Compendium Review #5

For chapters: 13 & 14
• Nervous System
• Diffusion and Action Potentials in Neurons--
rapid transmission of messages
• Reflex arc (simple somatic function)
and autonomic function
• Senses



***NERVOUS SYSTEM***
The nervous system has two major parts: the central nervous system (CNS), which includes the brain and spinal cord; and the peripheral nervous system (PNS), which consists of all the nerves throughout the entire body. It also has three functions: reception of input, integration of data, and it generates motor output. First off, there's the nervous tissue which has two types of cells: neurons, which transmit nerve impulses; and neuroglia, which nourish and support the neurons. The CNS, as stated above, has a spinal cord and the brain. The spinal cord contains the neuron cell bodies at the gray parts. At the white parts, myelinated axons can be found in tracts. The spinal cord is the conductor for the brain and its function is to carry out reflex actions. For instance, a shiver when you get cold. The brain, on the other hand, has many different parts to it. The cerebrum is connected by the corpus callosum. This part has two hemispheres, called, cerebral hemispheres. They say when you use your right hand, that means you are using the left part of your brain. The cerebrum is responsible for speech, sensation, reasoning memory, language, and learning skills. Sheltering the cerebrum is the cerebral cortex, which has four lobes for each cerebral hemisphere. These lobes are known as: frontal, parietal, occipital, and temporal.



The frontal lobe sends out the motor commands to the lower brain which then continue on with the message to the motor neurons. Next is the parietal lobe which receives the sensory info from the lower brain and passes on this information to the sensory neurons. The last three I will mention are the diencephalon, the cerebellum, and the brain stem. The diencephalon is named for the hypothalamus having control over homeostasis. The cerebellum is in charge of the skeletal muscle contractions, and the brain stem deals with the medulla oblongata and the pons which deal with regulating the body's heartbeat and breathing rate.
Fascinating points are that the amygdala controls whether a stimulus is granted our fear. A part of the brain called the hippocampus is actually functioning in storing and receiving memories!



Now that we discussed the central nervous system, here's the peripheral nervous system. The PNS deals with nerves and ganglia and nothing else. THe cranial nerves take the impulses to and from the brain while the spinal nerves take impulses to and from the spinal cord. The PNS, like the CNS, has different parts: the somatic system and the autonomic system. The somatic system deals with the skin, skeletal muscles, and our tendons. Most actions seen are due to reflexes while there are some actions that are decided upon in the cerebral cortex. The autonomic system has two parts as well: the sympathetic division and the parasympathetic division. First, the sympathetic division takes part in creating the responses that happen during stressful times. THe parasympathetic division deals with responses that happen during times of comfort and relaxation.




***DIFFUSION AND ACTION POTENTIALS IN NEURONS--***
**RAPID TRANSMISSION OF MESSAGES**



Taking a neuron apart, one would observe that it is made up of dendrites, a cell body, and an axon. Like anything else there are different types of neurons. One is a sensory neuron. These take the nerve impulses from the sensory receptors to the central nervous system. Next would be the interneurons which occur inside the central nervous system. The motor neurons take nerve impulses from the central nervous system to the effectors, otherwise called the muscles or glands of a body. The dendrites receive messages from sensory receptors and other neurons. While the axon conducts nerve impulses. As stimuli is noticed the sensory receptors send a nerve impulse that goes from the PNS to the CNS. It's there that the information is turned into motor output.



This nerve impulse though complex, is fairly simple. The more sodium outside the axon and the more potassium insie the axon, there is no impulse, thus it is referred to as resting potential. On the flip side, there's action potential. This is where there is a change in the polarity of the axonal membrane just as the impulse is happening. The sodium gates open and the sodium moes inside of the axon to where polarization is no more. The potassium does the same, though going outside of the axon, creating polarization to occur once more. Axon length is another factor that varies for each type of neuron. Most are covered in a protective lipid substance called a myelin sheath. It serves as a bridge for fiber growth in nerve regeneration. It also forms the neuroglia cells (Schwann cells) which wrap themselves around the axon, forming layers upon layers. There are gaps, howerer, and there are also unmyelinated axons which are relatively short. There is still an area that is between the sending neuron and the receiving neuron; this is a synaptic cleft and neurotransmitters from the senidng neuron are released to cross this synaptic cleft and grab hold of the receiving neuron.


***REFLEX ARC (SIMPLE SOMATIC FUNCTION) AND AUTONOMIC FUNCTION***



The PNS, like the CNS, has different parts: the somatic system and the autonomic system. The somatic system deals with the skin, skeletal muscles, and our tendons. Most actions seen are due to reflexes while there are some actions that are decided upon in the cerebral cortex. The autonomic system has two parts as well: the sympathetic division and the parasympathetic division. First, the sympathetic division takes part in creating the responses that happen during stressful times. The parasympathetic division deals with responses that happen during times of comfort and relaxation. In the parasympathetic division, the preganglionic fiber is long and the postganglionic fiber is short. The ganglia is either near or inside the organ, making this so. The parasympathetic division deals with the body's rest and digestive system. It promotes the responses for rest like slowing the heartbeat and the stages the eyes go through to get to a full sleep; it stimulates the organs when it's time for the digestion process.


***SENSES***



The body's senses are more than just the average five--smell, taste, touch, sight, hand hearing. There are sensory receptors too! The sensory receptors have four different types: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors. These sensory receptors create the nerve impulses that are sent to the spinal cord/brain. Sensation is the term titled for what happens when these nerve impulses reach the cerebral cortex. There are also proprioceptors and cutaneous receptors. The proprioceptors are mechanoreceptors that deal with reflexes and help maintain equilibrium and posture. The cutaneous receptors are found in the skin and are meant for touch, pressure, temperature, and the one that's most expected--pain. About the senses-> Chemoreceptors are stimulated by molecules found in the environment, therefore they conduct taste and smell. For taste, microvilli of taste cell shave receptor proteins that cause to brain to distinguish between sweet, sour, salty, and bitter tastes. There are different parts of the mouth with different microvilli to help the brain better determine these tastes. As for smell, the cilia of olfactory cells have receptor proteins for molecules that cause the brai to distinguish odors. Vision is much more complex. Vision takes the eyes, the optic nervers, and the visual areas of the cerebral cortex in order to work. The eye has three layers by itself: The sclera (outer layer), the choroid (middle, pigmented layer), and the retina (inner layer). The sclera protects and supports the eyeball. The choroid absorbs stray light rays. The retina contains the rod cells and the cone cells. These cells are sensory receptors for dim light and bright light and color, respectively. The lens functions to take the rays of light to the retina, focusing them. The visual pathway begins when the light hits the photoreceptors in the retina. Impulses from the eyes to the optic chiasma are taken from the optic neres and then are taken to the thalamus then the vision area in the occipital lobe of the brain. Hearing is just as complex as sight. It depends of the ear, the cochlear nere, and the auditory areas of the cerebral cortex. Again, this sense's parts are: the outer ear, the middle ear, and the inner ear. The outer ear consists of the pinna and auditory canal that send sound waves to the middle ear. The middle ear then amplifies sound waves. It is the tympanic membrain and the ossicles are responsible for this amplification. The inner ear includes the semicircular canals that find the rotational equilibrium. The utricle and the saccule look to find the gravitational equilibrium and the cochlea has in it, the spiral organ which holds the mechanoreceptors. The pathway for hearing starts with the outer ear and continues inside.