There are two types of cells in the nervous system: neurons and glial cell. Neurons are the nerve cells that handle the information processing function. There are about 100 billion neurons. The average neuron is a complex structure with as many as 10,000 physical connections with other cells. On the other hand, glial cells provide support, nutritional benefits, and other functions in the nervous system. For every neuron there are about 10 glial cells (King, 2013).
Anatomy of Neurons
1.1 Cell body contains a nucleus. Like the nucleus of any cell, the nucleus of neuron contains DNA (deoxyribonucleic acid), the chemical that contains the genetic blueprint that directs the development of the neuron (Pastorino & Doyle-Portillo, 2013)
1.2 Cell Membrane is like a fence that surrounds the entire neuron, giving it shapes and keeping the cell’s internal fluid inside; it is said to be semi-permeable.
1.3 Dendrites are branch like structures (from the Greek word for tree branch). They receive incoming signals from other neurons
1.4 Axon a long tail-like structure growing out of the other end of the cell, caries signals away from the cell body
1.5 Myelin Sheath consists of layer of cells containing fat, encases and insulates most axons. By insulating axons, myelin sheaths up transmission of nerve impulses
1.6 Terminal Buttons are located at the ends of the axon where neurotransmitters are stored before being released into synapse
• All-or-none Principle was introduced by Ramon y Cajal who believed that neutral transmission always operated according this principle. This means that neurons transmitted signals to other neurons only when depolarization was strong enough to trigger an action potential. He further believed that all action potentials are all the same once they get started. This idea dominated neuroscience for 100 years, but it is now known that neurons often transmit messages through graded electrical potential that vary in magnitude.
• Depolarization. This is the process during which positively charge ions flow into the axon, making it less negatively charge
• Action Potential. It refers to a brief electrical signal that travels the length of the axon
Synapses and Neurotransmitters
• Synapses are tiny spaces between neurons; the gaps between neurons are referred to as synaptic gap. Most synapses lie between the axon of one neuron and the dendrites or cell body of another neuron
• Each Axon branches into numerous fibers that end in structures called terminal buttons. Stored in very tiny synaptic vesicles (sacs) within the terminal buttons are chemical substances called neurotransmitters.
• Neurotransmitters are chemical substances that are stored in very tiny sacs within the terminal buttons and involved in transmitting information across a synaptic gap to the next neuron.
Researchers have identified more than 100 neurotransmitters in the brain alone, each with unique chemical make-up. (G.B. Johnson, 2012 in King, 2013). Below are seven of them that have major effects in behavior:
1. Acetylcholine (Ach) usually stimulates the firing of neuron and is involved in the action of the muscles, learning, and memory (Kalmbach, Hedrick, & Waters, 2012 in King, 2013).
2. GABA (gamma aminobutyric acid) is found throughout the central nervous. Low levels of GABA are linked with anxiety.
3. Dopamine helps control voluntary movement and affects sleep, mood, attention, learning, and the ability to recognize rewards in the environment (Meyer, 2012 in King, 2013)
4. Endorphins are natural opiates that mainly stimulate the firing of neurons. It shields the body from pain and elevate feelings of pleasure (King, 2013)
5. Oxytocin is a hormone and neurotransmitter that plays an important role in the experience of love and social bonding
6. Norepinephrine inhibits the firing of neurons in the central nervous system, but it excites the heart muscles, intestines, and urogenital tract. This helps control alertness. Too little norepinephrine is associated with depression, and too much trigger agitated, manic states. (King, 2013)
7. Serotonin is involved in the regulation of sleep, mood, attention, learning. Lowered levels of serotonin are associated with depression.
The Structure of the Nervous System
Our nervous system is the vast, interconnected network of all the neurons in our body. Every single facet of our body’s functioning and our behavior is monitored and influenced by the nervous system. The nervous system is arranged in a series of interconnected subsystems, each with its own specialized tasks. At the broadest level, the nervous system is divided into the brain and spinal cord, known as central nervous system (CNS), and the remaining components of the nervous system referred collectively as the peripheral nervous system (PNS).
Functions of the Peripheral Nervous System
1.1 PNS must ensure that the CNS is informed about what is happening inside and outside our body. To this end, the PNS is equipped with sensory neurons that convey information to the CNS from the outside world, such as sights and sounds, as well as information from our internal world, such as aches and pains. Once the information has reached CNS, it is carried across interneurons as the brain processes the information.
1.2 The PNS takes over as it acts out the directives of the CNS. The PNS with motor neurons that carry signals from CNS to our muscles. For example, when you see a juicy apple, the sensory neurons of your eyes send this information upward to the part of the brain that processes visual information. Here the brain recognizes the apple, and you decide to eat the apple. The brain then sends signals downward to the motor neurons of your hand and arm, which, in turn, direct you to reach out and grasp the apple with your hand. In this fashion, the sensory pathways send sensory information to the spinal cord and brain, and the motor pathways carry “others” away from the brain and spinal cord to the rest of the body.
