Human_Physiology_Medical_School_Crash_Course_Unabridged_007_A_T

This chapter discusses the physiology of the central nervous system, which consists of the brain and spinal cord. The central nervous system sends signals to the body and receives sensory information from the peripheral nervous system. The peripheral nervous system consists of motor and sensory nerves. Disruptions in brain or spinal cord function can lead to paralysis. The brain has three primary sections: forebrain, midbrain, and hindbrain. The forebrain, the largest part of the brain, is responsible for memory, intelligence, speech, movement, and emotions. Different lobes in the brain have different functions. The brain consists of two hemispheres, with the left side being more analytical and the right side being more creative. The brain also has glial cells that support and feed neurons. Neurons communicate through synapses and neurotransmitters. The brain is adaptable in young children but less so as it ages. Memory and movement are important functions of the brain. Chapter 4, Central Nervous System Physiology. This chapter is all about the physiology of the central nervous system. The central nervous system consists of the brain and spinal cord. They are highly physiologically active with billions of neurons and billions of connections that participate in sending signals to the body and receive sensory information from the peripheral nervous system. In this chapter, we will discuss the relationship between the central nervous system, the peripheral nervous system, and the various functions of the brain. The brain and the peripheral nervous system. The brain and spinal cord make up the central nervous system. The peripheral nervous system consists of the nerves that exit the spinal cord and those that travel to every part of the body. The peripheral nervous system consists of the motor nervous system or efferent system and the sensory nervous system or the afferent nervous system. The efferent system receives signals from the brain that tell the body which muscle to move, which organ systems to activate, and which bodily functions should occur. The afferent nervous system consists of sensory nerves that receive input from sensory organs that pick up signals from the organs and tissues of the body such as the skin, the taste buds, the olfactory cells, the ears, and the eyes. These signals go to the brain where they are interpreted. The interpretation is recorded by the brain and the individual makes cognitive sense of the sensory input. These types of interactions go on millions of times per day such that the body and organs are always functioning and sensory information is always taking place. If there is a disruption in part of the brain function or disruption in the spinal cord, the signals don't get transferred from the peripheral nervous system to the central nervous system and the body can become paralyzed. Paralysis involves lack of motor nerve connections between the body and the brain as well as a lack of sensory input going to the brain. In a sense, the brain is like a central computer system that has control over every body function. The spinal cord is the main highway that the nerve impulses travel to and from. It is located in the back and runs from the brain to the upper lumbar area where it sends spinal nerves out to make peripheral nerves that ultimately arrive at every part of the body. As an example, the brain receives peripheral nerve input when a person touches an extremely hot stovetop. The afferent nervous system sends an immediate impulse to the brain via the spinal cord where the brain interprets the sensation of heat. It sends an immediate signal via the efferent motor system where it tells the muscles to immediately back away from the hot item. Basic brain structure. In the very basic sense, the brain consists of three primary sections including the forebrain, the midbrain, and the hindbrain. These are what evolution has provided for us as part of the central nervous system. The forebrain is the biggest and most complex aspect of the brain. It is made up of the cerebrum and a few other structures in the brain. The cerebrum is the part of the brain that allows for thought. It has billions of neurons and is structurally unique in that it has many folds that allow for it to take up less space inside the skull. The cerebrum allows a person to have memory, intelligence, speech, movement, feeling, personality, and emotion. There are different lobes in the brain that are related to the various sensory and motor processes that need to happen throughout the day. There are four main lobes in the brain. The frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. Each brain lobe has different functions. For example, the frontal lobe controls movement, speech, behavior, intellectual functioning, memory, emotions, problem-solving, planning, and decision making. The parietal lobe controls the senses. The temporal lobe controls memory, emotions, and speech. The occipital lobe controls vision. There are two halves to the brain. These are the left half and the right half. These are also known as hemispheres. They are connected to one another by the corpus callosum, which is a nerve fiber bundle that allows for communication between the two sides. While the two sides look the same, they have completely different functions. The right side of the brain is more subjective, creative, and intuitive. Listening to music or drawing requires right brain function. The