Structure and Function of the Brain

Development of the Human Brain

The mental processes and behaviors studied by psychology are directly controlled by the brain, one of the most complex systems in nature.

Learning Objectives

Explain the structure of the major layers of the brain

Key Takeaways

Key Points

  • The study of psychology focuses on the interaction of mental processes and behavior on a systemic level, and therefore is intimately related to understanding the brain.
  • One of the most complex systems in nature, the brain is composed of systems that must all work together to keep the human body functioning.
  • The brain is split up into three major layers: the hindbrain, the midbrain, and the forebrain.

Key Terms

  • neural tube: An embryo’s predecessor to the central nervous system.

The human brain is one of the most complex systems on earth. Every component of the brain must work together in order to keep its body functioning. The brain and the spinal cord make up the central nervous system, which alongside the peripheral nervous system is responsible for regulating all bodily functions.

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The central nervous system: 1. Brain  2. Brain stem  3. Spinal cord

Psychology seeks to explain the mental processes and behavior of individuals by studying the interaction between mental processes and behavior on a systemic level. Therefore, the field of psychology is tightly intertwined with the study of the brain.

The Structure of the Brain

The developing brain goes through many stages. In the embryos of vertebrates, the predecessor to the brain and spinal cord is the neural tube. As the fetus develops, the grooves and folds in the neural tube deepen, giving rise to different layers of the brain. The human brain is split up into three major layers: the hindbrain, the midbrain, and the forebrain.

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The embryonic brain: The layers of the embryonic brain. The telencephalon and diencephalon give rise to the forebrain, while the metencephalon and myelencephalon give rise to the hindbrain.

Hindbrain

The hindbrain is the well-protected central core of the brain. It includes the cerebellum, reticular formation, and brain stem, which are responsible for some of the most basic autonomic functions of life, such as breathing and movement. The brain stem contains the pons and medulla oblongata. Evolutionarily speaking, the hindbrain contains the oldest parts of the brain, which all vertebrates possess, though they may look different from species to species.

Midbrain

The midbrain makes up part of the brain stem. It is located between the hindbrain and forebrain. All sensory and motor information that travels between the forebrain and the spinal cord passes through the midbrain, making it a relay station for the central nervous system.

Forebrain

The forebrain is the most anterior division of the developing vertebrate brain, containing the most complex networks in the central nervous system. The forebrain has two major divisions: the diencephalon and the telencephalon. The diencephalon is lower, containing the thalamus and hypothalamus (which together form the limbic system); the telencephalon is on top of the diencephalon and contains the cerebrum, the home of the highest-level cognitive processing in the brain. It is the large and complicated forebrain that distinguishes the human brain from other vertebrate brains.

Lower-Level Structures

The brain’s lower-level structures consist of the brain stem, the spinal cord, and the cerebellum.

Learning Objectives

Outline the location and functions of the lower-level structures of the brain

Key Takeaways

Key Points

  • The brain’s lower-level structures are the oldest in the brain, and are more geared towards basic bodily processes than the higher-level structures.
  • Except for the spinal cord, the brain’s lower-level structures are largely located within the hindbrain, diencephalon (or interbrain), and midbrain.
  • The hindbrain consists of the medulla oblongata, the pons, and the cerebellum, which control respiration and movement among other functions.
  • The midbrain is interposed between the hindbrain and the forebrain. Its ventral areas are dedicated to motor function while the dorsal regions are involved in sensory information circuits.
  • The thalamus and hypothalamus are located within the diencephalon (or “interbrain”), and are part of the limbic system. They regulate emotions and motivated behaviors like sexuality and hunger.
  • The spinal cord is a tail-like structure embedded in the vertebral canal of the spine, and is involved in transporting sensorimotor information and controlling nearby organs.

Key Terms

  • ventral: On the front side of the human body, or the corresponding surface of an animal, usually the lower surface.
  • proprioception: The sense of the position of parts of the body relative to neighbouring parts of the body.
  • dorsal: With respect to, or concerning the side in which the backbone is located, or the analogous side of an invertebrate.

