areas. The basal ganglia are associated with a variety of functions, including control of voluntary motor movements, procedural learning, habit learning, conditional learning, eye movements, cognition, and emotion.
The basal ganglia include the caudate, putamen, globus pallidus, substantia nigra, and subthalamic nucleus. The caudate and putamen are often referred to collectively as the striatum, and the globus pallidus and substantia nigra are each made up of multiple nuclei.
📔 The Corpus Striatum is the largest subcortical brain structure of the basal ganglia. This structure receives afferents from several cortical and subcortical structures and projects to various basal ganglia nuclei.
Most parts of the cerebral cortex project to the striatum (caudate nucleus + putamen), and then to the globus pallidus, substantia nigra, and thalamus, and from there to the motor cortex.
Much of the information the basal ganglia receives comes from the cerebral cortex and travels first to the caudate or putamen, the main input nuclei of the basal ganglia. The globus pallidus and substantia nigra are the main output nuclei, and they send projections out from the basal ganglia to the cerebral cortex, mostly by way of the thalamus, as well as to nuclei in the brainstem.
Functions of Basal Ganglia
The basal ganglia connect the motor cortex, premotor cortex, cerebellum, red nucleus, and spinal cord. They are responsible primarily for motor control, as well as other roles such as motor learning, executive functions and behaviors, and emotions. It is also closely connected to the limbic system and the prefrontal cortex (motivational and emotional influences in our motor system).
💡 Did you know that the caudate nucleus is related to pessimism? Stimulating the brain’s caudate nucleus generates a negative outlook that clouds decision-making. – Graybiel (2018), MIT
The basal ganglia control the beginning and end of movement, posture, muscle tone, balance, and even spontaneous laughter.
Malfunction of Basal Ganglia
Huntington's disease is a hereditary disease in which basal ganglia degenerate. It begins with occasional involuntary jerking or spasms, then progresses to more pronounced involuntary movements (chorea and athetosis), mental deterioration, and death.
Parkinson's disease is a progressive brain disorder that affects nerve cells in the basal ganglia and the substantia nigra (nerve cells in the substantia nigra produce the neurotransmitter dopamine and are responsible for relaying messages that plan and control body movement.). It causes unintended or uncontrollable movements, such as shaking, stiffness, and difficulty with balance and coordination. Symptoms usually begin gradually and worsen over time.
Limbic System
The limbic system is located within the cerebrum of the brain, immediately below the temporal lobes, and buried under the cerebral cortex. It is the part of the brain involved in our behavioral and emotional responses (especially behaviors we need for survival, like feeding, reproduction, or fight-or-flight responses), as well as long-term memory, and olfaction.
The set of anatomical structures considered part of the limbic system is controversial. Some structures are located in Cortical and subcortical areas (telencephalon), while others are diencephalic structures.
Parts of the limbic system in the telencephalon:
Amygdala: located deep within the temporal lobes and takes part in a number of emotional processes, especially fear and defensive aggression (fight or flight), as well as emotional learning.
Hippocampus: involved mainly with memory. It interprets how important an experience is and, if it is relevant, it stores it.
Anterograde amnesia is a type of memory loss that occurs when you can't form new memories.
Fornix: connects the hypothalamus with the cerebrum. It also connects the hippocampus with the mamillary body and the hypothalamus.
Diencephalic structures:
Thalamus: the main (and first) relay station for the brain (motor pathways, limbic pathways, and sensory pathways except for olfaction all pass through this central structure). Sensory information travels through nerve fibers from your body through brain structures to your thalamus. Specialized areas of your thalamus, called nuclei, are each responsible for processing different sensory or motor impulses received from your body and then sending the selected information through nerve fibers to the related area of your cerebral cortex for interpretation.
Hypothalamus: keeps your body balanced in a stable state called homeostasis (vegetative nervous system, endocrine system, survival-related behavior, etc.). It's the main link between our endocrine system and our nervous system. It consists of diverse nuclei and interconnected fibers and it is linked to the pituitary gland (or hypophysis).
Epithalamu: contains the habenular nuclei and their interconnecting fibers, the habenular commissure, the stria medullaris, and the pineal gland.
Pineal gland (epiphysis): small endocrine gland that produces melatonin, a serotonin-derived hormone that modulates sleep patterns in both circadian and seasonal cycles.
Subthalamus: related to motor control. It connects to the globus pallidus (basal nucleus of the telencephalon). It is located ventral to the thalamus, medial to the internal capsule, and lateral to the hypothalamus. It is a region formed by several grey matter nuclei and their associated white matter structures