Chapter 13- Nervous System
13.1- Overview of the Nervous System
13.2- The Central Nervous System
13.3- The Limbic System and Higher Mental Functions
13.4- The Peripheral Nervous System
13.5- Drug Abuse
13.1 Overview of the Nervous System
The nervous system has 2 major divisions that are connected and work together they are the central nervous system (CNS) and the peripheral nervous system (PNS). PNS consists of nerves while CNS consists of the brain and spinal cord. The three specific functions of the nervous system are 1- the nervous system gets sensory input, 2- CNS adds up all the input it gets from the body and 3- generates motor output, nerve impulses go from CNS by way of the PNS to the muscles and glands.
Neurons are cells that transmit nerve impulses and neuroglia are what support and nourish neurons. Both are cells that make up nervous tissue. Sensory neurons, interneurons an motor neurons are the three types of neurons. The sensory neuron takes messages from a sensory receptor to the CNS. Special structures that detect changes in the environment are sensory receptors. Interneurons lie entirely within the CNS. They get input from both sensory neurons and other interneurons. They add up all the received info before communicating with motor neurons. Motor neurons take nerve impulses from the CNS to an effector or muscle fiber or gland. Effectors are what carry out responses to environment changes. Neurons are made up of three parts- cell boy, dendrites and an axon. Nucleus and other organelles make up the cell body. Short extensions that get signals from sensory receptors and neurons make up dendrites. The portion of the neuron which conducts nerve impulses is known as the axon.
Types of neurons are shown below:
Myelin sheath is what protects axons. For the PNS it is made b neuroglia known as schwann cells- contact myelin in plasma membranes. Oligodendrocytes in the CNS do this. When cells wrap around axons the myelin sheath is made. Nodes of ranvier are the gaps where no myelin sheath is. Short axons do not have myelin sheath. White matter in CNS has myelinated axons, gray area does not. Loss of myelin sheath can result in MS or leukodystrophies.Resting potential occurs when the axon is not conducting an impulse. -65 mV inside the neuron is more negative than outside.
Sodium-potassium pump transports sodium ions (Na+) out of potassium ions (K+) into the axon. This is what maintains unequal distribution of both across the membrane. Always more positive ions outside the membrane. Rapid change in polarity is action potential- this happens if the axonal membrane depolarizes to threshold. Gates will open letting K+ and Na+ pass through the membrane. Sodium gates open first- Na+ goes to the axon causing potential changes -65mV to +40mV which is depolarization- the inside of the neuron goes negative to positive. Potassium gates open next and K+ goes to the axon and repolarization occurs +40 mV back to -65mV. Nerve impulses going from nerve to nerve is called salutatory conduction. The refractory period keeps the sodium gates closed keeping action potential from going backwards.
Axon terminals are small swellings at the end of axon branches. The synapse is the region of the close proximity, small gaps that separate sending neurons from receiving neurons are the synaptic cleft. Neurotransmitters are what helps get nerve impulses over the cleft. Once neurotransmitters initiate responses they leave the cleft. Acetylcholinesterase is an enzyme that inactivates neurotransmitters.
Synapse structure & function shown below:
Serotonin is a neurotransmitter that is involved with sleeping, emotions and perception, norepinephrine is too but it is involved with waking, dreaming and mood. Lower levels of these are linked with depression. Neuromodulators are molecules that block the release of neurotransmitters. Substance P is released when in pain, endorphins block the release of this. Both are neuromodulators.
13.2- The Central Nervous System
The spinal cord and the brain which make up the CNS are protected by bone; the vertebrae protects the spine the skull protects the brain. Protective membranes that wrap and protect both of these are known as meninges. Cerebrospinal fluid fills the spaces between the meninges and it cushions and protects the central nervous system. Interconnecting chambers that produce and serve as a reservoir for the fluid are known as ventricles. The brain has four of these. Water on the brain is a condition called hydrocephalus. Gray matter is one type of nervous tissue that contains cell bodies and short nonmyelineated fibers. The other type is known as white matter which contains mylineated axons that run together in bundled called tracts.