The brain’s lower-level structures consist of the brain stem and spinal cord, along with the cerebellum. With the exception of the spinal cord, these structures are largely located within the hindbrain, diencephalon (or interbrain), and midbrain. These lower dorsal structures are the oldest parts of the brain, having existed for much of its evolutionary history. As such they are geared more toward basic bodily processes necessary to survival. It is the more recent layers of the brain (the forebrain) which are responsible for the higher-level cognitive functioning (language, reasoning) not strictly necessary to keep a body alive.

The Hindbrain

The hindbrain, which includes the medulla oblongata, the pons, and the cerebellum, is responsible some of the oldest and most primitive body functions. Each of these structures is described below.

Medulla Oblongata

The medulla oblongata sits at the transition zone between the brain and the spinal cord. It is the first region that formally belongs to the brain (rather than the spinal cord). It is the control center for respiratory, cardiovascular, and digestive functions.

Pons

The pons connects the medulla oblongata with the midbrain region, and also relays signals from the forebrain to the cerebellum. It houses the control centers for respiration and inhibitory functions. The cerebellum is attached to the dorsal side of the pons.

Cerebellum

The cerebellum is a separate region of the brain located behind the medulla oblongata and pons. It is attached to the rest of the brain by three stalks (called pedunculi), and coordinates skeletal muscles to produce smooth, graceful motions. The cerebellum receives information from our eyes, ears, muscles, and joints about the body’s current positioning (referred to as proprioception). It also receives output from the cerebral cortex about where these body parts should be. After processing this information, the cerebellum sends motor impulses from the brain stem to the skeletal muscles so that they can move. The main function of the cerebellum is this muscle coordination. However, it is also responsible for balance and posture, and it assists us when we are learning a new motor skill, such as playing a sport or musical instrument. Recent research shows that apart from motor functions the cerebellum also has some role in emotional sensitivity.

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Human and shark brains: The shark brain diverged on the evolutionary tree from the human brain, but both still have the “old” structures of the hindbrain and midbrain dedicated to autonomic bodily processes.

The Midbrain

The midbrain is located between the hindbrain and forebrain, but it is actually part of the brain stem. It displays the same basic functional composition found in the spinal cord and the hindbrain. Ventral areas control motor function and convey motor information from the cerebral cortex. Dorsal regions of the midbrain are involved in sensory information circuits. The substantia nigra, a part of the brain that plays a role in reward, addiction, and movement (due to its high levels of dopaminergic neurons) is located in the midbrain. In Parkinson’s disease, which is characterized by a deficit of dopamine, death of the substantia nigra is evident.

The Diencephalon (“interbrain”)

The diencephalon is the region of the embryonic vertebrate neural tube that gives rise to posterior forebrain structures. In adults, the diencephalon appears at the upper end of the brain stem, situated between the cerebrum and the brain stem. It is home to the limbic system, which is considered the seat of emotion in the human brain. The diencephalon is made up of four distinct components: the thalamus, the subthalamus, the hypothalamus, and the epithalamus.

Thalamus

The thalamus is part of the limbic system. It consists of two lobes of grey matter along the bottom of the cerebral cortex. Because nearly all sensory information passes through the thalamus it is considered the sensory “way station” of the brain, passing information on to the cerebral cortex (which is in the forebrain). Lesions of, or stimulation to, the thalamus are associated with changes in emotional reactivity. However, the importance of this structure on the regulation of emotional behavior is not due to the activity of the thalamus itself, but to the connections between the thalamus and other limbic-system structures.

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Limbic system, brain stem, and spinal cord: An image of the brain showing the limbic system in relation to the brain stem and spinal cord.

Hypothalamus

The hypothalamus is a small part of the brain located just below the thalamus. Lesions of the hypothalamus interfere with motivated behaviors like sexuality, combativeness, and hunger. The hypothalamus also plays a role in emotion: parts of the hypothalamus seem to be involved in pleasure and rage, while the central part is linked to aversion, displeasure, and a tendency towards uncontrollable and loud laughing. When external stimuli are presented (for example, a dangerous stimuli), the hypothalamus sends signals to other limbic areas to trigger feeling states in response to the stimuli (in this case, fear).

