I have divided the first major topic for unit III into four subtopics. Each subtopic is handled separately. This review is organized in the following format: subtopic outline, discussion of the subtopic, and finally references for that subtopic.
Topics covered in this review:
I. The Nervous System: Structure and Function
II. Nerve Impulses: Communication in the Nervous System
III. The Senses: Structures and Function
IV. Drug Abuse: Impairment of the Nervous System
Discussion 1.1 - Organs and Division of the Nervous System
Two Divisions of the Nervous System
The nervous system consists of the brain, the spinal cord, and the nerves. The brain and the spinal cord make up the CNS (Central Nervous System) while the nerves make up the PNS (Peripheral Nervous System).
The CNS and PNS Work Together
The CNS is responsible for receiving sensory information brought in by the nerves of the PNS, integrating the information, evaluating the information, and initiating a motor response if one is needed. The nerves of the PNS then relay the instructions for the motor response to the effector muscle (or in some cases gland). This communication between the brain and the body is facilitated by the spinal cord. At times, the spinal cord handles the initiation of response alone, as in the case of reflex response.
Discussion 1.2 - The Central Nervous System and the Brain
The Brain
The major organ of the nervous system is the brain. The brain is located in the cranial cavity and is surrounded first by cerebrospinal fluid, then a protective tissue layer called the meninges, and finally by the bones of the skull. Rather than being a continuous ball of tissue as one might suspect, the brain tissue is actually separated by four ventricles (or chambers) that also contain cerebrospinal fluid. The image below shows the ventricles of the brain.
The Cerebrum
The cerebrum is the major structure of the brain which has specialized regions that control and integrate motor function, sensory function, and thought.
White and Gray Matter
The brain is made up of two different kinds of tissue: gray matter and white matter. The difference of color of the two types of brain tissue is due to the presence of myelin surrounding the long axons of white matter. Since, the axons of the cells of gray matter are short, they do not contain myelin and thus have a gray appearance.
Hemispheres of the Brain
(Image right) The brain is divided into two halves or hemispheres. The two hemispheres are separated by a the longitudinal fissure that runs the length of the brain. Despite this division the two hemispheres of the brain are connected by the corpus callosum.
Lobes of the Brain
The brain is further divided into lopes by the sulci which are more shallow fissures than the longitudinal fissure. The lobes of the brain (frontal lobe, parietal lobe, occipital lobe, and temporal lobe) are illustrated in the image to the lower left. Each lobe of the brain is specialized for particular functions. This is illustrated in the image to the lower right.
http://hopes.stanford.edu/basics/braintut/ab4.html
The Cerebral Cortex
The cerebrum is covered in a thin layer of gray matter called the cerebral cortex. The cerebral cortex is responsible for the wrinkled appearance of the brain. The cerebral cortex is responsible for a number of functions including sensation, voluntary movement, and conscious thought.
Sensory and Motor Areas of the Cerebral Cortex
The primary motor and primary somatosensory areas of the cortex are highlighted in the image below. The image below also shows how each part of the body is controlled by a separate portion of the primary motor and primary somatosensory areas. There are also specialized areas of the cortex associated with vision, smell, and hearing.
Association Areas of the Cerebral Cortex
Surrounding the motor, somatosensory, and sensory areas of the cortex are association areas responsible integrating information. The motor association area (premotor area) is responsible for coordinating motor activity for learned motor activities. The association areas of the somatosensory area is responsible for processing the sensory information that is being brought in to the cortex from the skin and muscles. The visual and auditory association areas takes new visual and auditory information and compares it with stored information. In the image to the right, the primary areas are in the darker color and the association areas are in the lighter color.
http://users.fmrib.ox.ac.uk/~stuart/thesis/chapter_3/image3_14.gif
Processing Areas of the Cerebral Cortex
The processing centers of the brain take the information received from the association areas, analyze the information, and use it to determine our actions. This is where critical thinking occurs. The processing areas of the cerebral cortex are largely responsible for characteristic human activities such as higher level thinking and speech.
