# SL Psychology/How the brain works
## Content
### Overview

One of the most complex systems known to us today, the brain is made of systems and parts that all work together to keep the human body functioning. The nervous system, the core of all processes, has two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made of the brain and the spinal cord. The PNS is the brain's connection to nerves. It links the CNS to sense receptors, muscles, and glands. Without the CNS and the PNS, we would not be able to function. So, when explaining how the brain works physically, it is essential that we understand the core of all function.

### Peripheral Nervous System

The PNS is the link from the brain to the rest of the body. Because the PNS is so extensive, it is necessary that it be made of other systems that all work together to maintain the PNS. The somatic nervous system, to start with, controls the movement of skeletal muscles. The autonomic nervous system controls the glands and the muscles of internal organs. This system controls basically all movement that we don't actually think about, such as a heartbeat, digestion, and glandular activity. This system is also made of two parts: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system arouses our bodies for defensive action. It does this by accelerating your heartbeat, slowing your digestion, raising your blood sugar, dilating your arteries, and cooling you with perspiration. As soon as your body realizes that it no longer needs to be ready for defensive action, your parasympathetic nervous system takes over. This system calms your body by undoing everything that the sympathetic nervous system just did, such as decreasing your heartbeat.

### Central Nervous System

The CNS is made of the spinal cord and brain. It links the PNS to the brain, thereby allowing the brain to be aware of the physical actions taking place. The spinal cord itself controls muscular reflexes. When you touch something hot, for example, you pull your hand away and then feel pain. Your spinal cord sends out a message to your muscles to pull your hand away, long before the pain sensation has reached your brain. The CNS also is made of the neural networks in the brain which interpret information, allowing the brain to take the necessary actions. These neural networks are cluster of neurons, which can communicate with each other through chemical changes in the brain.

### Neurons

Neurons are nerve cells. Although there are many different types of neurons, they all have the same essential parts. All neurons have dendrites that receive information, an axon that passes the message from the cell body to other neurons, muscles, or glands. The myelin sheath insulates the axon and helps to speed impulses. A neural impulse is an electrical signal that travels down the axon. Impulses are made from an exchange of ions between neurons. At the resting potential, the neuron has a negative charge in its axon and is surrounded by a positive charge. When the action potential is reached, the axon is depolarized, positive charges travel down the axon, and then trigger the release of neurotransmitters. After the impulse is sent, the neuron goes through a refractory period where the positive charges are pushed back outside the axon. The neurotransmitters that are sent are received on the dendrites of the receiving neuron. These neurotransmitters can either be excitatory or inhibitory. When there are more excitatory neurotransmitters and too few inhibitory neurotransmitters, the neuron's threshold is reached and the action potential is triggered. Neurons have an all-or-none response. The strength of the stimulus does not affect the action potential’s speed. In other words, it doesn't matter by how much the excitatory neurotransmitters outnumber the inhibitory neurotransmitters. As long as the number reaches the neuron's threshold, the action potential is triggered.

### Synaptic Gaps and Neurotransmitters

In between neurons is the synaptic gap. This gap separates the axon terminal from the receiving neuron. It is across this gap that neurotransmitters must be sent. Neurotransmitters are chemical messengers that are sent by neurons. They cross the synaptic gap to bind to receptor sites. Once they are bound to the receptor sites, neurotransmitters unlock channels in the receptor neuron. This allows ions to flood the axon of the neuron and the process is repeated. When a neuron releases too many neurons, it re-absorbs the excess neurotransmitters. This process is known as reuptake. Neurotransmitters can produce many different reactions in the brain and throughout the body. Acetylcholine, for example, enables muscle function, learning, and memory. Another neurotransmitter, dopamine, allows for movement, learning, attention, and emotion. Serotonin helps control mood, hunger, sleep, and arousal. Norepinephrine controls alertness and arousal. These neurotransmitters can be mimicked by certain drugs and chemicals. When the drugs are used for a long time, however, the brain becomes dependent on them and stops producing certain neurotransmitters naturally.

