This is a storyboard from "Telegraph Line" one of the Schoolhouse Rock videos. This one is about the nervous system. I Introduction
II Overview of Nervous System
- Neurons and the Nerve Impulse
III Central Nervous System
- Spinal Cord
- Brain
IV The Limbic System
V Peripheral Nervous System
- Somatic System
- Autonomic System
VI Sensation
- Sensory Receptors
VII Senses
- Taste
- Smell
- Sight
- Hearing
- Equilibrium
VIII Conclusion
An illustration showing the brain, spinal cord and nerves. Taken from Human Biology, page 248. Sylvia S Mader 
An illustration showing the types of neurons, their dendrites, and an axon sheathed in myelin. From Human Biology, page 249, Sylvia S. Mader.
The Nerve Impulse: I learned from the neuroscience for kids website that most of the information we have regarding the nerve impulse comes from studying the giant axons on squid. I guess because of its size, this axon is easy to study. Basically, nerve impulses send information within the nervous system, and it is sent electrochemically. Electrically charged chemicals within the body are called ions. Sodium and potassium ions are each have one positive charge and are the important ions in the nervous system.
When an axon is not not conducting an impulse, it is said to be at its resting potential. During resting potential the charge on an axon will be -65 mV. At this time the inside of the neuron is negative as compared to the outside. Sodium ions are more numerous on the outside than the inside of the axon. Ions will try to balance out on either side of the membrane, but the membrane only allows certain ions to pass through. This unequal distribution of ions is caused by the sodium-potassium pump which transports sodium out of the axon and potassium in.
When a neuron responding to a stimulus sends information down an axon it is known as the action potential. At this point depolarization occurs; depolarization simply means that the charge inside the axon goes from negative to positive. To make this happen, a "gate" or rather a protein structure in the membrane opens to allow sodium to enter into the axon. Then another "gate opens to allow potassium to flow to the outside of the axon. When the potassium flows out this changes the membrane charge from -65 mV to +40 mV. After an action potential has passed the negative charge of a resting potential is restored.
This graphic taken from www.millerandlevine.com/.../898-899-rewrite.html illustrates the sodium-potassium pump
The above graphic nicely illustrates an action potential. Taken from health.howstuffworks.com/nerve4.htm
An action potential will travel the length of an axon. In an axon that is myelinated the action potential will go from one Ravier node to another. This type of conduction is called saltatory conduction. Saltatory conduction is faster and uses less energy. Action potenials along nonmyelinated axons move more slowly. A refactory period occurs after an action potential has passed, preventing it from moving backward along the axon.
An illustration of an action potential moving down a myelinated axon. Taken from Human Biology, page 251, Sylvia S. Mader
At the end of each axon are small swellings called axon terminals. These axon terminals are located either close to a dendrite or the actual cell body of a neuron. The area between the axon and neuron/dendrite is called a synapse. A small gap called the synaptic cleft separates a sending neuron from a receiving one. Nerve impulses are not able to jump this gap so neurotransmitters must aid impulses in getting from one neuron to another. Neurotransmitters are molecules that are stored in synaptic vesicles in the axon terminals. When an action potential reaches the axon terminal, calcium ions enter the terminal, causing the synaptic vesicles to merge with the sending membrane. Then these molecules are released to bind to receptors along the membrane of the receiving neuron.Depending on the type of neurotransmitter, the response of the receiving neuron can be either excitation or inhibition. Neurotransmitters are removed from the cleft after doing their job. There are over 100 known neurotransmitters. A few examples include dopamine, serotonin, acetylcholine, and glutamate.
A graphic showing synapse structure and function. This example shows an excitatory neurotransmitter. Human Biology, page 252, Sylvia S. MaderIII Central Nervous System
The central nervous system (CNS) is composed of the brain and spinal cord. It is in the CNS that sensory information is received and motor control is initiated. The brain is protected by the skull and the spinal cord is protected by vertebrae. For extra protection they are also surrounded by membranes called meninges to provide a cushion. Two types of tissue are found in the CNS. Gray matter includes cell bodies and short nonmyelinated fibers, while white matter has myelinated axons running together in bundles called tracts.
Spinal Cord
Like the brain, the spinal cord contains both gray and white matter. The cord extends from the base of the brain and is protected by the vertebral column. Spinal nerves extend from the spinal cord. There are intervertebral disks between the vertebrae. Cerebrospinal fluid is contained within the central canal. Within the white matter there are ascending tracts that take information to the brain and descending tracts. I have often heard that the right side of the brain controls the left part of the body and vice versa. Apparently this is true as many of these tracts cross as they enter and exit the brain. As far as I can tell, the spinal cord basically hooks up the body to the brain. This is why a spinal cord injury can cause paralysis. the spinal cord is involved in reflex actions as well.
Brain
The brain contains four ventricles, the two lateral ventricles, the third ventricle and the forth ventricle. The ventricles are interconnecting chambers that produce cerebrospinal fluid and serve as a reservoir for it as well. The other parts of the brain I will be discussing are the cerebrum, the diencephalon, the cerebellum, and the brain stem. I should also mention that like the spinal cord, the brain is composed of white and gray matter.