Divisions of the Peripheral Nervous System
2.1 Somatic Nervous System: The somatic nervous system is the branch of the PMS that governs sensory and voluntary motor action in the body.
2.2 Autonomic Nervous System: The neurons of the autonomic nervous system control the smooth muscles of the internal organs, the muscles of the heart, and the glands. By automatically regulating organ functions, the autonomic nervous system frees up our conscious resources and enables us to respond quickly and efficiently to the demand placed on us by the environment.
The autonomic nervous system can be further subdivided into the parasympathetic nervous system (governs organs in calm situations) and the sympathetic nervous system (governs organs during time of stress).
2.3 Parasympathetic Nervous System. When the parasympathetic nervous system is active, heart rate, blood pressure, and respiration are kept at normal levels. Blood is circulated to the digestive tract and other internal organs so that they can function properly, and your pupils are not overly dilated. Your body is calm, and everything is running smoothly.
2.4 Sympathetic Nervous System. During times of stress, the sympathetic system takes over primary regulation of our organ functions from the parasympathetic system. The actions of the sympathetic nervous system are offered to as “ fight or flight response.” The sympathetic nervous system evolved to protect us from danger. When it is activated, heart rate increases, breathing become engorged with blood, the pupils dilate, and the hair on the back of the neck stands up. All of these changes help to prepare us to defend our body from threat.
The Central Nervous System: The Brain and the Spine
The brain is composed largely of neurons and glia cells. These structures are organized into three regions: the hindbrain, midbrain, and the forebrain. The hindbrain sits directly above the spinal cord and is named for its position at the bottom of the brain. The hindbrain is the most “primitive” part of the brain, involved in the most basic life-sustaining functions. The hindbrain make up a good portion of the brainstem, a series of brain structures that are essential for life. Even small amounts of damage to the brainstem can be life threatening.
The forebrain resides in the top part of the skull and regulates complex mental processes such as thinking and emotional control. It is the largest region of the brain and includes structures that regulate many emotional, motivational, and cognitive processes. Without this well-developed forebrain, humans would not have the mental abilities such as problem solving, thinking, remembering, and using language.
Between the hindbrain and the forebrain is the midbrain, which act as a connection between the more basic functions of the hindbrain and the complex mental processes of the forebrain, without the midbrain, the hindbrain could not supply the forebrain with the neutral impulses it needs to remain active and to keep us conscious.
1. The Hindbrain
The hindbrain consists of three structures: the medulla, the pons, and the cerebellum.
1.1 The medulla sits at the top of the spinal column at the point where the spinal cord enters the bases of the skull. The medulla regulates heartbeat and respiration, and even minor damage to the medulla can result in death from heart or respiratory failure. It also plays a role in sneezing, coughing, vomiting, swallowing and digestion.
1.2 The pons sits above the medulla, where the brainstem bulges inside the skull. Like the medulla, the pons is crucial to life. The pons plays a role in respiration, consciousness, sleep, dreaming, facial movements, sensory processes, and the transmission of neural signals from one part of the brain to another. The pons acts as a “bridge” for neural signals; in particular, sensory information coming from the right and left sides of the body crosses through the pons before moving on to other parts of the brain. If the pons becomes damaged, the “bridge” is out, and serious sensory impairments can result.
1.3 The cerebellum is the large, deeply grooved structure at the base of the brain. The cerebellum is necessary for balance, muscle tone, and the performance of motor skills. It may also play a critical role in the learning of motor skills and the execution of certain behaviors. Damage to the cerebellum leads to loss of balance and coordination.
2. The Midbrain
The midbrain structures connect the hindbrain with more sophisticated forebrain. For psychologists, one of the most interesting midbrain structures is the reticular formation. The reticular formation, located near the pons, is a network of neurons that extends from the hindbrain region into midbrain. The reticular formation serves primarily to regulate arousal levels, thereby playing an important role in attention, sleep, and consciousness. The reticular formation functions as a type of “on switch” for the high-level thinking centers of the forebrain. Additionally, the reticular formation appears to play role in regulating cardiovascular activity, respiratory functioning, and body movement.
3. The Forebrain
The forebrain contains several groups of structures that functions as subsystems. The structures of the limbic system govern emotional and motivational processes, and other forebrain structures govern sensory processing and motivation. The wrinkled and folded external surface of the brain, the cerebral cortex, governs high-level processes such as cognition and language. The forebrain is divided into right and left cerebral hemispheres. For the most part, forebrain structures are duplicated in the right and left hemispheres.
3.1 The Limbic System
The series forebrain structures collectively the limbic system regulate some of our basic emotional reactions. Two limbic structures are located deep in the central region of the brain, above the hindbrain and beneath the cerebral cortex: the amygdala and the hippocampus.
3.1.1 The amygdala is an almond-shaped structure located almost directly behind the temples. The amygdala governs the emotions of fear and aggression.
3.1.2 The hippocampus plays a role in the transfer of information from short to long term memory.
3.2 The Thalamus
The thalamus plays a role in the attention we pay to things the stimulate our senses, and it functions as a relay station for information coming from our senses to the brain. The thalamus also plays a role in keeping specific areas of the cortex activated during rapid eye movement or REM sleep, when much of our dreaming occurs.