left side of the brain is more analytic, logical, and objective. This is the side involved in mathematics and logical thinking. Some individuals focus on their left brain more and are more analytical in their thinking, while others are more creative, focusing more on the right side of the brain. Others are considered whole-brained because they use both sides of their brain to some degree. The outer aspect of the cerebrum is the gray matter or cortex. This is where the nerve cells live. Information is taken in by the senses and comes to the cortex of the brain via the spinal cord. The information is sent to the cerebrum and other parts of the brain to be processed. In the inner aspect of the forebrain is the hypothalamus, the pituitary gland, and the thalamus. The thalamus carries messages from the sensory organs and interprets them. The hypothalamus is near the thalamus and controls automatic body processes and things like sleep, appetite, and thirst. The hypothalamus also has a great deal of influence over the pituitary gland, which has many jobs, including stress response initiation, sexual maturity and function, mineral and water balance, metabolism, and growth. The midbrain is located beneath the middle aspect of the forebrain. It is not a conscious part of the brain, but acts as a coordinator, sending messages back and forth between the brain, the spinal cord, and the peripheral nervous system. The hindbrain is on the back part of the cerebrum. It is not a conscious part of the brain. The pons, the medulla, and the cerebellum are all part of the hindbrain. It is a smaller version of the cerebrum and is responsible for coordination, movement, equilibrium, and balance. The cerebellum in the hindbrain is the second biggest part of the brain. It is located behind the brain. There are two hemispheres in the cerebellum, which controls many body functions, including the collection of sensory information, walking behaviors, the ability to talk, reflexes, balance, posture, and movement. The pons and the medulla, along with the midbrain, are often referred to as the midbrain. It sends signals in and out of the brain and has influences on memory, automatic body functions, breathing, heart rate, blinking, digestion, swallowing, blood pressure, thirst, and hunger. The brainstem is also a part of this aspect of the brain and controls heart rate, breathing, and other things that are automatically done without thinking. Basic functions of the nervous system. The main backbone of the nervous system depends on miniature cells known as nerve cells or neurons. The brain has billions of neurons, each of which has a specialized job. For example, sensory neurons take up information from the various sense organs of the body. Motor neurons send information away from the brain and back to the rest of the body. Besides neurons, the brain has glial cells. While the neurons carry electrical signals to make the nervous system work, the glial cells support, protect, and feed the neurons. There are four different types of glial cells called microglial cells, ependymal cells, oligodendrocytes, and astrocytes. Neurons cannot be repaired or replaced when damaged, but glial cells can replicate and play a big role in the health of the brain. Neurons close to one another do not directly touch each other, but connect via tiny spaces known as synapses. Electrical signals pass from one nerve cell down through axons. The end of the axon has neurotransmitters that cross the synapse and attach to receptors on the receiving neuron. The neurotransmitters are chemical proteins that have different effects on the nervous system. Some neurotransmitters include dopamine, epinephrine, norepinephrine, serotonin, acetylcholine, and gamma-aminobutyric acid or GABA. The nervous system contains all the neurons necessary for life at the time of birth. Many of these neurons are not connected to one another, but are waiting for environmental experiences to connect them to one another. As an individual learns and develops, the connections are made between the existing neurons so that pathways of learning memory and intelligence are established. The brain is extremely adaptable in young children. When a part of the brain is injured in children, there is plenty of brain left that can take over some of the lost function. As the brain ages, however, the brain must work harder to establish new neural pathways, so it is difficult to develop new abilities or alter behavioral patterns. This is why many doctors believe that one should continually challenge oneself so that more neural pathways can be established and maintained. Memory is an important but complex aspect of brain function. The events that happen to us and the sensory information that comes from these events are initially processed in the cortex of the brain. If the information is to be long-term, the data is sent to the hippocampus and the amygdala, where it is processed for long-term storage. It can be retrieved at any time. As these messages travel across the brain, they also make pathways to parts of the brain that help an individual remember things. The brain is also responsible for movement. As you can recall, the left side of the brain controls the movements of the right side of the body, while the right side of the brain controls the left side of the body. If a part of the brain is injured on the right side, the individual can suffer from left-sided neglect, in which they don't pay attention to the left side of the body. In