Wernicke's Area and Broca's Areas
There are two processing areas found in the left cerebral cortex that are associated with the ability to speak. Wernicke's area is associated with understanding the written and spoken word and Broca's area controls the motor responses necessary for speech such as movement of the tongue and lips and hand movements needed to write. Broca's area is also responsible for grammatical functions as well.
http://www.d.umn.edu/cla/faculty/troufs/anth1602/video/Story_Hominid.html
Central White Matter
Below the cerebral cortex is white matter which composes the rest of the cerebrum. White matter actually develops as an individual grows. There are "tracts" that connect the white matter with the cerebral cortex and other brain structures. These tracts cross over from one side of the brain to the other. This is why it is said that the left side of the brain controls the right side of the body and vice versa.
The Diencephalon 
Hypothalamus
The diencephalon is an area that surrounds the third ventricle and contains the hypothalamus and thalamus as well as part of the pituitary gland and the pineal gland. The hypothalamus is responsible for maintaining homestasis in the body. It regulates basic body functions like sleep, hunger, thirst, body temperature, and water balance. The hypothalamus also controls the pituitary gland which is part of the endocrine system.
Thalamus
The thalamus receives sensory and somatosensory information (except olfactory information) coming into the brain via the cranial nerves and tracts of nerves coming up from the spinal cord. The thalamus integrates the information and then sends it to the cerebrum. The thalamus is also involved in memory and emotions as well as being responsible for stimulating the cerebrum.
Pineal Gland
The pineal gland is also located in the diencephalon and is responsible for secreting the hormone melatonin. It is believed that melatonin is involved in the body's circadian rhythm and possibly controls the onset of puberty.
The Cerebellum
The cerebellum is the portion of the brain that is located just dorsal to the brain stem and is separated from it by the fourth ventricle. The cerebellum is made of two tree like structures composed of white matter that is covered in a highly folded sheet of gray matter. The two portions of the cerebellum are connected by a thin portion of tissue in the middle. The cerebellum takes sensory input from the cerebral cortex to help determine the body's position in space and also receives motor instructions from the cerebral cortex in order to maintain balance, posture, and coordinated voluntary movement of the body. The cerebellum does this by processing the propriopositional information and motor commands from the cortex and then relays motor impulses through the brain stem to the skeletal muscles.
The Brain Stem
Midbrain
The brain stem is composed of the midbrain, the pons, and the medulla oblongata. (See image right) The midbrain is the structure responsible for relaying information between the cerebrum, spinal cord, and cerebellum. The midbrain is also controls the reflex responses related to vision, hearing, and touch.
Pons
The pons houses the axons that pass from the cerebellum to the rest of the central nervous system. The pons also helps to control respiration rate. It also controls reflex responses related to head movement due to visual or auditory stimuli.
Medulla Oblongata
The medulla oblongata also helps control breathing as well as the heartbeat, and blood pressure. The medulla oblongata also controls reflexes for vomiting, sneezing, hiccupping, coughing, and swallowing.
The Reticular Formation
The reticular formation is a network of gray matter that receives sensory information from higher portions of the brain and relays them to the spinal cord. The reticular formation is part of the reticular activating system that is responsible for stimulating the thalamus and causes an individual to be alert. The RAS also is involved in "tuning out" unnecessary stimulation from the environment.
The Limbic System
Amygdala and Hippocampus
The limbic system is a functional rather than structural system. It coordinates the efforts of the other brain structures in regards to emotion and memory. The two primary structures of the limbic system are the hippocampus and the amygdala. The amygdala is the seat of emotion and is especially active in the fight or flight response. It references past experience to determine when the fear response is appropriate. The hippocampus is a bridge between memory and the decision making portions of the forebrain. This allows us to call upon past experiences to make appropriate decisions. The hippocampus also is responsible for deciding what new information needs to be stored in memory and how it should be stored.
Memory
There are a number of different types of memory. We rely on memory to make decisions and to repeat learned tasks. The different types of memory are discussed below.
Short-term Memory
Short term memory uses the prefrontal area that is just behind the forehead. When we memorize a piece of information for a short period of time we are using our short-term memory.