Cerebrum: Another name for the cerebrum is the telencephalon. The cerebrum makes up the largest part of the brain in humans. This part of the brain is the last to receive sensory input and integrate information before commanding voluntary motor responses. The cerebrum also communicates and coordinates activities with other parts of the brain. A grove called the longitudinal fissure divides the cerebrum in to the left and right hemispheres. The hemispheres are divided into lobes by sulci. The frontal lobe is directly behind the forehead, the parietal lobe is behind it, the occipital lobe is at the back of the head, and the temporal lobe lies beneath the frontal and parietal lobes. Each of these lobes carry out certain functions as illustrated below.
The cerebral hemespheres are covered by thin outer layer of gray matter called the cerebral cortex. The cerebral cortex is the part of the brain that handles sensation, voluntary movement, and thought processes we associate with consciousness. Within the cerebral cortex are the primary motor and primary somatosensory areas. The primary motor area is found in the frontal lobe and certain sections control different parts of the body. Commands to skeletal muscles originate in the primary motor area. The primary somatosensory area is in the pariental lobe and receives sensory information from the skin and skeletal muscles.
IV The Limbic System
The limbic system is made up of several structures within the cerebrum including the amygdala and the hippocampus. The limbic system plays a role in both our emotions and higher mental functions like memory and speech. I thought it was kind of funny that both of these things come from the same part of the brain as our emotions often seem to get in the way of our minds. The amygdala is responsible for the sense of fear and also gives certain experiences like grief its emotional overtones. In contrast, the hippocampus plays an important role in learning and memory.
There are different kinds of memory. Short term memory is attributed to the prefrontal area of our brains which is effected by Alzheimer disease. Long term memory is broken up into two groups, semantic (numbers, words) and episodic (people, events). I am better at the former than the later. Skill memory involves all the motor areas and has to do with learned skills like riding a bike or driving a car.
V Peripheral Nervous System
The nerves that run throughout our bodies are part of the peripheral nervous system. the nerves that come from the brain are called cranial nerves of which we have 12 pairs. Cranial nerves can be sensory nerves or motor nerves. Nerves that arise from the spine are spinal nerves; there are 31 pairs of spinal nerves. Spinal nerves contain both sensory and motor fibers and are therefore known as mixed nerves. All nerves have the role of taking impulses to and from the CNS.
There are divisions to the peripheral nervous system, the somatic system and the autonomic system.
Somatic System
The somatic system is made up of nerves that serve the skin, skeletal muscles, and tendons. Nerves in this systom can take information from sensory receptors on the skin to the CNS and motor commands from the CNS to the skeletal muscles.
Sometimes the actions our bodies take are not voluntary, these involuntary responses are called reflexes. The illustration below shows the path of a reflex arc when a person touches a pin. This example involves the spinal cord. When skin is pierced by the pin, sensory receptors send nerve impulses along sensory axons through the dorsal-root ganglia to the spinal cord. The data is integrated by interneurons and sent to motor neurons. A nerve impulse is sent down a motor axon commanding skeletal muscles to contract. The result is that the hand moves away from the pin. Of course all of this takes place in a split second!
An example of a reflex arc taken from Human Biology, page 263, Sylvia S. Mader.
Autonomic System
The autonomic system is responsible for regulating the activity of the smooth muscles and glands. Basically this system functions automatically in an involuntary manner, innervates all internal organs, and uses two neurons and one ganglion for each impulse. The system is divided into two parts, the sympathetic division and the parasympathetic division. These two divisions generally produce opposite responses. The sympathetic division is responsble for the fight of flight response. Some of the involuntary aspects of this response include an increased heart rate an dilated bronchi. The parasympathetic system promotes responses that are associated with a relaxed state.
An illustration of the sympathetic and parasympathetic systems taken from http://www.scholarpedia.org/article/Autonomic_nervous_system
VI Sensation
Sensation occurs when sensory receptors responding to external or internal stimuli send nerve impulses to the cerebral cortex of the brain. Sensation is the conscious perception of stimuli.
Sensory Receptors
Dendrites that are specialized to pick up certain types of stimuli are called sensory receptors. Exteroreceptors detect stimuli from outside the body, while interoceptors receive stimuli from inside the body. Interoreceptors respond to change in blood pressure and other internal body conditions that have a direct bearing on homeostasis.
There are four categories of sensory receptors in humans, chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.
Chemorceptors: These receptors respond to chemical substances nearby. These receptors involve taste and smell. Pain receptors (nociceptors) are a type of chemoreceptors and are important because they alert us of danger through the sensation of pain.
Photoreceptors: These receptors are directly involved in the sense of sight. Photoreceptors are sensitive to light rays. Rod cells enable us to see in black and white, while cone cells give us color vision.
