The Nervous System
The nervous system is a network of cells carrying information to and from all parts of the body.
In neuroscience there is an emphasis on structure and function of neurons, nerves, and nervous tissue and is considered a branch of the life sciences.
The structure of the neuron is studied in biological psychology (AKA behavioral neuroscience) which is a branch of neuroscience that focuses on the biological bases of psychological processes, behavior, and learning.
The neuron is a specialized cell in the nervous system that sends and receives nervous system messages. There are various parts of the neuron:
- Dendrites – it is a branch-like structures that receive messages from other neurons.
- Soma – it is the cell body of the neuron and is responsible for maintaining the life of the cell.
- Axon – it is the long tube-like structure that carries the neural message to other cells.
There are other types of brain cells. Glial cells provide support for the neurons to grow on and deliver nutrients to neurons. They also remove waste products and dead neurons. The types of glial cells include oligodendrocytes and Schwann cells. They produce myelin to coat axons.
- Myelin sheath is a fatty substance produced by glial cells and coats the axons insulate, speed neural impulse. (Multiple sclerosis causes destruction of myelin sheath.)
- Oligodendrocytes produce myelin for brain and spinal cord.
- Schwann cells produce myelin for rest of body
Electrically Charged Neurons
Neurons are electrically charged with ions which are located inside and outside of the cell. There are more negatively charged ions located inside the cell while there are more positively charged ions located outside the cell. The difference in charges creates electrical potential.
- Resting potential is the state of neuron when its not firing a neural impulse
- Action potential is the release of the neural impulse and consists of the reversal of electrical charge within the axon
- All-or-none is when a neuron either fires completely or does not fire at all
Sending Messages to Other Neurons
The end of the axon has axon terminals which in turn has a saclike structure (synaptic vesicle) at each end.
Inside the synaptic vesicle are neurotransmitters. Next to the end of the axon is a little gap and then the dendrites of another neuron. The gap is called the synapse.
When an electrical message reaches the axon terminal, it releases neurotransmitters from the synaptic vesicle. The neurotransmitters float across the synapse. The dendrites have receptors sites which capture the neurotransmitter. Neurotransmitters fit like keys in a keyhole when they reach the receptor sites. The message is relayed to the next neuron.
Neurotransmitters can turn off cells (exhibitory) or turn on cells (inhibitory). This is important because it also for things such as sensory adaptation and reduced/no pain as injuries are healed.
Certain drugs mimic the shape of certain neurotransmitters and fit into the receptor sites.
Agonists are chemical substances that can mimic or enhance the effects of neurotransmitters on receptors sites of the next cell, which can result in an increase or decrease in the activity of the receiving cell, depending on what the effect of the original neurotransmitter (exhibitory or inhibitory) was going to be. Example: Original neurotransmitter (exhibitory) → increase excitement.
Antagonists are chemical substances that block or reduce a cell’s response to the action of other chemical or neurotransmitters. Example: Beta blockers are drugs used to control high blood pressure and are antagonist because it inhibits the heart’s contractions.
Neurotransmitters, Messengers of the Network
Acetylcholine is a type of neurotransmitter and is found between the synapse and neurons. It stimulates the skeletal muscle to contract but slows down the contractions of the heart muscle. If acetylcholine block from its site, a person can become paralyzed (curare, used in blow darts). Too much acetylcholine (like from a bite from a black widow spider) can result in convulsions and even death.
Serotonin is another neurotransmitter that can have either an exhibitory or inhibitory effect. It is associated with mood, sleep, and appetite. Low levels of serotonin are linked with depression.
Dopamine is a neurotransmitter that can have different effects depending on where in the brain it is released. Lack of dopamine in certain parts can result in Parkinson’s disease while too much in other parts can result in schizophrenia.
Endorphins are pain controlling chemicals in the brain. When the brain gets a message of pain, it triggers the release of endorphins which bind to the open receptor gates. This reduces pain over time. Morphine is the drug equivalent of endorphins. Endorphins are the reason heroin and other opium products are so addictive. When people take heroin their body stops producing endorphins (for a while) and that is why everything hurts so much.
Cleaning Up the Synapse
Neurotransmitters have to get out of the receptor site before the next stimulation can occur. These are the different ways they get out:
- Some neurotransmitters just diffuse away (dissolve)
- Reuptake where the neurotransmitters are suction cupped back into the vesicle
- Other neurotransmitters are not taken back into the cell like acetylcholine because it is response for muscle activity and movement.
