The Human Brain (OCR A Level Biology)

• 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 regions of the human brain

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

A cross-section of the brain showing regions found within the cerebrum.

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)