The Human Brain

❉  HUMAN BRAIN ❉

The human brain is the central organ of the nervous system, and with the spinal cord, comprises the central nervous system. It consists of the cerebrum, the brainstem and the cerebellum. The brain controls most of the activities of the body, processing, integrating, and co-ordinating the information it receives from the sensory nervous system. The brain integrates the instructions sent to the rest of the body.

CEREBRUM

• The cerebrum, the largest part of the human brain, consists of two cerebral hemispheres. Each hemisphere has an inner core composed of white matter, and an outer surface – the cerebral cortex – composed of grey matter. 
• The cortex has an outer layer, the neocortex, and an inner allocortex. 
• The neocortex is made up of six neuronal layers, while the allocortex has three or four.
•  Each hemisphere is divided into four lobes – the frontal, parietal, temporal, and occipital lobes. 
• The frontal lobe is associated with executive functions including self-control, planning, reasoning, and abstract thought, while the occipital lobe is dedicated to vision. 
• Within each lobe, cortical areas are associated with specific functions, such as the sensory, motor, and association regions. 
• Although the left and right hemispheres are broadly similar in shape and function, some functions are associated with one side, such as language in the left and visual-spatial ability in the right. 
• The hemispheres are connected by commissural nerve tracts, the largest being the corpus callosum.
• The cerebrum is connected by the brainstem to the spinal cord. 

BRAIN STEM

• The brainstem consists of the midbrain, the pons, and the medulla oblongata. 
• The cerebellum is connected to the brainstem by three pairs of nerve tracts called cerebellar peduncles. 
• Within the cerebrum is the ventricular system, consisting of four interconnected ventricles in which cerebrospinal fluid is produced and circulated. 
• Underneath the cerebral cortex are several structures, including the thalamus, the epithalamus, the pineal gland, the hypothalamus, the pituitary gland, and the subthalamus; the limbic structures, including the amygdalae and the hippocampi, the claustrum, the various nuclei of the basal ganglia, the basal forebrain structures, and three circumventricular organs.

CELLS of BRAIN

The cells of the brain include neurons and supportive glial cells. There are more than 86 billion neurons in the brain, and a more or less equal number of other cells. Brain activity is made possible by the interconnections of neurons and their release of neurotransmitters in response to nerve impulses. Neurons connect to form neural pathways, neural circuits, and elaborate network systems. The whole circuitry is driven by the process of neurotransmission.

BRAIN SKULL & DISORDERS 

• The brain is protected by the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier. 
• However, the brain is still susceptible to damage, disease, and infection. 
• Damage can be caused by trauma, or a loss of blood supply known as a stroke. 
• The brain is susceptible to degenerative disorders, such as Parkinson's disease, dementias including Alzheimer's disease, and multiple sclerosis. 
• Psychiatric conditions, including schizophrenia and clinical depression, are thought to be associated with brain dysfunctions. 
• The brain can also be the site of tumours, both benign and malignant; these mostly originate from other sites in the body.

BRAIN WEIGHT

• The adult human brain weighs on average about 1.2–1.4 kg which is about 2% of the total body weight, with a volume of around 1260 cm³ in men and 1130 cm³ in women. 
• There is substantial individual variation, with the standard reference range for men being 1,180–1,620 g and for women 1,030–1,400 g.