The meninges and central canal contain cerebrospinal fluid that protects the spinal cord. Spinal nerves are part of the peripheral nervous system. The right side of the brain controls the left side of the body, left side of the brain controls the right side of the body. Communication between the brain and peripheral nerves leaving the cord is provided with the spinal cord. Endorphins can stop pain otherwise the pain messages will pass through the gates to the brain. Paralysis occurs if the spinal cord is severed, meaning no voluntary muscle control. If the cord is severed in the thoracic area then the lower body is affected- the legs are paralyzed, which makes the person a paraplegic. Quadriplegic is when the injury happens in the neck area making all four limps paralyzed. Thousands of reflexes happen in the spinal cord.
The largest part of the brain is the cerebrum, which is the last part to receive sensory input and carry out integration before commanding voluntary motor responses. The cerebrum consists of two halves, the right cerebral hemisphere and the left hemisphere. It is split in half by a deep groove known as the longitudinal fissure. The hemispheres are divided into lobes by shallow grooves known as sulci.
The lobes of the hemisphere are shown below:
The cerebral cortex is known as an outer layer of gray matter that covers the cerebral hemispheres. Over one billion cell bodies are in the cortex. This part of the brain is associated with sensation, voluntary movement and the thought processes. Motor and sensory areas are in the cerebral cortex. The frontal lobe has the primary motor area. Sensory info from the skin and skeletal muscle goes to the primary somatosensory area. Integration occurs in association areas. The premotor area organizes motor functions like walking and talking at the same time. The processing centers of the cortex receive info from association areas and perform higher level functions. Prefrontal area gets info and plans our actions, the ability to think critically and formulate appropriate behaviors is because of integration in this area. Wernicke’s area and Broca’s area are partly responsible for humans speaking, both areas are found in the left cerebral cortex. Wernicke’s helps us to understand written and spoken words sending the info to Broca’s. White matter makes up most of the rest of the cerebrum. A region that encircles the third ventricle is known as the diencephalon. This is where the hypothalamus and thalamus are located. The hypothalamus helps in maintaining homeostasis and also is a link between the nervous system and the endocrine system. The thalamus receives all sensory input except smell. The thalamus integrates the info and sends it to the right part of the cerebrum. Also the thalamus helps in higher mental functions like memory and emotions. The fourth ventricle is what separates the cerebellum from the brain stem. Sensory input from the eyes, ears, joints and muscles is received from the cerebellum once this information is integrated the cerebellum sends motor impulses to the skeletal muscles. The relay station for the tracts passing between the cerebrum and spinal cord is the midbrain. Pons contains bundles of axons traveling between the cerebellum and the central nervous system. Reflex centers that regulate heart beat and breathing are in the medulla oblongata. The reticular formation is the network of nuclei and fibers that extend the length of the brain stem. The reticular activating system gets sensory signals and sends them up to higher centers.13.3- The Limbic System and Higher Mental Functions
The limbic system is responsible for why sexual behavior and eating seem pleasurable and why mental stress can cause high blood pressure. The amygdala which can cause experiences to have emotional overtones is part of the limbic system. Also the hippocampus is another part to the limbic system, which is crucial to learning and memory. The prefrontal area of our brains is responsible for short term memory, once something is memorized it is put into long term memory. Semantic memory is a mixture, like numbers, words, etc. Episodic memory is related to people or events. Skill memory is used in performing motor activities. Memories are stored in the hippocampus and are utilized in the prefrontal area. Language depends on semantic memory.
13.4- The Peripheral Nervous System
Nerves are designated as cranial nerves when they arise from the brain and spinal nerves when they arise from the spinal cord. Nerves are made up of axons. Twelve pairs of cranial nerves attached to the brain, some have only motor fibers, some only sensory fibers and others have both. The cell body of a sensory neuron is in a dorsal root ganglion. A collection of cell bodies outside the CNS is known as a ganglion. Spinal nerves consist of sensory and motor fibers and therefore are mixed nerves. The somatic system nerves serve the skin, skeletal muscles and tendons. Reflexes occur when automatic responses to a stimulus in the somatic system. Pain is not felt until the brain receives and interprets the information.