Skill Memory
Skill memory is used when we repeat a learned activity such as riding a bike or typing. We rely on our memory to perform the actions necessary without having to re-learn the activity every time. When we are first learning to perform a learned skill there are more areas of the cerebral cortex active than when repeating a skill that has been previously learned. In addition, performing learned skills involves the motor portions of the cerebrum that are beyond consciousness. We are literally able to perform the task without thinking about what we are doing.
Long-term Memory
Long-term memory uses semantic memory (memory of numbers, words . . .) and episodic memory (memory of events, people . . .). When you bring to mind a persons birthdate or events from the past you are utilizing long-term memory. Researchers believe that long-term memory is stored in pieces across the association areas of the cerebral cortex. The hippocampus coordinates these areas of stored memory with the prefrontal area that makes use of memories. Interestingly, researchers have found that there are more neurotransmitters released into the synapses (spaces between neurons) when the synapses have been heavily used. It is believed that this is somehow involved in memory storage. This tendency is called long-term potentiation.
Language and Speech
Memory is very important to the ability to speak and use language. It is necessary to store words and phrases for future use as well as grammar rules and sounds. The ability to see and hear words comes from sensory areas of the occipital and temporal lobes. The ability to understand speech and the written word is facilitated by Wernicke's area and Broca's area allows one to speak and write.
Discussion 1.3 - The Central Nervous System Part II: The Spinal Cord
Structure of the Spinal Cord
The Spinal cord is also part of the central nervous system. The spinal cord is located along the back of the body and is protected by the vertebrae of the vertebral column. Like the brain the spinal cord is surrounded by meninges and cerebrospinal fluid. Also like the brain, the spinal cord is composed of gray matter and white matter. The image below illustrates the structure of the spinal column which contains the spinal cord, the vertebra, and the spinal nerves that come off of the spinal cord. The center of the spinal cord contains the central canal that holds cerebrospinal fluid. Around the central canal is gray matter in the shape of the letter H. This is the location of many sensory and motor neurons as well as interneurons. The dorsal root of the spinal nerves house the sensory fibers coming into the gray matter and the ventral root of the spinal nerves house the motor fibers that are exiting the gray matter. Surrounding the gray matter (on the dorsal side) is white matter that holds tracts of nerves that carry information to the brain. The ventral side of the white matter contains tracts that carry information coming from the brain. Typically, the tracts switch sides just after exiting the brain, thus the right side of the brain controls the left side of the body and vice versa.
Function of the Spinal Cord
The Spinal cord functions as the communications center between the brain and the body. Voluntary motor impulses from the brain flow down the tracts to the spinal cord and then pass through the spinal nerves to the muscles. Spinal cord injuries often cause paralysis of the body corresponding to the point at which the spinal cord is damaged.
Reflex Actions and the Spinal Cord
The other function of the spinal cord is to initiate a number of our reflex responses. Theses are known as reflex arcs. When the body experiences a stimulus the sensory receptors send a signal to the spinal cord where interneurons process the information and relay instructions to motor neurons for the response. In the case of reflex arcs, the brain is not directly involved in the processing of sensory input or in the initiation of motor response.
The spinal cord also takes part in a number of control mechanisms for the internal organs. An example is low blood pressure. When the blood pressure drops the carotid arteries and the aorta signal the spinal cord and the impulses move up the cord to the cardiac control region of he brain where instructions are initiated that flow back down the cord and cause the blood vessels to constrict which raises blood pressure.
References
Image References
Images obtained from Aris site for Human Biology by Sylvia Mader (http://highered.mcgraw-hill.com/classware/selfstudy.do?isbn=0072986867 chapter resources - power point presentation), unless otherwise cited under image.
Discussion 2.1 Structure of Nervous Tissue
Function of Nervous Tissue
Nervous tissue is made up of neurons and neuroglia. The neurons are the functional cells of the nervous system, while the neuroglia are support cells that nourish the neurons. Neurons are responsible for relaying messages between the cells of the body. For example, neurons deliver motor impulses generated in the brain to the muscles of the body causing them to contract.
The nerves (composed of nervous tissue) make up the peripheral nervous system. As mentioned above, the primary responsibility of the PNS is to relay messages between the CNS and the body.