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Hypothalamus: An image of the brain showing the location of the hypothalamus.

The Spinal Cord

The spinal cord is a tail-like structure embedded in the vertebral canal of the spine. The adult spinal cord is about 40 cm long and weighs approximately 30 g. The spinal cord is attached to the underside of the medulla oblongata, and is organized to serve four distinct tasks:

  1. to convey (mainly sensory) information to the brain;
  2. to carry information generated in the brain to peripheral targets like skeletal muscles;
  3. to control nearby organs via the autonomic nervous system;
  4. to enable sensorimotor functions to control posture and other fundamental movements.

Basic parts of the brain, part 1, 3-D anatomy tutorial: http://www.anatomyzone.com 3D anatomy tutorial on the basic parts of the brain using the Zygote Body Browser (http://www.zygotebody.com). This is the FIRST part, please watch the second part as well! Join the Facebook page for updates: http://www.facebook.com/anatomyzone Follow me on twitter: http://www.twitter.com/anatomyzone Subscribe to the channel for more videos and updates: http://www.youtube.com/subscription_center?add_user=theanatomyzone

Cerebral Cortex

The cerebral cortex is the outermost layered structure of the brain and controls higher brain functions such as information processing.

Learning Objectives

Differentiate between the cortex and the cerebrum

Key Takeaways

Key Points

  • The cerebral cortex, the largest part of the brain, is the ultimate control and information-processing center in the brain.
  • The cerebral cortex is responsible for many higher-order brain functions such as sensation, perception, memory, association, thought, and voluntary physical action.
  • The cerebrum is the large, main part of the brain and serves as the thought and control center.

Key Terms

  • cerebral cortex: The grey, folded, outermost layer of the cerebrum responsible for higher brain processes such as sensation, voluntary muscle movement, thought, reasoning, and memory.
  • cerebrum: In humans, the part of the brain comprising the cerebral cortex and several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb.
  • myelin: A white, fatty material composed of lipids and lipoproteins that surrounds the axons of nerves and facilitates swift neural communication.

Cortex

The cerebral cortex, the largest part of the mammalian brain, is the wrinkly gray outer covering of the cerebrum. While the cortex is less than 1/4″ thick, it is here that all sensation, perception, memory, association, thought, and voluntary physical actions occur. The cerebral cortex is considered the ultimate control and information-processing center in the brain.

The cortex is made of layers of neurons with many inputs; these cortical neurons function like mini microprocessors or logic gates. It contains glial cells, which guide neural connections, provide nutrients and myelin to neurons, and absorb extra ions and neurotransmitters. The cortex is divided into four different lobes (the parietal, occipital, temporal, and frontal lobes ), each with a different specific function.

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Lobes of the brain: A diagram of the brain identifying the different lobes by color. Counterclockwise from bottom: It contains the parietal lobe (green), the occipital lobe (red), the temporal lobe (yellow), and the frontal lobe (blue).

The cortex is wrinkly in appearance. Evolutionary constraints on skull size brought about this development; it allowed for the cortex to become larger without our brains (and therefore craniums) becoming disadvantageously large. At times it has been theorized that brain size correlated positively with intelligence; it has also been suggested that surface area of cortex (basically, “wrinkliness” of the brain) rather than brain size that correlates most directly with intelligence. Current research suggests that both of these may be at least partially true, but the degree to which they correlate is not clear.

The “valleys” of the wrinkles are called sulci (or sometimes, fissures); the “peaks” between wrinkles are called gyri. While there are variations from person to person in their sulci and gyri, the brain has been studied enough to identify patterns. One notable sulcus is the central sulcus, or the wrinkle dividing the parietal lobe from the frontal lobe.

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Sulci and gyri: As depicted in this diagram of brain structures, sulci are the “valleys” and gyri are the “peaks” in the folds of the brain.

