The Brain

 The Brain

Brain physiology

The brain includes a number of major structures. The key components are neurons and the nerve cells that handle information processing.

Structure and function

Forebrain (top portion)

Cerebral cortex (outer layer - cap) has four lobes

Frontal, occipital, temporal, parietal lobes

Deeper in brain:

Hypothalamus, pituitary gland, amygdala,  hippocampus

The brain has two halves, or hemispheres. The top portion of the brain, farthest from the spinal cord, is known as the forebrain. Its outer layer of cells, the cerebral cortex, covers it like a cap. The cerebral cortex is responsible for about 80 percent of the brain’s volume and is critical in perception, thinking, language, and other important functions. 

 Each hemisphere of the cortex has four major areas, called lobes. Although the lobes usually work together, each has a somewhat different primary function.

Deeper in the brain, beneath the cortex, lie other key structures. These include the hypothalamus and the pituitary gland as well as the amygdala, which plays an important role in emotions, and the hippocampus, which is especially active in memory and emotion.

Functions of Lobes of the Cortex

Frontal         Involved in voluntary movement, thinking,  personality, and intentionality or purpose

Occipital Function in vision

Parietal         Active role in hearing, language  processing, and memory

Temporal Roles in registering spatial location,  attention, and motor control

Neurons — nerve cells handling information  processing at the cellular level

Axon, dendrites, synapses

Neurotransmitters: dopamine

Myelin sheath and myelination

Neural circuits

Lateralization — specialization of functions in one  hemisphere of cerebral cortex

Basically, an axon sends electrical signals away from the central part of the neuron. At the end of the axon are terminal buttons, which release chemicals called neurotransmitters into synapses, which are tiny gaps between neurons’ fibers. Chemical interactions in synapses connect axons and dendrites, allowing information to pass from neuron to neuron. Similarly, a message in the brain is sends across the synapse by a neurotransmitter.  

Most axons are covered by a myelin sheath, which is a layer of fat cells. The sheath helps impulses travel faster along the axon, increasing the speed with which information travels from neuron to neuron. The myelin sheath developed as the brain evolved. As brain size increased, it became necessary for information to travel faster over longer distances 

in the nervous system.  

Clusters of neurons known as neural circuits work together to handle particular types of information. The brain is organized in many neural circuits. This neural circuit uses the neurotransmitter dopamine and lies in the prefrontal cortex area of the frontal lobes. 

 To some extent, the type of information handled by neurons depends on whether they are in the left or right hemisphere of the cortex.

This specialization of function in one hemisphere of the cerebral cortex or the other is called 

lateralization. However, most neuroscientists agree that complex functions such as reading or performing music involve both hemispheres. Complex thinking in normal people is the outcome of communication between both hemispheres of the brain.

Infancy

Shaken Baby Syndrome

Born with about 100 billion neurons

Brain flexibility and resilience demonstrated in  deprived environments

Dramatic increases of neural connections

Brain areas do not mature uniformly; skills affected  by myelination and interconnections

Brain is developing so rapidly in infancy, the infant’s head should be protected from falls or other injuries and the baby should never be shaken. 

Shaken baby syndrome, which includes brain swelling and hemorrhaging. A recent analysis found that fathers were the most frequent perpetrators of shaken baby syndrome, followed by child care providers and by a boyfriend of the victim’s mother.

Studying the brain’s development in infancy is through Positron-emission tomography (PET) scans, magnetic resonance imaging (MRI), and electroencephalogram (EEG).

Among the researchers who studied the brain development was Charles Nelson and his colleagues found that  newborns 

produce distinctive brain waves that reveal they can distinguish their mother’s voice while they are asleep.

As an infant walks, talks, runs, shakes a rattle, smiles, and frowns, changes in its brain are occurring.

infant’s brain is waiting for experiences to determine how connections are made. Before birth, it appears that genes mainly direct basic wiring patterns. Neurons grow and travel to distant places awaiting further instructions. After birth, the inflowing stream of sights, sounds, smells, touches, language, and eye contact help shape the brain’s neural connections.

 Changing NeuronsNewborn’s brain is about 25 percent of its adult weight. By the second birthday, the brain is about 75 percent of its adult weight. Two key developments during these first two years involve the myelin sheath (the layer of fat cells that speeds up the electrical impulse along the axon) and connections between dendrites.Myelination, the process of encasing axons with a myelin sheath, begins prenatally and continues after birth (see Figure 4.11). Myelination for visual pathways occurs rapidly after birth, being completed in the first six months. Auditory myelination is not completed until 4 or 5 years of age. Some aspects of myelination continue even into adolescence.

Infancy

Myelination; visual and auditory

Rapid growth of myelin sheath, dendrite and  synapse connections

Blooming and pruning of connections in brain

At birth, greater activity in left hemisphere

Motor control begins about 2 months

Vision occurs about the fourth postnatal month

Brain involved in hearing and language.

Prefrontal cortex (higher-level thinking and self-regulation occur), after 3 years of age. 

Both heredity and environment are influence synaptic overproduction and subsequent pruning.

Changing Structures

At birth, the hemispheres already have started to specialize: 

Newborns show greater electrical activity in the left hemisphere than in the right hemisphere when they are making or listening to speech sounds.

►Primary motor and sensory areas develops.

►The frontal lobes are immature in the newborn. However, myelinated and interconnected.

►develop an ability to regulate their physiological states, such as sleep, and gain more control over their reflexes. 

►Cognitive skills (deliberate thinking) do not emerge until later in the first year.

Childhood

The brain and head grow more rapidly than any  other part of the body — growth curves

Some brain size increase due myelination and  number and size of dendrites

Greatest anatomical brain increases from ages 3  to 15 years; distinct bursts of growth

Ages 3 to 6; most rapid growth in frontal lobe

Age 6 to puberty; most dramatic growth in  temporal and parietal lobes

Promotes spatial relations and language

Brain pathways and circuitry promote cognitive  control (attention, thoughts, actions, choices)

Adolescence

Using fMRI brain scans, scientists have recently discovered that adolescents’ brains undergo significant structural changes.

Brain continues growth

Corpus callosum – connect the brain’s left and right hemispheres, axon fibers thicken & ability to process information.

Prefrontal cortex – increased reasoning, decision  making, self-control.

Amygdala – seat of emotions, matures earlier

– Positive link between volume and duration of  aggressive behavior toward parents

Developmental social neuroscience, which involves connections between development, the brain, and socioemotional processes.

Research on brain development and changes

Thicker prefrontal cortex, more brain connections linked  to peer pressure resistance

Early ‘turbo charged’ emotions – more risky behaviors,  drug use, legal system involvement?

Brain change – result of biology, experiences

Adulthood and aging

Brain loss: 5-10% of weight in ages 20 to 90

Dendrites decrease; myelin sheath damage

Shrinkage is not uniform; most in prefrontal cortex

General slowing of brain and spinal cord function

Begins in middle age, accelerates with age

Reductions in neurotransmitters

The adapting brain

Exercise and activities influence development

Remarkable repair capability

Neurogenesis – new cells generated

Dendrite growth; “rewiring” to compensate loss

Less lateralization with age, more adaptation