Structure of Neurons
Neurons are composed of three parts. There is the cell body that houses the nucleus and other organelles of the cell. Extending from the cell body on one side are a number of short extension called dendrites. Dendrites receive information either from sensory receptors or from other neurons. Extending from the other side of the cell body is a longer extension that conducts nerve impulses called the axon. The axon is what is referred to as the nerve fiber when it is found in nerves. Axons can be very long, extending almost the length of the entire body.
Types of Neurons
There are three different types of neurons that are specialized for three different functions in the body.
Sensory Neurons
Sensory neurons are responsible for taking instructions from sensory receptors in the body to the central nervous system. Sensory receptors are specialized cells that are located throughout the body that detect changes in the internal or external environment. Sensory receptors will be discussed in a later section of this review. Sensory neurons typically have long axons that begin at a sensory neuron and the cell body along the middle of the axon.
Interneurons
Interneurons are found in the central nervous system (brain and spinal cord) and serve as a processing center for information coming in from sensory neurons and from other interneurons and then relay the information to motor neurons. Interneurons have a number of dendrites that branch off of the cell body and then the axon extends away from the cell body in the other direction. Some interneurons have short unmyelinated axons.
Motor Neurons
Motor neurons are responsible for taking information from the CNS to the intended muscle or gland (effector). Motor neurons are typically structured like interneurons with dendrites, cell body, then axon. The axons of motor neurons are typically long and are covered by a myelin sheath. The image below shows the three different types of neurons.
Myelin Sheath
The image above also illustrates schwann cells. Schwann cells are specialized cells that contain a specialized lipid called myelin in the cell membrane. The schwann cells are wrapped around long axons and serves to protect and insulate them. As the image above demonstrates, there are points along long axons where there is no myelin sheath. These are known as the nodes of Ranvier.
Discussion 2.2 Nerve Impulses and Action Potential
Researchers are able to study the way in which neurons conduct nerve impulses with the use of a voltmeter. A voltmeter measures the potential difference in voltage between one side of the axon cell membrane and the other. An electrode is placed on the outside of the axon membrane and another electrode is placed on the inside of the axon cell membrane.
Resting Potential
When an axon is not conducting a nerve impulse (is at rest) the voltmeter read -65 millivolts. This means that the inside of the axon is more negative than the outside of the neuron. This is created by the action of the sodium potassium pump discussed in Unit I. Because the axon membrane is more permeable to potassium than to sodium there will be more positive ions gathered outside the cell membrane. In addition, there are negatively charged ions in the cytoplasm of the axon that also add to the negative charge within the axon. This phenomena is known at resting potential.
Action Potential
When a impulse is created either by a sensory neuron or the CNS the axons of the neurons must somehow relay the message along its long fibers (and sometimes along many neurons) to the intended target. This is accomplished by an even known as action potential. As the nerve impulse moves along the axon, there is a change in the polarity of the axon membrane. The polarity changes because of two events. First, sodium gates open and Na+ moves through the membrane into the axon. This causes the polarity of the membrane to change and the voltage goes from -60 mV to +40 mV (depolarization). Next, the potassium gates open and K+ flows out of the cell membrane. As the happens the axon is replolarized and the voltage goes from +40 mV back to -65 mV. The images below demonstrate action potential.
In order to move the impulse down the axon, this depolarization/ repolarization event or action potential moves down the axon. Once the action potential passes a point along the axon the sodium gates of that portion of the axon are not able to open for a brief time. This is called the refractory period and ensures that action potential keeps moving in the right direction and does not go backward.
In an unmyelinated axon the action potential moves down the short axon from one part of the membrane to the next. In long myelinated axons, the action potential moves from one node of Ranvier to the next. This is because the myelin sheath prevents conduction of the action potential in the region of the action it covers. This type of conduction is called saltatory conduction (from one node to the next).
Discussion 2.3 Nerve Impulses and Neurotransmitters
Neurotransmitters
When an action potential reaches the end of an axon (axon terminal) it has to get to the next neuron or to the effector muscle or gland. Neurotransmitters are how action potentials can continue from one neuron to the next and how the effector gland or cell is stimulated as well.