Cerebrum

Beneath the cerebral cortex is the cerebrum, which serves as the main thought and control center of the brain. It is the seat of higher-level thought like emotions and decision making (as opposed to lower-level thought like balance, movement, and reflexes).

The cerebrum is composed of gray and white matter. Gray matter is the mass of all the cell bodies, dendrites, and synapses of neurons interlaced with one another, while white matter consists of the long, myelin-coated axons of those neurons connecting masses of gray matter to each other.

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Grey matter and white matter: A sagittal cross-section of a human brain showing the distinct layers of grey matter (the darker outer layer) and white matter (the lighter inner layer) in the cerebrum.

Cerebral Hemispheres and Lobes of the Brain

The brain is divided into two hemispheres and four lobes, each of which specializes in a different function.

Learning Objectives

Outline the structure and function of the lobes and hemispheres of the brain

Key Takeaways

Key Points

  • The left hemisphere is dominant with regard to language and logical processing, while the right hemisphere handles spatial perception.
  • The brain is separated into the frontal, temporal, occipital, and parietal lobes.
  • The frontal lobe is associated with executive functions and motor performance.
  • The temporal lobe is associated with the retention of short- and and long-term memories. It processes sensory input, including auditory information, language comprehension, and naming.
  • The occipital lobe is the visual-processing center of the brain.
  • The parietal lobe is associated with sensory skills.

Key Terms

  • corpus callosum: A wide, flat bundle of neural fibers beneath the cortex that connects the left and right cerebral hemispheres and facilitates interhemispheric communication.
  • lateralization: Localization of a function, such as speech, to the right or left side of the brain.
  • visuospatial: Of or pertaining to the visual perception of spatial relationships.

Brain Lateralization

The brain is divided into two halves, called hemispheres. There is evidence that each brain hemisphere has its own distinct functions, a phenomenon referred to as lateralization. The left hemisphere appears to dominate the functions of speech, language processing and comprehension, and logical reasoning, while the right is more dominant in spatial tasks like vision-independent object recognition (such as identifying an object by touch or another nonvisual sense). However, it is easy to exaggerate the differences between the functions of the left and right hemispheres; both hemispheres are involved with most processes. Additionally, neuroplasticity (the ability of a brain to adapt to experience) enables the brain to compensate for damage to one hemisphere by taking on extra functions in the other half, especially in young brains.

Corpus Callosum

The two hemispheres communicate with one another through the corpus callosum. The corpus callosum is a wide, flat bundle of neural fibers beneath the cortex that connects the left and right cerebral hemispheres and facilitates interhemispheric communication. The corpus callosum is sometimes implicated in the cause of seizures; patients with epilepsy sometimes undergo a corpus callostomy, or the removal of the corpus callosum.

The Lobes of the Brain

The brain is separated into four lobes: the frontal, temporal, occipital, and parietal lobes.

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Lobes of the brain: The brain is divided into four lobes, each of which is associated with different types of mental processes. Clockwise from left: The frontal lobe is in blue at the front, the parietal lobe in yellow at the top, the occipital lobe in red at the back, and the temporal lobe in green on the bottom.

The Frontal Lobe

The frontal lobe is associated with executive functions and motor performance. Executive functions are some of the highest-order cognitive processes that humans have. Examples include:

  • planing and engaging in goal-directed behavior;
  • recognizing future consequences of current actions;
  • choosing between good and bad actions;
  • overriding and suppressing socially unacceptable responses;
  • determining similarities and differences between objects or situations.

The frontal lobe is considered to be the moral center of the brain because it is responsible for advanced decision-making processes. It also plays an important role in retaining emotional memories derived from the limbic system, and modifying those emotions to fit socially accepted norms.

The Temporal Lobe

The temporal lobe is associated with the retention of short- and long-term memories. It processes sensory input including auditory information, language comprehension, and naming. It also creates emotional responses and controls biological drives such as aggression and sexuality.

The temporal lobe contains the hippocampus, which is the memory center of the brain. The hippocampus plays a key role in the formation of emotion-laden, long-term memories based on emotional input from the amygdala. The left temporal lobe holds the primary auditory cortex, which is important for processing the semantics of speech.