Mechanoreceptors: These receptors are stimulated by sound, motion, and gravity. These stimuli result in pressure of some sort. Sound waves for example generate fluid-borne pressure waves that are detected by mechanoreceptors in the inner ear.
Mechanoreceptors involved in reflex actions are known as proprioceptors. These receptors help us maintain muscle tone and therefore keep the body's equalibrium and posture. Muscle spindles are able to send messages to the spinal cord and motor nerve impulses order muscle contraction in order to maintain muscle tone.
An illustration of a muscle spindle taken from Human Biology page 276, Sylvia S. Mader
Thermoreceptors: These receptors can be found in the hypothalamus and skin and respond to changes in temperature. There are both warmth and cold receptors in the body.
The skin contains cutaneous receptors that are sensitive to touch, pain, pressure and temperature. Although the book did not classify the different cutaneous receptors as one of the four sensory receptors it is easy to guess based on their functions.
An illustration of touch and pressure receptors found at www.octc.kctcs.edu/gcaplan/anat/Notes/API%20N...
VII Senses
Taste
The sense of taste is brought about by chemical receptors that respond to in the food we eat. The senses of taste and smell are connected and a lot of our perceived "tastes" are directly related to smell. Adults have about 3,000 taste buds which are found mainly on the tongue, although there are taste buds on other parts of the mouth as well. There are five primary types of taste, sweet, sour, bitter, salty, and umani. Some areas of the tongue are more sensitive to certain tastes than others.
A graphic showing the tungue and the sense of taste taken from embryology.med.unsw.edu.au/notes/tongue.htm
Smell
Humans have about 10 to 20 million olfactory cells. There are olfactory cells found inside olfactory epithelium up in the roof of the nasal cavity. As I mentioned before, our sense of taste is directly related to smell. There is a connection between olfactory cells and the limbic system which causes certain odors to bring up vivid memories.
An illustration showing where olfactory cells are located and how they work. Human biology, page 279, Sylvia S. Mader
Sight
Eyeballs are spherical and made up of three layers, the sclera, the choroid, and the retina. The outer layer, the sclera, includes the cornea which is the window of the eye. The middle layer, the choroid, includes the iris which regulates the size of the pupil. The pupil is a hole in the center of the iris that allows light to enter the eyeball. Behind the iris is the ciliary body on which the lens is attached. The ciliary body helps control the shape of the lens to provide near and far vision. Thelast layer, retina contains photoreceptors. As I mentioned before, rod cells do not see color and cone cells do. The optic nerve, located at the back of the eye is fomed out of sensory fibers from the retina. The optic nerve takes nerve impulses to the visual cortex giving us the sense of sight.
Rhodopsin is a molecule found in rods and is highly sensitive to light. Rhodopsin is made up of the protein opsin and the molecule retinal. When rods are exposed to light signals are sent to the retina. Rods are useful for night vision.
As I mentioned before, cones are responsible for color vision. Cones contain three pigments for blue, green , and red. These pigments contain retinal and opsin.
There are three layers to the retina. The rod and cones cells are contained in the layer of the retina closest to the choriod. Bipolar and ganglion cells in the other two layers of the retina and the sensory fibers of the ganglion cells become the optic nerve. Integration happens when signals are sent to bipolar and ganglion cells. Ganglion cells send nerve impulses via the optic nerve to the visual cortex.
Hearing
There are three parts to the human ear, outer, middle, and inner. The outer ear includes the pinna and the auditory canal. The middle ear begins at the eardrum (AKA tympanic membrane). Three bones called the ossicles are also found in the middle ear. The inner ear includes the semicircular canals, the vestibule, and the cochlea. Of these three parts it is the cochlea that has a role in hearing.
Mechanoreceptors consisting of hair cells in the inner ear are sensitive to mechanical stimulation. Hearing results when sound waves enter the auditory canal and hit the eardrum. The vibration creates pressure which is moved along and multiplied by the ossicles. The stapes (one of the ossicles) strikes the oval window and the pressure is passed on to the cochlea. These pressure waves then move on to the tympanic canal and across the basilar membrane. This is where the hair cells come in. They start to bend, and nerve impulses begin in the cochlear nerve and make their way to the brain.
A graphic showing the anatomy of the human ear, Human Biology, page 286, Sylvia S. Mader.
Equalibrium
Our sense of balance takes place in the inner ear, specifically in the semicircular canals and vestibule. Fluid in these parts of the inner ear is stimulated by movements of the head and/or body giving us a sense of movement and acceleration.
Illustrations of the inner ear and its relationship to balance and equalibrium. From the Power Point presentation and Human Biology, Sylvia S. Mader
VIII Conclusion
Overall I found the nervous system quite interesting. I will say that I obtained a basic grasp of the processes of the central nervous system and its connection to the peripheral nervous system. I can basically understand the "wiring" that links our body parts to our brain. However the five senses and how they are processed (especially sight) are still pretty confusing to me. I do understand the concept of the different neurons and how they are specialized to send messages to the brain, but it is still a mystery to me as to how are brains are able to process the images sent and translate them as well.
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