Neurotransmitters are specialized molecules that either excite or inhibit the target cell. There are numerous neurotransmitters in the human body (over 100). Neurotransmitters either have and excitatory or inhibitory effect on the receiving cell. Some well known neurotransmitters are acetylcholine, dopamine, and seratonin. Neurotransmitters function both in the CNS and PNS.
When an action potential reaches the axon terminal, calcium channels open allowing Ca2+ into the cell. The Ca2+ molecules signal synaptic vesicles to release the neurotransmitters. Neurotransmitters are released and then move into the synaptic cleft (gap between the axon terminal and the next cell). (1)
The neurotransmitters then move across the synaptic cleft by diffusion and bind with receptor sites on the membrane of the receiving cell. The receiving cell is either excited and the action potential continues or it is inhibited and the action potential is stopped or slowed. The neurotransmitter is then removed from the synaptic gap. This is accomplished by either the reuptake of the neurotransmitter by the releasing cell or enzymes in the receiving cells inactivating the neurotransmitter. The action of neurotransmitters is show in the images below.
Neuromodulators
There are other molecules called neuromodulators that prevent neurotransmitters from working on receiving cells or prevent the release of neurotransmitters altogether. Examples of neuromodulators are caffeine (blocks inhibitory brain neurotransmitters), and endorphins (block the release of substance P which is released by sensory neurons due to pain).
Integration of Signals
Since neurons have a number of axon terminals that branch out and synapse with numerous other neurons, it is possible for a cell to receive a number of different signals from a number of different axons. The cell can be receiving inhibitory and excitatory signals at the same time. The ultimate response of the cell is determined by integration of all the signals being received. If there are more excitatory signals or very rapid excitatory signals they will cancel out the inhibitory signals and the cell will fire an action potential. If there are more inhibitory signals than excitatory then the cell will not fire an action potential.
References
Image References
Images obtained from Aris site for Human Biology by Sylvia Mader (http://highered.mcgraw-hill.com/classware/selfstudy.do?isbn=0072986867 chapter resources - power point presentation), unless otherwise cited under image.
Topic References
http://www.williams.edu/imput/synapse/pages/IIA1.htm
Discussion 3.1 The Mechanics of Sensation
The dendrites of certain neurons are specialized to receive sensory information from inside the body or from the external environment. These specialized dendrites are known as sensory receptors. Exteroceptros are concerned with the outside environment and interoceptors are concerned with the inside of the body. There are four different kinds of sensory receptors in humans. They are chemoreceptors, photoreceptros, mechanoreceptors, and thermorecptors.
Some sensory receptors have nerve endings that are encapsulated while others are free nerve endings. Still other sensory receptors are specialized cells that are found in association with neurons but aren't actually part of the neuron.
Sensory receptors work by sending nerve impulses to the cerebral cortex. This information can stimulate a conscious response but sensory receptors also play an important role in reflex response as well. The strength of a sensory impulse is determined by its frequency. The stronger the stimulus the more frequent impulses are sent to the brain. Sensory receptors perform integration of sensory information to determine the nerve impulse that will be relayed to the brain. This is how sensory adaptation can occur. Once we are exposed to a stimulus for a time our sensory receptors send less and less signals to the brain regarding that stimulus.
Discussion 3.2 Types of Sensory Receptors
Chemoreceptors
Cehmoreceptors contain specialized receptors that allow them to respond to a particular chemical substance. Chemoreceptors are responsible for the senses of taste and smell as well as the regulation of pH in the blood.
Pain Receptors
Pain receptors are also known as nociceptors and are actually a kind of chemorecptor. Pain receptors are exposed dendrites that receive chemicals that are released from the tissues of the body when they are injured in some way.
Photoreceptros
Photoreceptors are able to respond to light rays and are responsible for stimulating specialized photoreceptor cells in the eyes that are responsible for vision.
Mechanoreceptors
Mechanoreceptors respond to physical stimulation such as pressure caused by sound waves and touch as well as the movement of the body. Pressorecptors, stretch receptors, and proprioreceptors are all types of mechanoreceptors.
