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Nervous System Ch 10 The Nervous System is composed of neural tissue, blood vessels and connective tissue. This system allows us to experience the world, think and to feel emotion. Neural tissue : 2 types a) neurons (nerve cells)- are specialized cells which react to physical and chemical stimuli b) neuroglia- which surround the neuron and help nourish them and remove ions & neurotransmitters that accumulate between neurons The nervous system is divided into “2” groups 1) Central Nervous System (CNS) = brain and spinal cord 2) Peripheral Nervous System (PNS) = all the other nerves that connect the CNS to other body parts. These nerves include cranial & spinal nerves There are 3 functions of the nervous system: sensory, integrative and motor Sensory : with the help of sensory receptors which detect changed inside and outside the body, they gather info from the environment ex. from light, sound, temperature, oxygen concentration. Integrative : the sensory receptors change their info into nerve impulses which by way of the PNS go to the CNS, where the signals are “integrated” - meaning they are brought together to create sensations, add to memory or help produce thoughts Motor : once conscious or subconscious decisions are made they are acted on by motor functions which use neurons to carry impulses from the CNS to effectors. Effectors are muscles (respond) or glands (secrete) that react when stimulated The PNS motor portion is divided into: 1) Somatic nervous system : involves conscious activities employing skeletal muscle 2) Autonomic nervous system : controls viscera, like heart and glands under unconscious action Neuron Anatomy Each neuron is composed of many branched dendrites which receive input from the body and a longer, single axon (nerve fiber) which carries info away from the neuron in the form of electrical signals called a nerve impulse ( See fig. 10.1 & 10.3 ) The cell body is also called the soma which contains the nucleus Between each neuron is a small space called a synapse. At the synapse, biochemicals called neurotransmitters help send electrical messages from neuron to neuron Nerves are just bundles of axons The larger axons of the PNS are enclosed in lipid-rich sheaths called Schwann cells. Schwann cells are neuroglial cells that wrap around the axon and protect it. They do not conduct impulses. The sheath is formed by layers of cell membranes which are wound tightly like a bandage wrapped around a finger. The layers are called myelin and the coating is considered a “myelin sheath”. “Nodes of Ranvier” are the narrow gaps in the myelin sheath between Schwann cells (see fig. 10.3 / 10.4) Axons with myelin sheaths are called myelinated and appear white, like those seen in the “white matter” of the brain and spinal cord (multiple sclerosis involves the loss of myelin) Axons lacking myelin are called unmyelinated and appear gray, like those seen in the “gray matter” of the brain and spinal cord Neurons classified by their “function” depend on whether they carry info into the CNS, carry info within the CNS or if they carry info out of the CNS ( see fig. 10.7 ) The 3 types of neurons based on function are: sensory, interneurons and motor neurons. 1) Sensory neurons ( or afferent neurons ): carry nerve impulses from the body to the brain or spinal cord 2) Interneurons: lie within the brain or spinal cord and transmit impulses from one part of the brain or spinal cord to another 3) Motor neurons (or efferent neurons ): carry nerve impulses out of the brain or spinal cord to effectors (muscles or glands) which either contract or secrete substances Cell Membrane Potential A cell membrane is electrically charged or polarized so that the inside is negatively charged with respect to the outside. This polarization is due to an unequal distribution of positive and negative ions on either side of the membrane. The outer surface of the neuron membrane consists of many positive ions. Sodium (Na+) and potassium ions (K+) are the major intracellular positive ions. (see fig. 10.12). Remember that there is a small gap between neurons called the synapse. Also a neuron that has positive ions on the outside of the membrane is said to be a polarized nerve. The difference in electrical charge between two points is measured in units called volts. This represents a “potential difference” because it represents stored electrical energy which can be used later to do work. The potential difference across the cell membrane is called the membrane potential, which is measured in millivolts. A resting neuron is one that is not being stimulated to send a nerve impulse and its membrane potential is called the resting potential. The resting potential has a value of -- 70 millivolts. Itʼs negative to indicate the excess negative charges on the inside of the cell membrane. Figure 1 shows a simplified neuron and an impulse traveling through it. Figure 2 shows a polarized neuron and the synapse. Fig. 1: Simplified neuron Fig. 2 Polarized neuron When the membrane of the dendrite is activated by a stimulus, Na+ will enter. These ions will diffuse along the membrane of the dendrite and soma in the intracellular fluid. When the ions reach the junction of the soma and the axon ( an area called the axon hillock), the membrane becomes very permeable to Na+. There is a tremendous influx of Na+ in the axon. The ions in the extracellular fluid of the axon will enter the axon in a domino fashion. Therefore, when one positive ion enters (a process called depolarization) an adjacent positive ion will then enter. This will then cause the next positive ion to enter and so forth. As each ion enters the neuron, a wave-like occurs. This wave-like activity is the impulse. This traveling wave of activity is known as the action potential. If a neuron is sufficiently depolarized the membrane reaches a level called the “threshold potential” because events are set in motion to allow the impulse to travel through the neuron. At threshold, an action potential is produced. Threshold can be defined as the amount of stimuli required to cause depolarization. If a person has a high threshold, they will therefore, require more stimuli in order to have depolarization and