situations where the left side is injured from a stroke, injury, or bleeding in the brain, the individual has paralysis and loss of sensation on the right side of the body. Basic body functions are controlled by the brain and peripheral nervous system. The autonomic nervous system is unconscious and controls things like digestion, breathing, shivering, and sweating. The sympathetic branch of the autonomic nervous system is what is activated during stress. If a person experiences stress, the body enters a fight-or-flight response, in which the brain sends signals to the adrenal glands to release epinephrine and norepinephrine in preparation for a fight-or-flight from danger. The parasympathetic system is the opposite of the sympathetic nervous system. It allows the body to be restful and manages digestion and excretion of waste. The sympathetic nervous system and the parasympathetic nervous system can work at the same time or can change places, depending on the person's mood, level of stress, and the external environment. The senses impact the brain. There are five basic senses. The sensation of sight comes from the eyes. Light goes into the eyes and is picked up by the retina. The optic nerve sends signals to the brain that interprets what the retina is seeing, and the individual recognizes the image as being something they have seen. Sounds are picked up by the ear. The inner ear takes the sound waves and sends a signal using the cochlear nerve to the brain, which processes it as something the person recognizes as something they have heard. Taste is picked up by the taste buds and is interpreted somewhat by the sense of smell. Taste buds can only pick up things as being bitter, salty, sour, or sweet. It is up to the sense of smell and the brain to interpret the sensations as a particular taste that is more complex than simply what the brain receives from the taste buds. The sense of smell can be operating in other body senses besides taste. The olfactory center and the olfactory nerve send signals to the brain. The brain can detect more than 10,000 smells and is extremely sensitive to smell. The sense of touch is picked up by the skin. There are more than 4 million sensory receptors on the skin. There are proportionately more receptors on the fingers, tongue, and lips. These gather information related to the brain from receptors that can detect pain, temperature, light touch, and pressure. It is up to the peripheral nerves to send the signal to the brain, which interprets the sensation as being something the person has encountered. Brain disorders. The brain controls every aspect of the body so that when a disorder occurs, there are abnormalities that can occur in parts of the body or in the memory and cognition aspects of the brain. There can be genetic reasons for brain dysfunction, mental disorders, stroke, bleeding in the brain, cancer, or brain injury that can negatively act on the brain's function. Brain tumors can affect brain function. A brain tumor is an abnormal growth of cancerous or non-cancerous tissue that takes the place of neurons. As the tumor grows, it can spread throughout the brain, interfering with many brain and bodily functions. These can be treated with surgery to remove the tumor. Chemotherapy rarely helps brain tumors as they don't cross the blood-brain barrier. Cerebral palsy is another brain disorder. It happens because of a developmental defect or damage to brain tissue prior to birth, as a result of birth trauma, or as a result of an injury in the first few years of the child's life. It mainly affects the motor aspects of the brain. Those with cerebral palsy may be cognitively normal or may have severe developmental delays. Mild cerebral palsy can result in minor weaknesses of the arms and legs. In serious cases, there is spasticity of the muscles and the patient has difficulty in all areas of living. Some cerebral palsy patients are wheelchair-bound. Epilepsy or seizure disorders can happen in the brain at any age. There are partial seizures in which the individual is awake and has motor or sensory symptoms affecting just a part of the body. There are generalized seizures or tonic-clonic seizures in which the patient has several minutes of unconsciousness, falls to the ground, and has jerking motions of the arms and legs. The episodes usually resolve after a few minutes, but sometimes patients have status epilepticus in which the seizure does not resolve on its own but needs medications to stop the seizure. There are absence seizures in which the patient blanks out for a few seconds or minutes but remains upright and doesn't have jerking motion. Myoclonus is another type of disorder similar to seizure disorders. It involves jerking of the muscles while the patient is entirely awake. Headaches can be from a brain disorder or a non-brain disorder. The most common headaches are not from a brain disorder but are caused by muscle tension in the shoulders, head, and neck. Migraine headaches have an unclear cause and can be inherited. Certain triggers can result in a migraine headache which is intense and associated with vision symptoms or other neurological symptoms. Cluster headaches are extremely intense headaches occurring on one side of the head. It may be associated with tearing, nasal drainage, and facial sweating on the side of the headache. Migraines can happen at any time but cluster headaches occur for a couple of weeks and then resolve for years or months before another set of cluster headaches