Prorioreceptors
Proprioreceptors are found in the muscles, joints, and tendons. Prorioreceptors send messages to the brain regarding the position of the body.
Cutaneousreceptors
Cutaneous (or touch) receptors are found in the dermis of the skin and allows us to sense touch, pressure, pain and temperature. The various cutaneous receptors are outlined in the image below.
Thermoreceptors
Thermoreceptors respond to changes in temperature. There are warmth receptors that respond when the temperature rises and cold receptors that respond when the temperature lowers.
Discussion 3.3 The Sense of Smell
The sense of smell is powered by chemoreceptors found in the plasma membrane of the cell and bind with molecules that are released into the environment. The sense of smell can respond to stimuli that is some distance from the body. Taste and smell are very closely related. Most of what we attribute to taste is actually picked up by our sense of smell.
There are between 10 to 20 million olfactory cells in the nose. These olfactory cells are neurons that end in cilia. They are located in the epithelium found in the top of the nasal cavity. Each cell only has one kind of receptor protein and their are only a few hundred different receptor proteins found in the olfactory cells as a whole. The varied number of smells we are able to distinguish is due to the combination of receptor proteins triggered by a particular smell.
Discussion 3.4 The Sense of Taste
Taste receptors or taste buds are located mainly on the tongue (a few are found on the roof of the mouth, pharynx, and epiglottis). The taste bud which can be felt on the tongue leads to a taste pore. Leading off of the taste pore are taste cells and supporting cells. Each taste cell ends in microvilli . The microvilli of the taste cells have receptor proteins that bind with molecules from food. This causes nerve impulses to be created that are sent to the gustatory or taste cortex where the signals can be processed and interpreted.
Discussion 3.5 Vision
Structure of the Eye
Sclera
The ability to see is dependant on both the eye and the brain. The eye is constructed of three layers. The first layer is the sclera. The sclera is the white fibrous part of the eye. The function of the sclera is to protect and support the eyeball. The sclera also contains the cornea which is the portion of the eye which refracts light.
Choroid
The second layer of the eye is the choroid which is thin and darkly colored. The choroid absorbs light rays that aren't absorbed by the photoreceptors. The iris is contained in the choroid. In the front portion of the choroid is the iris and pupil. The pupil is the hole found in the center of the eye that allows light into the eyeball. The iris is what controls the size of the pupil and thus the amount of light coming into the eyeball. The iris is the colored portion of the eye. Eye color is dependant upon how much pigment there is in the iris. The ciliary body is a thickening of the choroid behind the iris that contains the ciliary muscle. The ciliary muscle adjusts the shape of the lens to allow for near and far vision. The lens is the portion of the eye that focuses light rays and is attached to the ciliary body by ligaments called suspensory ligaments. The lens divides the eye into two compartments. The front or anterior compartment is filled with aqueous humor which is a clear liquid.
Retina
The third layer of the eye is the retina. The retina is contained in the back or posterior compartment of the eye that is filled with vitreous humor which is a clear gelatinous material. The back layer of the retina (closest to the choroid) contains the cells that are responsible for receiving visual stimuli. These cells or photoreceptors are rod cells and cone cells. The middle layer is made up of bipolar cells and the most anterior layer of the retina contains ganglion cells that eventually form the optic nerve. The optic nerve is responsible for carrying impulses to the visual cortex. A special region of the retina is the fovea centralis which is covered in a large number of cone cells. This is the region of clearest vision.
Rod and Cone Cells
The structure of rod and cone cells is illustrated below. The rod cells can function in very low light but do not perceive color. The rod cells however require bright light to function and do perceive color. Contained within rod cells is a pigment called rhodopsin which absorbs light. This causes the rhodopsin to be split into opsin and retinal which stimulates the closing of ion channels in the cell membrane of the rod cells and the initiation of impulses to other neurons in the retina. Rod cells are found all over the retina except the fovea while cones are found primarily in the fovea. Cones allow for the perception of color and detail. There are a number of different cones that contain different pigments similar to rhodopsin. The pigment of the different types of cones is structured in a slightly different way to allow for the absorption of different spectrums of light and thus the absorption of different colors.