### Parts of the Brain

Neurons send this information throughout the brain, including the brainstem, the thalamus, the cerebellum, the limbic system, and the cerebral cortex. The brainstem, the oldest and innermost region of the brain, contains the medulla and the reticular formation. The medulla controls heartbeat and breathing. The reticular formation helps control arousal and sends incoming information from the thalamus to specific parts of the brain. The thalamus is the “sensory switchboard.” It receives information from all of the senses except smell and routes the information to other areas of the brain. When it receives replies, the thalamus sends them to the cerebellum and the medulla. The cerebellum, which means little brain, coordinates voluntary movement of the body. The limbic system, near the cerebellum, influences emotions and motives, partly through hormones. The limbic system contains the amygdale which influences aggression and fear. It also includes the hypothalamus, which regulates thirst, hunger, and body temperature. The hypothalamus secretes hormones that affect both bodily functions and other hormonal glands. Lastly, the limbic system contains the hippocampus which lays down long-term memories. The cerebral cortex is the ultimate control and information-processing center in the brain. It contains the parietal lobes, the occipital lobes, the temporal lobes, and the frontal lobes. The cerebral cortex also contains glial cells, which guide neural connections, provide nutrients and myelin to neurons, and absorb extra ions and neurotransmitters. The frontal lobes in the cerebral cortex control speaking, muscle movements, and the ability to make plans and judgments. The parietal lobes include the sensory cortex which controls the information that the brain receives from our senses. The temporal lobes contain the auditory areas in the brain. The motor cortex, found in the frontal lobes, is in control of bodily movements.

### Specialization and Integration

All of the parts of the brain have their own specialities but need to work as a complete system to make sure that the body functions correctly. This is known as specialization and integration. An example of specialization and integration would be how we read aloud. Our visual cortex receives written words as stimulation. The angular gyrus then transforms the visual representations to an auditory code. This code is then sent to Wernicke’s area, which interprets the code and sends it to Broca’s area, which controls speech muscles via the motor cortex. The motor cortex then controls the muscles in the mouth to pronounce the word. All of these systems work together to produce something as simple as reading aloud; there is no one system that can produce the same effect...

### Endocrine System

The endocrine system controls the secretion of hormones throughout the body. These hormones influence our interest in sex, food, and aggression. The glands that secrete hormones are: the hypothalamus, which controls the pituitary gland; the pituitary gland secretes many different hormones, some of which are used to control other glands; the thyroid sends out hormones that are used to control metabolism; the hormones from the parathyroid control the amount of calcium found in the blood stream; the adrenal glands trigger the fight or flight response; and the pancreas controls the level of sugar in the blood.

### Circadian Rhythms

The body is also on a schedule of bodily rhythms. There are annual cycles, that affect seasonal affective disorder. There are also 24-hour cycles, which control when we want to sleep. This cycle is known as a circadian rhythm. During a 24-hour period, our body temperature goes up in the morning, peaks during the day, and begins to fall in early afternoon when many people take siestas. When we sleep, our bodies also go through a certain rhythm. Sleep is separated into five stages. The first stage is when you have just fallen asleep. During this stage that lasts up to five minutes, breathing is slowed and there are irregular brain wave patterns. During stage 2, which lasts for about twenty minutes, there are periods of active brain activity. This is the stage when sleep talking would occur. During stages 3 to 4, the brain emits delta waves for about thirty minutes. After stage 4, we seem to begin to cycle out of sleep, but instead of reaching stage one again we go into REM sleep, or rapid eye movement. This is when your heart rate is increased, we have irregular breathing patterns, and eye movement can be seen.
All of the parts of the brains and different systems allow our bodies to work efficiently and correctly. Without even one of these systems, our bodies would not work the same. Although each part of the brain and each system has a special job, they all work together to maintain our bodily functions and higher thought.
Bibliography:
Myers, D. G. (2004). Psychology. Holland, MI: Worth Publishers.