come back. Rarely headaches can come from brain hemorrhage or trauma to the brain. Individuals can get meningitis or encephalitis. Meningitis affects the lining of the brain called the meninges. It can be bacterial or viral illness and can be fatal if not treated with antibiotics or antiviral medications. Encephalitis affects the entire brain. The brain becomes inflamed from infection secondary to bacteria, fungi, or viruses. Failure to treat this condition can result in permanent brain damage. Mental illnesses can be caused by dysfunction of the brain. Chemical abnormalities in the brain can cause mental changes and difficulties with thought and memory. Structural brain abnormalities can also result in mental illnesses. Drug or alcohol abuse can result in certain mental illnesses. Some mental illnesses such as bipolar disorder and schizophrenia are highly hereditary and can manifest themselves as early as childhood. Common mental disorders affected by the brain include major depressive disorder, eating disorders, schizophrenia, and other psychotic disorders, anxiety disorders, bipolar disorder, and phobias. Head injuries can affect brain function. There are two types of head injuries. There can be external injuries affecting just the scalp and internal injuries that affect the skull and brain. Blood vessels in the meninges or dura surrounding the brain can bleed, resulting in subdural hematomas or epidural hematomas. There can be direct injury to the brain tissue. Any of these internal brain injuries can result in permanent cognitive, motor, or sensory problems with the brain. Individuals with chronic brain injury problems can have mental illnesses later in life. Key takeaways. The central nervous system consists of the brain and spinal cord. The afferent nerves take sensations and send them to the brain, while efferent nerves take messages to the body from the brain. There are three major parts of the brain, which are the midbrain, the forebrain, and the hindbrain. The right side of the brain controls motor function on the left side of the body, while the left side of the brain controls motor function on the right side of the body. Sensory organs pick up signals for touch, sight, smell, taste, and hearing. The cortex of the brain contains most of the nerve cells and controls functions of the brain. The cerebellum has many functions, including motor control and the maintenance of equilibrium and balance. Injuries and illnesses of the brain can be temporary or permanent. Brain illnesses can result in mental illnesses, cognitive deficits, and motor problems. Quiz. Number one. Which nerve is responsible for relaying sound to the brain? A. The cochlear nerve. B. The optic nerve. C. The vagus nerve. D. The olfactory nerve. Answer. A. The cochlear nerve connects the inner ear to the brain and controls the sensation of hearing and sound. Number two. Which part of the brain is considered the biggest? A. The brainstem. B. The cerebellum. C. The midbrain. D. The cortex. Answer. D. The cortex of the brain is considered the biggest part of the brain. Number three. What environmentally caused brain disorder results in motor problems and spasticity of the muscles with occasional cognitive deficits? A. meningitis. B. encephalitis. C. cerebral palsy. D. concussion. Answer. C. Cerebral palsy arises from injuries to the brain in utero during the birth process and in the first few years of a child's life. It causes motor deficits, spasticity, and cognitive deficits. Number four. What part of the brain involves the endocrine system? A. The thalamus. B. The hypothalamus. C. The amygdala. D. The cortex. Answer. B. The hypothalamus secretes hormones that control the pituitary gland. Both structures are part of the endocrine system. Number five. Which part of the brain controls balance, coordination, sensory input, and equilibrium? A. The brain stem. B. The forebrain. C. The cerebellum. D. The thalamus. Answer. C. The cerebellum is located in the back of the brain and controls balance, coordination, sensory input, and equilibrium. Number six. Which part of the brain processes memory? A. The amygdala. B. The hippocampus. C. The brain stem. D. The cerebellum. Answer. B. The hippocampus is responsible for processing short-term memory into long-term memory. Number seven. Which part of the brain controls breathing, heart rate, and other automatic functions of the body? A. The brain stem. B. The cerebellum. C. The amygdala. D. The hypothalamus. Answer. A. The brain stem controls breathing, heart rate, and other automatic functions of the body. Number eight. Which brain disorder happens with an extreme headache that lasts for a while and recurs in a pattern over several months or years? A. Cluster headaches. B. Migraine headaches. C. Epilepsy. D. Tension headaches. Answer. A. Cluster headaches cause extreme headaches that occur in a pattern over several months or years. Number nine. The child has episodes of blanking out with no motor dysfunction or complete loss of consciousness. What disorder does the child likely have? A. Tonic-clonic seizures. B. Myoclonus. C. Partial seizures. D. Absence seizures. Answer. D. The child most likely has absence seizures as evidenced by blanking out without complete loss of consciousness. Number ten. Which type of headache is considered the most common? A. Migraine headaches. B. Post-traumatic headaches. C. Tension headaches. D. Cluster headaches. Answer. C. Tension headaches are considered the most common type of headache.