Brain



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Chapter 2 : Brain



The Major Structures of the Brain arrow_upward


  • The human brain is a mass of pinkish-gray tissue containing a neural network involving approximately 10 billion nerve cells, called neurons.
  • Glial cells serve as the brain's support system, in addition to blood vessels and secretory organs.
  • It controls nearly every vital activity necessary for survival.
  • Emotions are controlled by the brain: anger, fear, joy, love, elation, contentment, and happiness find their origin inside the brain.
  • Furthermore, the brain receives and interprets the multitude of signals being sent by other parts of the body and the outside environment.
  • There are three major divisions of the brain: the forebrain, midbrain, and hindbrain.

  • Forebrain arrow_upward


  • For anatomical study the forebrain is divided into two subdivisions: the telencephalon and the diencephalon.
  • It is the forward most part of the vertebrate brain.
  • The primary structures of the telencephalon include the cerebral cortex, basal ganglia, and the limbic system.
  • The diencephalon is the second major division of the forebrain. It includes the thalamus and the hypothalamus.

  • Telencephalon:

  • Cerebral Cortex
    • The cerebral cortex is commonly referred to as gray matter.
    • This is based upon the appearance of the cortex which appears grayish brown (due to the predominance of cells).
    • The neurons of the cerebral cortex are connected to other neurons within the brain via millions of axons located beneath the cortex.
    • This area is white in color due to the concentration of myelin; it is often called white matter.
    • One of the most apparent visible features of the brain is the division between the left and right hemispheres of the cerebral cortex.
    • In general terms it is well understood that the left hemisphere controls linguistic consciousness, the right half of the body, talking, reading, writing, spelling, speech communication, verbal intelligence and memories, and information processing in the areas of math, typing, grammar, logic, analytic reasoning, and perception of details.
    • The right hemisphere is associated with 'unconscious' awareness, perception of faces and patterns, comprehension of body language and social cues, creativity and insight, intuitive reasoning, visual-spatial processing, and holistic comprehension.
    • Communication between the two hemispheres takes place through the corpus callosum, which, by the way, is more fully developed in women than men- likely giving rise to women's intuition.
  • Basal Ganglia
    • The basal ganglia are a collection of subcortical nuclei situated beneath the anterior portions of the lateral ventricles; they are involved with the control of movement.
    • Parkinson's disease has an effect upon the basal ganglia resulting in poor balance, rigidity of the limbs, tremors, weakness, and difficulty with initiating movements.
    • Some anatomists consider the amygdala (primary component of the limbic system) a part of the basal ganglia given its location.
  • The Limbic System
    • The limbic system is a collection of brain structures involved with emotion, motivation, multifaceted behavior, memory storage and recall.
    • The hippocampus (sea horse) and the amygdala (almond), along with portions of the hypothalamus, thalamus, caudate nuclei, and septum function together to form the limbic system.

    Diencephalon:

  • The diencephalon is the second major division of the forebrain.
  • The principle structures include the thalamus and hypothalamus.
  • Thalamus:
    • The thalamus is the relay station for incoming sensory signals and outgoing motor signals passing to and from the cerebral cortex.
    • With the exception of the olfactory sense, all sensory input to the brain connected to nerve cell clusters (nuclei) of the thalamus.
    • The thalamus consists of two large connected lobes.
    • The Massa intermedia serve as a bridge connecting the two lobes of the thalamus.
    • It is comprised of gray matter and is deemed a non-critical part of the brain; absence of which is outwardly unnoticeable.
  • Hypothalamus:
    • The hypothalamus is comprised of distinct areas and nuclei which control vital survival behaviors and activities; such as: eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity.
    • It is located just beneath the thalamus and lies at the base of the brain.
    • The autonomic nervous system and endocrine system are controlled by the hypothalamus.
    • The anterior pituitary gland is directly connected to the hypothalamus via a special system of blood vessels.
    • Neurosecretory cells released by the hypothalamus act upon the anterior pituitary gland which then secretes its hormones.
    • Most hormones secreted by the anterior pituitary gland control other endocrine glands.
    • Because of this the anterior pituitary gland is sometimes referred to as the “Master Gland”.
    • Hormones of the posterior pituitary gland are also governed by the hypothalamus.

    Midbrain, the Mesencephalon arrow_upward


  • The midbrain is a portion of the brain located just above the medulla and pons and contains basic vision and hearing functions.
  • Two primary parts comprise the midbrain: the tectum and the tegmentum.
  • Tectum:
    • The primary structure of the tectum includes the superior colliculi and the inferior colliculi.
    • The superior colliculi form part of the visual system.
    • The inferior colliculi are part of the auditory system.
    • The structures appear as four small bumps located on the brain stem.
    • Function in mammals relates to visual reflexes and reaction to moving stimuli.
  • Tegmentum:
    • The tegmentum is situated below the tectum.
    • The reticular formation, periaqueductal gray matter, and the red nucleus and substantia nigra are part of the tegmentum.
    • The reticular formation is comprised of more than 90 nuclei and an interconnected neural network located at the core of the brain stem.
    • It receives sensory information and is involved with attention, sleep and arousal, muscle tonus, movement, and various vital reflexes.

    The Hindbrain arrow_upward


  • The portion of the embryonic brain from which the metencephalon and myelencephalon develop.

  • Metencephalon

  • Cerebellum (little brain):
    • The cerebellum's primary function involves control of bodily movements.
    • It serves as a reflex center for the coordination and precise maintenance of equilibrium.
    • Voluntary and involuntary bodily movements are controlled by the cerebellum.
  • Pons:
    • The pons appears as a large bulge in the brain stem between the mesencephalon and the medulla oblongata.
    • The pons contains a portion of the reticular formation as well as nuclei believed important in the role of sleep and arousal.

    Myelencephalon

  • The myelencephalon is comprised of one structure: the medulla oblongata (oblong marrow).
  • It is the origin of the reticular formation and consists of nuclei which control vital bodily functions.
  • The medulla oblongata is the control center for cardiac, vasoconstrictor, and respiratory functions.
  • Reflex activities, including vomiting, are controlled by this structure of the hindbrain.
  • Appearing as a pyramid-shaped enlargement of the spinal cord, damage to this area typically results in immediate death.


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