One specific portion of the temporal lobe, Wernicke’s area, plays a key role in speech comprehension. Another portion, Broca’s area, underlies the ability to produce (rather than understand) speech. Patients with damage to Wernicke’s area can speak clearly but the words make no sense, while patients with damage to Broca’s area will fail to form words properly and speech will be halting and slurred. These disorders are known as Wernicke’s and Broca’s aphasia respectively; an aphasia is an inability to speak.

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Broca’s and Wernicke’s areas: The locations of Broca’s and Wernicke’s areas in the brain. The Broca’s area is at the back of the frontal lobe, and the Wernicke’s area is roughly where the temporal lobe and parietal lobe meet.

The Occipital Lobe

The occipital lobe contains most of the visual cortex and is the visual processing center of the brain. Cells on the posterior side of the occipital lobe are arranged as a spatial map of the retinal field. The visual cortex receives raw sensory information through sensors in the retina of the eyes, which is then conveyed through the optic tracts to the visual cortex. Other areas of the occipital lobe are specialized for different visual tasks, such as visuospatial processing, color discrimination, and motion perception. Damage to the primary visual cortex (located on the surface of the posterior occipital lobe) can cause blindness, due to the holes in the visual map on the surface of the cortex caused by the lesions.

The Parietal Lobe

The parietal lobe is associated with sensory skills. It integrates different types of sensory information and is particularly useful in spatial processing and navigation. The parietal lobe plays an important role in integrating sensory information from various parts of the body, understanding numbers and their relations, and manipulating objects. Its also processes information related to the sense of touch.

The parietal lobe is comprised of the somatosensory cortex and part of the visual system. The somatosensory cortex consists of a “map” of the body that processes sensory information from specific areas of the body. Several portions of the parietal lobe are important to language and visuospatial processing; the left parietal lobe is involved in symbolic functions in language and mathematics, while the right parietal lobe is specialized to process images and interpretation of maps (i.e., spatial relationships).

The Limbic System

The limbic system combines higher mental functions and primitive emotion into one system.

Learning Objectives

Summarize the structural elements and functions of the limbic system

Key Takeaways

Key Points

  • The limbic system, located just beneath the cerebrum on both sides of the thalamus, is not only responsible for our emotional lives but also many higher mental functions, such as learning and formation of memories.
  • The primary structures within the limbic system include the amygdala, hippocampus, thalamus, hypothalamus, basal ganglia, and cingulate gyrus.
  • The amygdala is the emotion center of the brain, while the hippocampus plays an essential role in the formation of new memories about past experiences.
  • The thalamus and hypothalamus are associated with changes in emotional reactivity.
  • The cingulate gyrus coordinates smells and sights with pleasant memories, induces an emotional reaction to pain, and helps regulate aggressive behavior.
  • The basal ganglia is a group of nuclei lying deep in the subcortical white matter of the frontal lobes; its functions include organizing motor behavior and coordinating rule-based, habit learning.

Key Terms

  • cerebrum: The seat of motor and sensory functions, as well as higher mental functions such as consciousness, thought, reason, emotion, and memory.
  • medial: Pertaining to the inside; closer to the midline.
  • corpus callosum: In mammals, a broad band of nerve fibres that connects the left and right hemispheres of the brain.

The limbic system is a complex set of structures found on the central underside of the cerebrum, comprising inner sections of the temporal lobes and the bottom of the frontal lobe. It combines higher mental functions and primitive emotion into a single system often referred to as the emotional nervous system. It is not only responsible for our emotional lives but also our higher mental functions, such as learning and formation of memories. The limbic system is the reason that some physical things such as eating seem so pleasurable to us, and the reason why some medical conditions, such as high blood pressure, are caused by mental stress. There are several important structures within the limbic system: the amygdala, hippocampus, thalamus, hypothalamus, basal ganglia, and cingulate gyrus.

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The limbic system: All the components of the limbic system work together to regulate some of the brain’s most important processes.

