OCR A Level Biology

Revision Notes

5.5.9 The Human Brain

The Human Brain

  • The brain, alongside the spinal cord, is part of our central nervous system (CNS)
  • The brain is made of billions of interconnected neurones and is responsible for controlling complex behaviours
  • Within the brain are different regions that carry out different functions
  • These regions include:
    • The cerebrum
    • The hypothalamus
    • The pituitary gland
    • The cerebellum
    • The medulla oblongata

The cerebrum

  • Largest part of the brain in humans (accounts for about 80% of the total mass of the brain)
  • Carries out a large variety of functions involved with conscious activities, including:
    • Vision
    • Hearing
    • Speech
    • Thinking
    • Memory
  • Consists of five lobes
  • Divided into two halves, known as the cerebral hemispheres
    • The hemispheres are joined together by a band of nerve fibres, known as the corpus callosum
    • The right hemisphere controls the left side of the body and the left one controls the right side
  • Has a thin outer layer known as the cerebral cortex or ‘grey matter
    • The cerebral cortex consists of the cell bodies of neurones
    • It is highly folded, which increases its surface area and allows it to contain a greater number of neurones
    • With more neurones in the brain, more connections between neurones can be made
    • This is important, as the more connections between neurones in the brain, the greater the ability of the brain to carry out more complex behaviours
  • Beneath the cerebral cortex or grey matter layer is the ‘white matter’
    • The white matter consists of the myelinated axons of neurones

The hypothalamus

  • Area in the middle of the lower part of the brain
  • It is found just above the pituitary gland (which it is connected to)
  • Monitors the blood as it is flowing through it and in response, releases hormones (involved in homeostasis) itself or stimulates the pituitary gland to release certain hormones
  • Main functions include:
    • Regulating body temperature – by monitoring blood temperature and initiating a homeostatic response if this temperature gets too high or too low
    • Osmoregulation – by monitoring how concentrated the blood is and if it gets too concentrated, stimulating the posterior pituitary gland to release anti-diuretic hormone (ADH), which causes increased water retention in the kidneys. The hypothalamus also generates a feeling of thirst, causing us to increase our water intake
    • Regulating digestive activity – controls the secretion of enzymes in the gut and peristalsis. The hypothalamus also generates a feeling of hunger, causing us to increase our food intake if blood nutrient concentrations get too low
    • Controlling endocrine functions – the hypothalamus releases chemicals that cause the pituitary gland to release certain hormones that control a variety of processes (e.g. metabolism, growth and development, puberty sexual functions, sleep, mood)

The pituitary gland

  • At the bottom of the brain, below the hypothalamus
  • Produces a range of hormones
    • Some of these directly influence and regulate processes in the body but some stimulate the release of further hormones from specific, remote locations in the body (e.g. from other glands)
  • Divided into two sections: the anterior pituitary and posterior pituitary
    • Anterior pituitary – produces and releases certain hormones
    • Posterior pituitary – stores and releases hormones produced by the hypothalamus (e.g. ADH and oxytocin)

The cerebellum

  • Lies below the cerebrum
  • Controls motor coordination
    • This includes balance
    • Balance is a highly complex function that requires coordination between multiple parts, including the eyes, semicircular canals in the ears, and many muscles
  • Functions only subconsciously (i.e. all of the actions it controls are involuntary)

The medulla oblongata

  • Also known as the medulla
  • Found at the very base of the brain, where it joins the spinal cord
  • Contains three ‘centres’ that control different functions:
    • The cardiac centre – controls heart rate
    • The vasomotor centre – controls blood pressure by controlling the contraction of smooth muscles in arteriole walls
    • The respiratory centre – controls breathing rate (contains an inspiratory centre and an expiratory centre)
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