A reflex arc is shown below:
The autonomic system is what regulates the activity of cardiac and smooth muscles and glands. This system is in the PNS. Sympathetic and parasympathetic divisions make up the autonomic system. Both function automatically and involuntary, they innervate all internal organs, and they utilize two neurons and one ganglion for each impulse. Preganglionic fibers of the sympathetic division come from the middle of the spinal cord and terminate in ganglia that lie near the cord. This division is important in emergency situations. The parasympathetic division consists of cranial nerves and fibers that come from the bottom part of the spinal cord. All the internal responses we associate with a relaxed state is in this division, also known as the housekeeper division. Both the somatic system and autonomic system are in the peripheral nervous system.
13.5- Drug Abuse
Drugs which consist of natural molecules or synthetically derived affect the nervous system, they alter the mood and our emotional state. They increase or decrease the action of a particular neurotransmitter and affect the limbic system. Dopamine has a role in mood. Psychological dependence on a drug is obvious when a person craves a drug, spends time looking for the drug and uses it regularly. The person is shows physical dependence or is addicted when they need more and more to reach the same high and they show withdrawal symptoms when they stop taking the drug. The most socially excepted form of drug use is alcoholism. A long time use of alcohol can harm the liver and make it function less. If alcohol blood level gets to high coma or death can result. Alcohol acts as a depressant in the CNS. Brain size can decrease with chronic use of alcohol. Nicotine is a stimulant, when it reaches the CNS it binds to the neurons and dopamine is released. In the PNS skeletal muscle activity increases. Cocaine use leads to a rush of well being that can last five to thirty minutes. The use of this drug brings on lack of appetite, sleeplessness and an increased sex drive. Crack is the street name for cocaine and is the form of smoking it. Dopamine is made less to make up for the excess of synapses with continued use. Methamphetamine is known as speed, which is powder form or crystal meth or ice which is in crystal form. Heroin acts as a depressant in the nervous system. Heroin is delivered to the brain and converted to morphine. Heroin can be smoked, injected or snorted. When marijuana reaches the CNS mild euphoria can occur. Regular use of marijuana can lead o cravings. This can have an effect on memory, balance, motor coordination and orientation.
Chapter 14: Senses
14.1- Sensory Receptors and Sensations
14.2- Proprioceptors and Cutaneous Receptors
14.3- Senses of Taste and Smell
14.4- Sense of Vision
14.5- Sense of Hearing
14.6- Sense of Equilibrium
14.1- Sensory Receptors and Sensations
The dendrites specialized to detect certain types of stimuli are known as sensory receptors. Sensory receptors that detect stimuli from outside the body are known as exterocceptors. Interoceptors get stimuli from inside the body and are involved in homeostasis. Four types of sensory receptors are chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors. A response to chemical substances is chemoreceptors. Pain receptors are a type of chemoreceptors, they alert us to possible danger. Response to light energy is photoreceptors which are in our eyes. Mechanical forces stimulate our mechanoreceptors (located in our ear) usually with some kind of pressure. They also help to keep our balance. Thermoreceptors are located in the hypothalamus and the skin and respond in temperature changes. Warmth receptors respond when temperatures rise and cold receptors respond in lower temperatures.
Sensations occur when nerve impulses get to the cerebral cortex. All sensory receptors initiate nerve impulses but the sensation depends on what part of the brain receives the impulse. Integration is carried out before the sensory receptors initiate the nerve impulse. A decrease in response to a stimulus is known as sensory adaptation. Sensory receptors functioning helps maintain homeostasis.
14.2- Proprioceptors and Cutaneous Receptors
Three types of general sensory receptors are known as proprioceptors, cutaneous receptors and pain receptors. Maintaining muscle tone and the body’s equilibrium and posture is due to proprioceptors which are mechanoreceptors involved in reflex. Nerve impulses occur when the muscle relaxes and undue stretching of the muscle spindle occurs. The dermis, which is a layer of the skin, contains cutaneous receptors. This makes the skin sensitive to touch, pain, pressure and temperature. Pain receptors are in the skin and many internal organs. Referred pain is when stimulation of internal pain receptors is felt as pain from the skin as well as internal organs.
