During embryonic development, the brain first forms as a
tube, the anterior end of which enlarges into three hollow swellings that form
the brain, and the posterior of which develops into the spinal cord. Some parts
of the brain have changed little during vertebrate evolutionary history.
Vertebrate evolutionary trends include
- Increase in brain size relative to body size.
- Subdivision and increasing specialization of the forebrain, midbrain, and hindbrain.
- Growth in relative size of the forebrain, especially the cerebrum, which is associated with increasingly complex behavior in mammals.
The Brain Stem and Midbrain
The brain stem is the smallest and from an evolutionary viewpoint, the oldest and most primitive part of the brain. The brain stem is continuous with the spinal cord, and is composed of the parts of the hindbrain and midbrain. The medulla oblongata and pons control heart rate, constriction of blood vessels, digestion and respiration.
The midbrain consists of connections between the hindbrain and forebrain. Mammals use this part of the brain only for eye reflexes.
The Cerebellum
The cerebellum is the third part of the hindbrain, but it is not considered part of the brain stem. Functions of the cerebellum include fine motor coordination and body movement, posture, and balance. This region of the brain is enlarged in birds and controls muscle action needed for flight.
The Forebrain
The forebrain consists of the diencephalon and cerebrum. The thalamus and hypothalamus are the parts of the diencephalon. The thalamus acts as a switching center for nerve messages. The hypothalamus is a major homeostatic center having both nervous and endocrine functions.
The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. Fish have no cerebral cortex, amphibians and reptiles have only rudiments of this area.
The cortex in each hemisphere of the cerebrum is between 1
and 4 mm thick. Folds divide the cortex into four lobes: occipital, temporal,
parietal, and frontal. No region of the brain functions alone, although major
functions of various parts of the lobes have been determined.
The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobe conducts three functions:
- motor activity and integration of muscle activity
- speech
- thought processes
The Spinal Cord
The spinal cord runs along the dorsal side of the body and links the brain to the rest of the body. Vertebrates have their spinal cords encased in a series of (usually) bony vertebrae that comprise the vertebral column.
The gray matter of the spinal cord consists mostly of cell
bodies and dendrites. The surrounding white matter is made up of bundles of
interneuronal axons (tracts). Some tracts are ascending (carrying messages to
the brain), others are descending (carrying messages from the brain). The spinal
cord is also involved in reflexes that do not immediately involve the brain.