Function of the Eye and Vision
As light passes into the eye the cornea and the lens focus the light onto the retina. The lens shape is changed by the function of the ciliary muscle in to accommodate for the distance of the field of vision (rounds for close vision and flattens for distance vision). When light reaches the retina the pigments in the rods and cones absorb light and trigger the ion channels that in turn trigger nerve impulses to be sent to the bipolar cells. Once the signal reaches the bipolar cells the impulses are integrated and then sent on to the optic nerve and the visual cortex.
Discussion 3.6 Hearing
Structure of the Ear
The ear is divided into three regions: the outer ear, middle ear, and inner ear. The outer ear is made up of the external pinna which is composed of connective tissue. The epithelia of the innermost part of the outer ear forms the auditory canal. The epithelium of the auditory canal is lined with hair and sweat glands. The innermost portion of the auditory canal contains specialized glands that secrete earwax which helps keep small debris out of the inner ear.
The middle ear and outer ear are divided by the tympanic membrane or eardrum. The middle ear contains three bones called the malleus, incus, and stapes. These bones are called the ossicles. The middle ear also contains an opening to the nasopharynx called the auditory tube or Eustachian tube which allows for equalizing air pressure in the ear.
The middle ear and inner ear are divided by bone. The bone has two openings which are the round window and the oval window. The inner ear unlike the outer ear and middle ear, is filled with fluid. The inner ear also contains the semicircular canals and vestibule which assist in maintaining equilibrium and the cochlea. (balance and equilibrium will be discussed in the next section of this review). The cochlea is a bony structure shaped like the shell of a snail and contains the cochlear nerve which relays auditory signals to the brain. The cochlea contains three canals (image below). The middle canal is the cochlear canal and contains the spiral organ (organ of corti). The organ of corti is composed of hair cells which rest on top of the basilar membrane. The hair cells have tiny hairlike projections called stereocilia that extend upward into a jelly like membrane called the tectorial membrane. The cochlear nerve has sensory receptors that reach into the basilar membrane and contact the hair cells.
Function of the Ear in Hearing
Sound waves first are directed by the outer ear into the auditory canal and cause the tympanic membrane to vibrate. The vibration of the tympanic membrane then causes the malleus to absorb the vibrations which are then passed to the incus and the stapes. The movement of the vibrations from one structure to the next serves to increase the pressure of the original vibration by approximately twenty times. The stapes then strikes the membrane covering the oval window and causes it to vibrate. This vibration is passed to the fluid of the inner ear to the vestibular canal and then the tympanic canal and then across the basilar membrane. When this happens the basilar membrane vibrates up and down causing movement of the stereocilia of bend. This cause nerve impulses to be generated by the hair cells that travel via the cochlear nerve to the auditory cortex in the brain.
Discussion 3.7 Equilibrium and Balance
As mentioned above, the inner ear contains the vestibule and semicircular canals. The semicircular canals contain mechanoreceptors that are able to detect the rotation or tilting of the head. Each of the semicircular canals is positioned to detect movement on a different plane. The bottom of each canal (ampulla) contains hair cells with stereocilia embedded in a gelatinous membrane called the cupula. The movement of fluid within the semicircular canal cause the stereocilia to bend and thus stimulates the relay of impulses by the vestibular nerve to the brain.
Two other structures located at the base of the semicircular canals are the utricle and saccule. The utricle relays information to the brain concerned with back and forth movements and the bending of the head. The saccule is concerned with vertical movements. Each of these sac like structures contain the same construction as the semicircular canals, but in the utricle and saccule the re are also calcium carbonate granules called otoliths that rest on the membrane in which the stereocilia are embedded. The otoliths move around with the various movements of the head and further stimulate the vestibular nerve.
References
Image References
Images obtained from Aris site for Human Biology by Sylvia Mader (http://highered.mcgraw-hill.com/classware/selfstudy.do?isbn=0072986867 chapter resources - power point presentation), unless otherwise cited under image.