14.3- Senses of Taste and Smell
Chemical senses are taste and smell. Chemoreceptors are present in taste cells. Four primary types of taste are sweet, salty, sour and bitter. Taste cells end in microvilli, this helps distinguish the taste. Adults have about 3,000 taste buds. 80-90% of what we taste is due to smell. High in the roof of the nasal cavity is where 10 and 20 million olfactory cells are located, our sense of smell depends on these cells. Olfactory cells are connected with the limbic system, certain smells can take us back to a person or place.
14.4- Sense of Vision
There are three layers to the eyeball which is 2.5 cm in diameter. The layers are the sclera which is the outer layer made of white and fibrous except the cornea, the choroid which is the middle layer that absorbs light rays that have not yet been absorbed by photoreceptors and the retina which is what helps us to see black and white, color and acute vision. The iris is what the choroid becomes toward the front of the eyeball and is what regulates the pupil. The pupil is what allows light in. The shape of the lens for near and far vision is controlled by the ciliary body. The posterior compartment of the eye (behind the lens) is separated from the anterior compartment (in front of the lens) by the lens. Aqueous humor is a clear watery fluid that fills the anterior compartment, this fluid is made daily. Tiny ducts are how this fluid is released, if they become blocked then glaucoma can occur, this can lead to blindness. A clear gelatin material known as vitreous humor is located in the posterior compartment. The fovea centralis is located in the retina and is where cone cells are packed. The optic nerve is what takes nerve impulses to the visual cortex. The lens, humors and cornea help focus things on the retina. Visual accommodation involves the lens rounding up bringing the image to focus on the retina during close vision. Rod cells and cone cells are photoreceptors. Rhodopsin is a molecule made of protein opsin and retinal, a light absorbing molecule. Rods help with peripheral vision and perception of motion. Carrots, being high in vitamin A help our night vision. The three kinds of cones that help us with color vision are the B (blue), G (green) and R (red) pigments. The retina is made up of three layers of neurons. Layer closest to the choroid has rod cells and cone cells, middle layer is bipolar cells and the inner layer has ganglion cells.
Structure & function of the retina is shown below:
A blind spot occurs for the left eye left of center and for the right eye right of center. Blind spots occur because there are no rods or cones where the optic nerve exits making no vision possible. The blind spot only happens for one eye, not when using both eyes. Optic nerves carry nerve impulses from the eyes to the optic chiasma, which is X shapped, crossing over optic nerves.
Some abnormalities of the eye include color blindness and misshapen eyeballs, 5-8% of males are affected with color blindness. Also nearsighted which means you can see better close up than at a distance, and farsighted which means you can see better at a distance than close up. When light rays cannot be evenly focused on the retina it is known as astigmatism.
14.5- Sense of Hearing
Hearing and balance are the two sensory functions of the ear. Hair cells located in the inner ear are the sensory receptors, mechanoreceptors. Three divisions of the ear include the outer ear, pinna the external flap and auditory canal, the middle ear at the tympanic membrane or the eardrum and the inner ear which unlike the first two divisions contains fluid instead of air.
Anatomy of the human ear is shown below:
The semicircular canals and vestibule of the inner ear work for equilibrium and the cochlea works for hearing. The outer ear gets the sound, middle ear magnifies the sound and strikes the oval window causing a vibration making pressure pass to the fluid in the cochlea.
14.6- Sense of Equilibrium
Nerve impulses are taken to the brain stem and the cerebellum by way of the vestibular nerve. The vestibular nerve helps maintain equilibrium. Rotational equilibrium is detected by the mechanoreceptors in the semicircular canals when movement of the head occurs. Gravitational equilibrium is when the mechanoreceptors in the utricle and saccule detect movement of the head.