therefore in order to respond to something. A neuron with a low threshold will have depolarization with very little stimuli. Whenever a doctor gives a patient a drug that numbs the feeling, the drug actually raises the threshold of the neuron to the point where depolarization is not occurring. Without depolarization, an impulse will not occur. Without an impulse, the brain does not sense any pain. Many times a single depolarizing stimuli is not sufficient for the membrane to reach threshold. But, if another stimuli of the same type arrives before the first subsides, the potential change is greater. This is called summation. It allows several “subthreshold potentials” to combine and reach threshold. Refractory period: a period of time following the passage of a nerve impulse where a threshold stimulus will not trigger another impulse. Takes about 10 to 30 milliseconds to return to resting state. This time is required for the membrane to change itʼs ion (Na+) permeability. All-or-None Principle : This principle states that if a stimulus is strong enough to cause depolarization, the entire neuron will depolarize in succession. An impulse will travel the length of the neuron. If a stimulus is not strong enough to cause depolarization, an impulse will not even begin. An impulse will not travel part way through an axon and then quit. It goes all the way or not at all. Fig. 3: Depolarized neuron To further understand how an impulse travels through a neuron, lets use the domino analogy again. Think of the positive ions as dominoes. When one positive ion enters the neuron, it causes the next ion to enter and then the next, and so on. When you push one domino over, it causes the next domino to fall and so on. As you watch each domino fall, you see a wave-like activity. Figure 3 shows a depolarized neuron in sequence. To help with your understanding, some of the dendrites have been left out. Figure 3A shows a polarized neuron. The neuron is stimulated and therefore, one positive ion begins to enter the dendrite. This is the beginning of depolarization. This ion diffuses to the axon region. Figure 3B shows depolarization of the axon. The dotted arrow shows the movement of the impulse down the axon. Figure 3C shows more ions entering. The dotted arrow shows the impulse traveling a little farther down the axon. This process continues until the impulse reaches the end of the axon. At the end of the axon, there will be an influx of positive ions but, in this case, it will be calcium ions. Calcium ions target the presynaptic vesicle and cause the vesicle to release the neurotransmitter into the synapse area. Figure 4 shows the release of the neurotransmitter. (see fig. 10.15 & 10.18) Fig. 4: The release of the neurotransmitter As soon as the neurotransmitter ( acetylcholine in this example ) is released from the presynaptic vesicle, it enters into the synaptic region. It flows across the synapse and comes in contact with the membrane of the next neuron in line. As soon as it stimulates the membrane of the next neuron, it causes depolarization of the next neuron in sequence and the impulse continues on to its destination. In order to use the neuron a second time, all the positive ions must leave the neuron and go back to the extracellular regions outside the neuron. Also, the neurotransmitter must be decomposed so it too leaves the synapse. Basically, everything needs to go back to its original position. The process of returning the ions to the extracellular region is called repolarization. This is an active process that requires ATP. The dendrite end releases an enzyme called acetylcholinesterase. This enzyme will decompose acetylcholine to form acetate and choline. Acetate and choline can then be reabsorbed into the presynaptic vesicle to recombine and make more acetylcholine. Figure 5 shows repolarization and the breakdown of the neurotransmitter. Fig. 5: Repolarization After repolarization and after the breakdown of acetylcholine, everything has returned to the original state. The positive ions are back to the outside of the membrane to the polarized condition. The acetylcholine is decomposed so the synapse region is clear once again. Now the same set of neurons can be used again. One Way Transmission The nervous system is designed to ultimately protect the body from harm. To ensure this, it is imperative that the impulse travel one direction through a neuron to arrive at the correct destination. If impulses were allowed to travel in any direction within a neuron, chaos would result. The impulse must travel toward the axonʼs presynaptic region in order to be able to successfully cross the synapse to continue its journey to its destination. Neurotransmitters The body produces about 30 different neurotransmitters. They are the chemical messengers important to the transmission of a nerve impulse. Most are made in the cytoplasm of the synaptic knobs and are stored in the synaptic vesicles (see fig. 10.18) Neurotransmitters are chemicals modified from amino acids or are themselves short chains of amino acids. Acetylcholine is one of the most common in skeletal muscle contractions. Other common ones include: epinephrine, norepinephrine, dopamine and serotonin (see Table 10.4) To Myelin or not to Myelin ( Myelinated vs Unmyelinated ) Unmyelinated axons conduct impulses over their entire surface. Myelinated axons function differently. Because myelin serves to protect and insulate the axon, it prevents all the flow of ions where it covers the axon ( see fig. 10.16). However, at the nodes of Ranvier, between the Schwann cells, the myelin is absent. At these nodes there are sodium and potassium channels that allow for action potentials. The action potentials appear to jump from node to node ( really the entire myelin covered Schwann cell is stimulated ). This type of conduction is called saltatory conduction. Nerve impulses along myelinated axons happen many times faster than conduction on unmyelinated axons. Also the larger the diameter of the axon, the faster an impulse travels through it. Ex. a thick, myelinated motor neuron of a skeletal muscle travels ~ 120 meters per second but one which is thin and unmyelinated, like those of a sensory neuron might only travel ~ 0.5 meters per second.