The chart below describes various drugs that act on the nervous system and their effects. Drug abuse is a major problem in this country and it impacts all facets of society.
| Drug | Impact on Body/ Nervous System | Additional Information |
| Alcohol | Is a depressant and increases the functioning of GABA as well as the release of beta-endorphins in the hypothalamus. In low to moderate amounts causes the individual to feel relaxed, lowers inhibitions, interferes with concentration and motor abilities and possibly vomiting. In larger amounts can cause coma or death. Changes the structure of proteins in the brain and internal organs. Can cause liver cirrhosis is consumed in excess overtime aw well as damaging the frontal lobes. | 65% of people in the US drink occasionally and 5% of those who drink to excess
80% of college students drink
Treatment for alcoholism focuses on behavior modification.
|
| Nicotine | Is a stimulant and acts especially on the midbrain. Causes the release of dopamine. Causes the same effects as acetylcholine in the PNS causing the increase of skeletal muscle contraction, increased heart rate, and increased blood pressure and increase digestive system activity. | 70 million people in the US smoke cigarettes
Highly addictive
70% of people who smoke become addicted
90% of those who try to quit smoking fail.
There are various forms of treatment to help individuals quit smoking. |
| Cocaine | Acts as a stimulant on the CNS and prevents the re-uptake of dopamine by the neurons which causes an overwhelming feeling of well being. It also causes sleeplessness, loss of appetite, increased sex drive, tremors, and paranoia. As the drug wears off it causes fatigue, depression, irritability, memory loss, and confusion. Eventually the body begins to make less and less dopamine to compensate for the effects of continued cocaine abuse. Cocaine can cause heart attack or respiratory arrest. | 35 million people use cocaine in its various forms
Produced from a shrub called Erythroxylon coca.
"Crack" is cocaine that is smoked. 8 million people smoke crack in the US.
There are no effective treatments for cocaine addiction. |
| Methamphetamine | Methamphetamine simulates dopamine and causes the same effects on the body as cocaine. Often also causes agitation and violent behavior. Prolonged use can lead to amphetamine psychosis which is characterized by paranoia, hallucination, self absorption, and irritability | Produced by adding a methyl group to amphetamine.
Used by over 9 million people in this country.
Is especially popular because it can be produced cheaply from common ingredients.
Generally smoked or snorted. |
| Heroin | Acts as a depressant on the CNS. Has pain killing properties (morphine and codeine are related). Heroin is converted in the brain to morphine which binds to opioid receptors and creates a feeling of euphoria. Depresses the breathing, blocks pain pathways, creates mental confusion and activates the the reward circuit. Can cause nausea and vomiting | Produced from the poppy plant.
Can be injected, snorted, and smoked.
About 300,000 people in the US use heroin over the course of a year.
There are pharmacological treatments available. |
| Marijuana | Effects are caused by the compound THC (tetrahydrocannabinol) found on the hemp plant. THC acts like the neurotransmitter anadamide in the brain. Causes a feeling of euphoria as well as altered vision and impaired judgement. May cause hallucinations, anxiety, and depression in those who use it heavily. | Produced from the flowers of the hemp plant.
Generally smoked.
|
References
Image References
Images obtained from Aris site for Human Biology by Sylvia Mader (http://highered.mcgraw-hill.com/classware/selfstudy.do?isbn=0072986867 chapter resources - power point presentation), unless otherwise cited under image.
Eventhough it usually can not be heard by a physician with a doppler yet, during the third week of development (fifth week of pregnancy) the heart begins to develop and during the fourth week the heart begins to beat. At this point the heart is composed of two tubes that are fused together with veins entering from posterior and arteries exiting from the anterior of the heart structure. Until the heart begins to beat (something most of us tend to equate with life) it is difficult to think of a growing embryo as having any relationship to a human being. 
By this point in development a fetus has its unique fingerprint pattern that will not change for the rest of its life. 
Though this date varies for everyone, around the 20th week of pregnancy the fetal bones have hardened enough for fetal kicks to be felt. The image to the right is of a 20 week old fetus sucking its thumb.
![scan0001[1]](http://lh6.ggpht.com/_lRGislJejZ0/STheZhybgtI/AAAAAAAACBQ/483Gwy-l6lM/s1600-h/scan0001%5B1%5D%5B10%5D.jpg)