Mechanoreceptors for equilibrium are shown below:
The autonomic system is what regulates the activity of cardiac and smooth muscles and glands. This system is in the PNS. Sympathetic and parasympathetic divisions make up the autonomic system. Both function automatically and involuntary, they innervate all internal organs, and they utilize two neurons and one ganglion for each impulse. Preganglionic fibers of the sympathetic division come from the middle of the spinal cord and terminate in ganglia that lie near the cord. This division is important in emergency situations. The parasympathetic division consists of cranial nerves and fibers that come from the bottom part of the spinal cord. All the internal responses we associate with a relaxed state is in this division, also known as the housekeeper division. Both the somatic system and autonomic system are in the peripheral nervous system.
13.5- Drug Abuse
Drugs which consist of natural molecules or synthetically derived affect the nervous system, they alter the mood and our emotional state. They increase or decrease the action of a particular neurotransmitter and affect the limbic system. Dopamine has a role in mood. Psychological dependence on a drug is obvious when a person craves a drug, spends time looking for the drug and uses it regularly. The person is shows physical dependence or is addicted when they need more and more to reach the same high and they show withdrawal symptoms when they stop taking the drug. The most socially excepted form of drug use is alcoholism. A long time use of alcohol can harm the liver and make it function less. If alcohol blood level gets to high coma or death can result. Alcohol acts as a depressant in the CNS. Brain size can decrease with chronic use of alcohol. Nicotine is a stimulant, when it reaches the CNS it binds to the neurons and dopamine is released. In the PNS skeletal muscle activity increases. Cocaine use leads to a rush of well being that can last five to thirty minutes. The use of this drug brings on lack of appetite, sleeplessness and an increased sex drive. Crack is the street name for cocaine and is the form of smoking it. Dopamine is made less to make up for the excess of synapses with continued use. Methamphetamine is known as speed, which is powder form or crystal meth or ice which is in crystal form. Heroin acts as a depressant in the nervous system. Heroin is delivered to the brain and converted to morphine. Heroin can be smoked, injected or snorted. When marijuana reaches the CNS mild euphoria can occur. Regular use of marijuana can lead o cravings. This can have an effect on memory, balance, motor coordination and orientation.
Chapter 14: Senses
14.1- Sensory Receptors and Sensations
14.2- Proprioceptors and Cutaneous Receptors
14.3- Senses of Taste and Smell
14.4- Sense of Vision
14.5- Sense of Hearing
14.6- Sense of Equilibrium
14.1- Sensory Receptors and Sensations
The dendrites specialized to detect certain types of stimuli are known as sensory receptors. Sensory receptors that detect stimuli from outside the body are known as exterocceptors. Interoceptors get stimuli from inside the body and are involved in homeostasis. Four types of sensory receptors are chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors. A response to chemical substances is chemoreceptors. Pain receptors are a type of chemoreceptors, they alert us to possible danger. Response to light energy is photoreceptors which are in our eyes. Mechanical forces stimulate our mechanoreceptors (located in our ear) usually with some kind of pressure. They also help to keep our balance. Thermoreceptors are located in the hypothalamus and the skin and respond in temperature changes. Warmth receptors respond when temperatures rise and cold receptors respond in lower temperatures.
Sensations occur when nerve impulses get to the cerebral cortex. All sensory receptors initiate nerve impulses but the sensation depends on what part of the brain receives the impulse. Integration is carried out before the sensory receptors initiate the nerve impulse. A decrease in response to a stimulus is known as sensory adaptation. Sensory receptors functioning helps maintain homeostasis.
14.2- Proprioceptors and Cutaneous Receptors
Three types of general sensory receptors are known as proprioceptors, cutaneous receptors and pain receptors. Maintaining muscle tone and the body’s equilibrium and posture is due to proprioceptors which are mechanoreceptors involved in reflex. Nerve impulses occur when the muscle relaxes and undue stretching of the muscle spindle occurs. The dermis, which is a layer of the skin, contains cutaneous receptors. This makes the skin sensitive to touch, pain, pressure and temperature. Pain receptors are in the skin and many internal organs. Referred pain is when stimulation of internal pain receptors is felt as pain from the skin as well as internal organs.