The Amygdala

The amygdala is a small almond-shaped structure; there is one located in each of the left and right temporal lobes. Known as the emotional center of the brain, the amygdala is involved in evaluating the emotional valence of situations (e.g., happy, sad, scary). It helps the brain recognize potential threats and helps prepare the body for fight-or-flight reactions by increasing heart and breathing rate. The amygdala is also responsible for learning on the basis of reward or punishment.

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The amygdala: The figure shows the location of the amygdala from the underside (ventral view) of the human brain, with the front of the brain at the top of the image.

Due to its close proximity to the hippocampus, the amygdala is involved in the modulation of memory consolidation, particularly emotionally-laden memories. Emotional arousal following a learning event influences the strength of the subsequent memory of that event, so that greater emotional arousal following a learning event enhances a person’s retention of that memory. In fact, experiments have shown that administering stress hormones to individuals immediately after they learn something enhances their retention when they are tested two weeks later.

The Hippocampus

The hippocampus is found deep in the temporal lobe, and is shaped like a seahorse. It consists of two horns curving back from the amygdala. Psychologists and neuroscientists dispute the precise role of the hippocampus, but generally agree that it plays an essential role in the formation of new memories about past experiences. Some researchers consider the hippocampus to be responsible for general declarative memory (memories that can be explicitly verbalized, such as memory of facts and episodic memory).

Damage to the hippocampus usually results in profound difficulties in forming new memories (anterograde amnesia), and may also affect access to memories formed prior to the damage (retrograde amnesia). Although the retrograde effect normally extends some years prior to the brain damage, in some cases older memories remain intact; this leads to the idea that over time the hippocampus becomes less important in the storage of memory.

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Hippocampus: This image shows the horned hippocampus deep within the temporal lobe.

The Thalamus and Hypothalamus

Both the thalamus and hypothalamus are associated with changes in emotional reactivity. The thalamus, which is a sensory “way-station” for the rest of the brain, is primarily important due to its connections with other limbic-system structures. The hypothalamus is a small part of the brain located just below the thalamus on both sides of the third ventricle. Lesions of the hypothalamus interfere with several unconscious functions (such as respiration and metabolism) and some so-called motivated behaviors like sexuality, combativeness, and hunger. The lateral parts of the hypothalamus seem to be involved with pleasure and rage, while the medial part is linked to aversion, displeasure, and a tendency for uncontrollable and loud laughter.

The Cingulate Gyrus

The cingulate gyrus is located in the medial side of the brain next to the corpus callosum. There is still much to be learned about this gyrus, but it is known that its frontal part links smells and sights with pleasant memories of previous emotions. This region also participates in our emotional reaction to pain and in the regulation of aggressive behavior.

The Basal Ganglia

The basal ganglia is a group of nuclei lying deep in the subcortical white matter of the frontal lobes that organizes motor behavior. The caudate, putamen, and globus pallidus are major components of the basal ganglia. The basal ganglia appears to serve as a gating mechanism for physical movements, inhibiting potential movements until they are fully appropriate for the circumstances in which they are to be executed. The basal ganglia is also involved with:

  • rule-based habit learning (e.g., initiating, stopping, monitoring, temporal sequencing, and maintaining the appropriate movement);
  • inhibiting undesired movements and permitting desired ones;
  • choosing from potential actions;
  • motor planning;
  • sequencing;
  • predictive control;
  • working memory;
  • attention.

Neuroplasticity

Neuroplasticity is the brain’s ability to create new neural pathways to account for learning and acquisition of new experiences.

Learning Objectives

Explain how neuroplasticity occurs

Key Takeaways

Key Points

  • ” Neuroplasticity ” refers to changes in neural pathways and synapses that result from changes in behavior, environmental and neural processes, and changes resulting from bodily injury.
  • Neuroplasticity has replaced the formerly held theory that the brain is a physiologically static organ, and explores how the brain changes throughout life.
  • Neuroplasticity occurs on a variety of levels, ranging from minute cellular changes resulting from learning to large-scale cortical remapping in response to injury.
  • Synaptic pruning, or apoptosis, is the programmed neuron cell death that takes place during early childhood and adolescence.
  • Pruning strengthens important connections and eliminates weaker ones, creating more effective neural communication.