3.3 The Hypothalamus
Nestled below the thalamus is the hypothalamus (hypo means “below”). The hypothalamus maintains homeostasis in the body, a state of internal equilibrium across a variety of bodily systems. In maintaining homeostasis, the hypothalamus is responsible for monitoring and regulating body temperature, thirst, hunger, sleep, autonomic nervous system functioning, and some sexual and reproductive functions, and can change hormone levels in the bloodstream. To maintain homeostasis, the hypothalamus must ultimately motivate us to engage in certain behaviors. For example, when our body needs fuel, the hypothalamus motivates us with hunger. Without the hypothalamus, we would not know when to engage in the behaviors that keep our bodily systems in balance.
The most noticeable structure on the external surface of the brain is the cerebral cortex, or simply the cortex. The cortex is the thin (approximately 2mm thick), wrinkled layer of tissue that covers the outside of the cerebral hemispheres, or the two sides of the brain the Cortex is arguably the most sophisticated part of the brain and is responsible for the highest levels of processing: cognition and mental processes such as planning, decision making, perception, and language. It is the cortex that gives us our humanness.
The Lobes of the Cortex and Lateralization in the Brain
The human cortex is divided into four distinct physical regions called lobes. These are the frontal lobe, parietal lobe, occipital lobe, and the temporal lobe.
1. Frontal Lobe: cortical area directly behind the forehead that plays a role in thinking, planning, decision making, language, and motor movement.
2. Parietal Lobe: Cortical area on top sides of the brain that plays a role in visual processing.
3. Occipital Lobe: Cortical area at the back of the brain that plays a role in visual processing.
4. Temporal Lobe: Cortical area directly below the ears that play a role in auditory processing and language.
Lateralization in the brain is evident in that the right and left hemispheres process somewhat different types of information. For example most people process language largely in the left hemisphere although some people have major language centers in the right hemisphere, and some have major language centers in both hemisphere.
Two examples that illustrate hemispheric specialization of language:
1. Broca’s Aphasia : When damage is severe in Broca’s area in the left frontal lobe, people are unable to produce understandable speech.
2. Wernicke’s Aphasia : When damage is in the Wernicke’s area in the left temporal lobe, people are unable to understand spoken language.
The hemispheres coordinate the information they process through corpus callosum, a dense band of neurons that sits just below the cortex along the midline of the brain.
Endocrine System: Chemical Messenger of the Body
The endocrine system is another biological system in the body that also plays an important role in the communication and the regulation of bodily processes. This system consists of a number of glands – that secrete two kinds of chemical substances.
1. Neuropeptides. This allows the endocrine glands to communicate with one another. They play important roles in stress regulation, social bonding, emotion, and memory.
2. Hormones. These are chemical substances, produced by the endocrines glands, the influence internal organs. The release of neuropeptides and hormones by the endocrine glands is regulated by several systems of the brain through the hypothalamus. Thus the endocrine glands gives the brain additional ways to control the body’s organs. This is particularly true during physical stress or emotional arousal. At these times, neuropeptides and hormones influence such things as metabolism, blood pressure, blood-sugar level, and sexual functioning.
The Seven Endocrine Glands
1. Pituitary Gland. This is located near the hypothalamus, which directly controls its functioning. This is considered the body’s master gland because its secretions help regulate the activity of the other glands in the endocrine system. Further, it secretes hormones that control blood pressure, thirst, and body growth.
2. Adrenal Glands. This pair of glands sit atop the kidney. They play important role in emotional arousal and secrete hormones important to metabolism. This secretes three hormones that are important in our reaction to stress:
3. Islets of Langerhans. These are embedded in the pancreas. This regulates the level of sugar in the blood by secreting two hormones that have opposing actions such as:
3.1 Glucagon which causes the liver to convert its stored sugar into blood sugar and to dump it into the bloodstream.
3.2 Insulin which reduces the amount of blood sugar by helping the body’s cells absorb sugar in the form of fat.
4. Gonads. These are glands that produce sex cells and hormones important in sexual arousal and that contribute to the development of secondary sex characteristics. There are two sex glands or gonads – the ovaries in females, the testes in males. The gonads produce sex cells – ova in females and sperm in males. The most important sex hormones are estrogen in female and testosterone in males.
5. Thyroid Gland. This is located just below the larynx, or voice box. It plays an important role in the regulation of metabolism. It secretes hormone called thyroxin which is necessary for proper mental development in children and helps determine weight and level of activity in adults.
6. Parathyroid Glands. These are four small glands embedded in the thyroid glands. They secrete parathormone, which is important in the functioning of the nervous system. Parathormone regulates ion levels in neurons and controls excitability of the nervous system.
7. Pineal Gland. It is located between the cerebral hemispheres, attached to the top of the thalamus. Its primary secretion is melatonin. Melatonin is important in the regulation of biological rhythms, including menstrual cycles in females and the daily regulation of sleep and wakefulness.
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