14.3- Senses of Taste and Smell
Chemical senses are taste and smell. Chemoreceptors are present in taste cells. Four primary types of taste are sweet, salty, sour and bitter. Taste cells end in microvilli, this helps distinguish the taste. Adults have about 3,000 taste buds. 80-90% of what we taste is due to smell. High in the roof of the nasal cavity is where 10 and 20 million olfactory cells are located, our sense of smell depends on these cells. Olfactory cells are connected with the limbic system, certain smells can take us back to a person or place.
14.4- Sense of Vision
There are three layers to the eyeball which is 2.5 cm in diameter. The layers are the sclera which is the outer layer made of white and fibrous except the cornea, the choroid which is the middle layer that absorbs light rays that have not yet been absorbed by photoreceptors and the retina which is what helps us to see black and white, color and acute vision. The iris is what the choroid becomes toward the front of the eyeball and is what regulates the pupil. The pupil is what allows light in. The shape of the lens for near and far vision is controlled by the ciliary body. The posterior compartment of the eye (behind the lens) is separated from the anterior compartment (in front of the lens) by the lens. Aqueous humor is a clear watery fluid that fills the anterior compartment, this fluid is made daily. Tiny ducts are how this fluid is released, if they become blocked then glaucoma can occur, this can lead to blindness. A clear gelatin material known as vitreous humor is located in the posterior compartment. The fovea centralis is located in the retina and is where cone cells are packed. The optic nerve is what takes nerve impulses to the visual cortex. The lens, humors and cornea help focus things on the retina. Visual accommodation involves the lens rounding up bringing the image to focus on the retina during close vision. Rod cells and cone cells are photoreceptors. Rhodopsin is a molecule made of protein opsin and retinal, a light absorbing molecule. Rods help with peripheral vision and perception of motion. Carrots, being high in vitamin A help our night vision. The three kinds of cones that help us with color vision are the B (blue), G (green) and R (red) pigments. The retina is made up of three layers of neurons. Layer closest to the choroid has rod cells and cone cells, middle layer is bipolar cells and the inner layer has ganglion cells.
Structure & function of the retina is shown below:
A blind spot occurs for the left eye left of center and for the right eye right of center. Blind spots occur because there are no rods or cones where the optic nerve exits making no vision possible. The blind spot only happens for one eye, not when using both eyes. Optic nerves carry nerve impulses from the eyes to the optic chiasma, which is X shapped, crossing over optic nerves.Some abnormalities of the eye include color blindness and misshapen eyeballs, 5-8% of males are affected with color blindness. Also nearsighted which means you can see better close up than at a distance, and farsighted which means you can see better at a distance than close up. When light rays cannot be evenly focused on the retina it is known as astigmatism.
14.5- Sense of Hearing
Hearing and balance are the two sensory functions of the ear. Hair cells located in the inner ear are the sensory receptors, mechanoreceptors. Three divisions of the ear include the outer ear, pinna the external flap and auditory canal, the middle ear at the tympanic membrane or the eardrum and the inner ear which unlike the first two divisions contains fluid instead of air.
Anatomy of the human ear is shown below:
The semicircular canals and vestibule of the inner ear work for equilibrium and the cochlea works for hearing. The outer ear gets the sound, middle ear magnifies the sound and strikes the oval window causing a vibration making pressure pass to the fluid in the cochlea.14.6- Sense of Equilibrium
Nerve impulses are taken to the brain stem and the cerebellum by way of the vestibular nerve. The vestibular nerve helps maintain equilibrium. Rotational equilibrium is detected by the mechanoreceptors in the semicircular canals when movement of the head occurs. Gravitational equilibrium is when the mechanoreceptors in the utricle and saccule detect movement of the head.
Mechanoreceptors for equilibrium are shown below:
Work Cited:
Mader, Sylvia S. Human Biology 1oth ed. McGraw Hill Companies, 2008.
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