The Vertebrate Brain

The brain develops from a hollow neural tube. Forebrain, midbrain, and hindbrain form from three successive regions of tube.

The hindbrain is the region where spinal cord and brain join. It has three parts:

• Medulla oblongata
• Cerebellum
• Pons

The medulla oblongata has influence over respiration, blood circulation, motor response coordination, and sleep / wake responses.

The cerebellum acts as a reflex center for maintaining posture and coordinating limbs. The pons ("bridge") possesses bands of axons that pass between brain centers.

The midbrain lies between the hindbrain and forebrain. The midbrain originally coordinated reflex responses to visual input. The roof of the midbrain, the tectum, still integrates visual and auditory signals in vertebrates such as amphibians and reptiles. In mammals it is now mostly a pathway switching center.

This video (Audio - Important) describes the parts of the brain stem.

The forebrain has undergone the greatest evolution. It is composed of four regions:

• Olfactory lobes
• Cerebrum
• Thalamus
• Hypothalamus and pituitary gland

The large olfactory lobes dominated early vertebrate forebrains.

The cerebrum integrates sensory input and selected motor responses. The thalamus (below the cerebrum) relays and coordinates sensory signals. The hypothalamus monitors intemal organs and influences responses to thirst, hunger, and sex.

The reticular formation is an ancient mesh of interneurons that extends from the uppermost part of the spinal cord, through the brain stem, and into the cerebral cortex.

The Cerebrum

The cerebrum is the largest and most complex part of the human brain. The outer layer (cerebral cortex) is highly folded. The human cerebrum (housed in a chamber within the skull bones) is divided into left and right cerebral hemispheres by a longitudinal fissure. The two halves communicate with each other by means of nerve tracts called the corpus callosum.

The left hemisphere deals with speech, math, and analytical skills. The right half controls nonverbal skills, such as music.

The thin surface (cerebral cortex) is gray matter, composed of the cell bodies of interneurons, which lie above the axons below.

Each cerebral hemisphere is divided into four lobes:

• The occipital lobe, which is located in the rear, has centers for vision.
• The temporal lobe, near each temple, is a processing center for hearing and houses centers for influencing emotional behavior.
• The parietal lobe contains the somatosensory cortex. It is the main receiving area for signals from the skin and joints.
• The frontal lobe includes the motor cortex, which coordinates instructions for motor responses.

The limbic system is evolutionarily related to the olfactory lobes. The limbic system controls emotions and has role in memory. It includes the following areas:

• Hypothalamus
• Amygdala
• Cingulate gyrus
• Hippocampus
• Parts of the thalamus

This video (Audio - Important) describes the limbic system.

Cerebrospinal fluid fills the canals and cavities of the brain and spinal cord, bathing the cells and tissues. The blood-brain barrier protects the brain and spinal cord by forcing materials to pass through cells which act as regulators of substances reaching the brain.

This animation (Audio - Important) describes the parts of the brain.

Memory

Memory is the brain's capacity to store and retrieve information about past sensory input.

Memory is stored in stages:

• Temporary storage in cerebral cortex
• Short-term memory
• Long-term memory

Association is the linkage of information into larger packages that can be sent to other brain regions for storage. Information becomes stored in "memory traces" that are chemical and structural changes in brain regions.

Short-term memory lasts from seconds to hours and is limited to small amounts of information.

Long-term memory is more permanent and seems to be limitless. Persons suffering from retrograde amnesia lose short-term memory, but long-term memory remains intact.

Information is moved into long-term storage with the cooperation of epinephrine, that increases a person's state of arousal.