The Central Nervous System (CNS)
The central nervous system is composed of the brain and spinal cord.
The brain is the core of the CNS and makes sense of the messages sent to it.
The spinal cord is a long bundle of neurons and is divided into areas (inside and outside the cord). The outer section carries messages from the body to the brain and from the brain down to the body. It is like a pipeline.
The inside section is responsible for certain reflexes – very fast, life-saving reflexes. There are three types of neurons:
- Afferent (sensory) neurons – carry the message to the spinal cord (touch a flame and a message of pain is sent)
- Efferent (motor) neurons – carry the message from the spinal cord to the muscles and glands (cause finger to pull back)
- Interneurons – connect afferent to motor neurons (pain message received and sends out message to pull back)
If the message had to go to the brain, a lot more physical damage would have been done to the body. Having this reflex arc allows for a very fast response time.
It was once thought that damage to a person spinal cord was permanent because neurons were believe to not be capable of repairing themselves. The brain actually exhibits a great deal of neuroplasticity, the ability to constantly change both the structure and function of many cells in the brain in response to experience and even trauma. Neurons can adopt new functions when old ones die.
The Peripheral Nervous System (PNS)
The word peripheral the refers to things that are not in the center. It is made up of all the nerve and neurons that are not contained in the brain and the spinal. This system allows the brain and the spinal cord to communicate with the sensory systems of the eyes, ears, skin, and mouth and allow the brain and spinal cord to control the muscle and glands of the body. The PNS can be divided into two major systems, the somatic nervous system and the autonomic nervous system.
- The somatic nervous system is made up of the sensory pathway, which is all the nerves carrying the messages from the senses to the central nervous system and the motor pathway (allows movement of the body).
- The autonomic nervous system consists of large groups of neurons located near the spinal column. This system controls internal organs, glands, and involuntary muscles. The ANS is divided into two systems, the sympathetic division and the parasympathetic division.
- The sympathetic division allows people to deal with stress and is sometimes referred to as the fight-or-flight system. It gets the body ready to deal with stress. The adrenal glands are stimulated. The heart, muscles, and lungs are mostly affected. If the stress ends then the parasympathetic division is activated. A person that collapses from a stressful situation is due to the fact that the parasympathetic division over responds.
- The parasympathetic division does the opposite of the sympathetic division by trying to restore the body to its normal functioning after a stressful event. It allows the body to put back all the energy it burned (that is why people are sometimes hungry after a stressful event). The parasympathetic division is also responsible for the day-to-day bodily functioning (breathing, digestion, etc.). It is active most of the day.
Peeking Inside the Brain
Researchers study people with brain damage to see how it is affecting them. The problem is no two case of brain damage are identical.
When it come to animal studies, deep lesioning is done to study their brain. A thin wire is inserted into the brain and neurons in that area are killed with an electrical shock. The animal is then studied to see what impact the deep lesioning. Researcher can also use a reduced electrical shock to see what it may activate.
An electroencephalograph (EEG) studies the living brain by recording the electrical activity of the cortex. Electrodes are placed on the scalp and provide readings. It can detect things like sleeping, seizures, and the presence of tumors. It can also detect which areas of the brain are being activated during certain cognitive tasks. There are beta, alpha, theta, and delta waves.
A CT (computed tomography) scan takes an X-ray of the brain which is aided by a computer. It can show stroke damage, tumors, injuries, and other abnormalities.
A MRI (magnetic resonance imaging) scan can provide more details than a CT scan. It uses magnetic fields to scan the brain.
A PET (positron emission tomography) scan allows researchers to see the brain in action. A radioactive glucose is injected in the person and researcher can see which cells are using it up.
A functional MRI involves a computer tracking the changes in oxygen levels of the blood. It allows researchers to see what parts of the brain are active.
From the Bottom Up
The medulla controls life-sustaining functions such as heartbeat, breathing, and swallowing. (Christopher Reeves) It is in this part that the sensory nerves cross.
The pons allow for the coordination of movement of the left and right sides of the body. They influence sleep, dreaming, and arousal.
The reticular formation allow people to ignore constant, unchanging information and to become alert to changes in information. (Example: The constant hum of an air conditioner.) It also keeps people alert and aroused. It keeps people awake and alert. (Example: If you get cut off all of a sudden on the highway of if your baby cries at night.) It is also associated with comas.