Key Terms

  • neuron: A cell of the nervous system that conducts nerve impulses; consisting of an axon and several dendrites. Neurons are connected by synapses.
  • plastic: Capable of being molded; malleable, flexible, plaint.
  • synapse: The junction between the terminal of a neuron and either another neuron or a muscle or gland cell, over which nerve impulses pass.
  • apoptosis: The process of programmed cell death.

Neuroplasticity

The brain is constantly adapting throughout a lifetime, though sometimes over critical, genetically determined periods of time. Neuroplasticity is the brain’s ability to create new neural pathways based on new experiences. It refers to changes in neural pathways and synapses that result from changes in behavior, environmental and neural processes, and changes resulting from bodily injury. Neuroplasticity has replaced the formerly held theory that the brain is a physiologically static organ, and explores how the brain changes throughout life.

Neuroplasticity occurs on a variety of levels, ranging from minute cellular changes resulting from learning to large-scale cortical remapping in response to injury. The role of neuroplasticity is widely recognized in healthy development, learning, memory, and recovery from brain damage. During most of the 20th century, the consensus among neuroscientists was that brain structure is relatively immutable after a critical period during early childhood. It is true that the brain is especially ” plastic ” during childhood’s critical period, with new neural connections forming constantly. However, recent findings show that many aspects of the brain remain plastic even into adulthood.

Plasticity can be demonstrated over the course of virtually any form of learning. For one to remember an experience, the circuitry of the brain must change. Learning takes place when there is either a change in the internal structure of neurons or a heightened number of synapses between neurons. Studies conducted using rats illustrate how the brain changes in response to experience: rats who lived in more enriched environments had larger neurons, more DNA and RNA, heavier cerebral cortices, and larger synapses compared to rats who lived in sparse environments.

A surprising consequence of neuroplasticity is that the brain activity associated with a given function can move to a different location; this can result from normal experience, and also occurs in the process of recovery from brain injury. In fact, neuroplasticity is the basis of goal-directed experiential therapeutic programs in rehabilitation after brain injury. For example, after a person is blinded in one eye, the part of the brain associated with processing input from that eye doesn’t simply sit idle; it takes on new functions, perhaps processing visual input from the remaining eye or doing something else entirely. This is because while certain parts of the brain have a typical function, the brain can be “rewired”—all because of plasticity.

Synaptic Pruning

“Synaptic (or neuronal or axon ) pruning” refers to neurological regulatory processes that facilitate changes in neural structure by reducing the overall number of neurons and synapses, leaving more efficient synaptic configurations. At birth, there are approximately 2,500 synapses in the cerebral cortex of a human baby. By three years old, the cerebral cortex has about 15,000 synapses. Since the infant brain has such a large capacity for growth, it must eventually be pruned down to remove unnecessary neuronal structures from the brain. This process of pruning is referred to as apoptosis, or programmed cell death. As the human brain develops, the need for more complex neuronal associations becomes much more pertinent, and simpler associations formed at childhood are replaced by more intricately interconnected structures.

Pruning removes axons from synaptic connections that are not functionally appropriate. This process strengthens important connections and eliminates weaker ones, creating more effective neural communication. Generally, the number of neurons in the cerebral cortex increases until adolescence. Apoptosis occurs during early childhood and adolescence, after which there is a decrease in the number of synapses. Approximately 50% of neurons present at birth do not survive until adulthood. The selection of the pruned neurons follows the “use it or lose it” principle, meaning that synapses that are frequently used have strong connections, while the rarely used synapses are eliminated.

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Neuron growth: Neurons grow throughout adolescence and then are pruned down based on the connections that get the most use.

Synaptic pruning is distinct from the regressive events seen during older age. While developmental pruning is experience-dependent, the deteriorating connections that occur with old age are not. Synaptic pruning is like carving a statue: getting the unformed stone into its best form. Once the statue is complete, the weather will begin to erode the statue, which represents the lost connections that occur with old age.