The cerebellum controls all involuntary, rapid, and fine motor movement. It allows for a person sitting upright, walking, diving, dancing, typing, etc. Learned reflexes, skills, and habits are stored here. Because of this you don’t always have to think about posture, muscle tone, and balance.
Structures Under the Cortex
The limbic system is involved in emotions, motivation, and learning.
The thalamus is a relay station for incoming sensory information. It processes sensory information before sending it on to the part of the cortex that deals with that kind of sensation. All the senses go through the thalamus except smell.
The hypothalamus regulates body temperature, thirst, hunger, sleeping and waking, sexual activity, and emotions.
The hippocampus is important in storing long-term (permanent) memories. It is also associated with memories of where objects are located. Deterioration of this can lead to people forgetting where they live.
The amygdala is responsible for fear response and memory of fear. Having damage to this area can result in a loss of fear for certain animals (example: a rat is unafraid to be next to a cat).
The cortex is what most people think of when the see the brain. It is full of wrinkles. It is wrinkled because it allows for more surface area. Corticalization is the wrinkling of the brain over time and is a real measure of human intelligence.
The Lobes and Their Specialties
The cerebral hemispheres is the name for the two sections of the cortex. They are connected by the corpus callosum which is made up of neural fibers and helps the two hemispheres communicate.
The occipital lobe is located in the back of the brain and is the area that processes visual information. It makes sense of visual stimuli. This is why people can temporarily lose vision or see “stars” when they are hit hard in the back of the head.
The parietal lobes are located on the top and back of the brain and contains the somatosensory cortex. This area of the brain processes information pertaining to touch, temperature, and body position.
The temporal lobes are located on the temples of the head (thus the name) and are associated with auditory (sound) information. Language as well as the sense of taste are also connected to this area.
The frontal lobes are located in the front of the brain (thus the name) and deal with higher mental functions such as planning, personality, memory storage, and complex decision-making. It also helps control emotions. These lobes also contain the motor cortex which control the body’s voluntary movements.
- The association areas are devoted to making connections between sensory information and stored memories, images, and knowledge. Most of the association areas are located in the frontal lobes.
- The Broca’ area of the brain allows people to speak fluently and smoothly. It is located in the left frontal lobe. Damage to this area can result in people having difficulty getting words out smoothly. Broca’s aphasia is an inability to use or understand either written or spoken language.
- The Wernicke’s area of the brain involves understanding the meaning of words. A person with Wernicke’s aphasia would be able to speak fluently and pronounce words, but the words would be the wrong ones entirely.
The Cerebral Hemispheres
Even though both hemispheres may appear nearly identically they are different. For instance, in most people seems confined to only the left hemisphere. The cerebrum is the upper part of the brain consisting of the two hemispheres and the structure connecting them.
Roger Sperry studied the two hemispheres of the brain. He even cut the corpus callosum to see how it would affect people with epilepsy.
The left hemisphere specializes in the language, speech, handwriting, calculations, sense of time and rhythm, and basically any kind of thought that requirements analysis. It breaks information down into smaller parts before processing.
The right hemisphere appears to specialize in global, widespread processing involving perception, visualization, spatial perception, recognition of patterns, faces, emotions, melodies, and expression of emotion. This hemisphere processed information all at once.
Ever recognize someone but cannot put a name to the face? This is because the right hemisphere recognizes the face but the left side is having problems finding the information you need (the name).
The Chemical Connection
Endocrine glands secrete their chemicals (hormones) directly into the bloodstream which are carried to their target organs. Hormones affect behavior and emotions by controlling muscles and organs such as the heart, pancreas, and sex organs.
The pituitary gland is located in the brain and is a master gland which means it controls or influences all other endocrine glands. This gland can affect pregnancy, milk production for nursing, salt and water in the body, and growth.
The pineal gland secretes melatonin mostly at night which can affect the sleep-wake cycle.
The thyroid gland secretes thyroxin which regulates metabolism.
The pancreas secretes insulin and glucagons and is connected to diabetes (too little) and hypoglycemia (too much). It is also related to feelings of hunger.
The gonads are the sex glands, including the ovaries in females and the testes in males. They secrete hormones related to sexual behavior and reproduction.
The adrenal glands releases epinephrine and norepinephrine when people are in stress. It produces a chemical that is a type of steroid. An adrenal hormone called cortisol is released when people are under physical and psychological stress. Cortisol release glucose into the bloodstream during stress, providing energy for the brain, and provides energy to the muscles.