BioGen Lecture 7 PDF

Human Biology_Mader Dr. Ermira Hodo  In the nervous system: 1. Reception of stimuli is associated with sensory neurons; 2. Integration is associated with interneurons; 3. Motor output is associated with motor neurons. *All neurons use the same methods to transmit nerv...

Human Biology_Mader Dr. Ermira Hodo  In the nervous system: 1. Reception of stimuli is associated with sensory neurons; 2. Integration is associated with interneurons; 3. Motor output is associated with motor neurons. *All neurons use the same methods to transmit nerve impulses along neurons and across synapses. Dr. Ermira Hodo The Central Nervous System The central nervous system consists of: 1) The brain 2) The spinal cord - The brain is divided into portions, each with specific functions, and the spinal cord communicates with the brain. - The spinal cord provides input to, and output from, the brain. Dr. Ermira Hodo The Limbic System and Higher Mental Functions The limbic system involves many parts of the brain (amygdala and hippocampus) It gives emotional overtones to the activities of the brain, and it is important in the processes of learning and memory. Dr. Ermira Hodo The Peripheral Nervous System The peripheral nervous system consists of nerves that project from the CNS. Cranial nerves project from the brain. The spinal cord gives rise to spinal nerves.  Sarah case as an example to understand the nervous system functioning One day Sarah notice the traffic red light to be more orange than normal. She had headache on the same day. After few weeks Sarah lost sight from one eye, lost the sensation of the feet The neurologist ordered Magnetic Resonance Imaging (MRI) and Somatosensory Evoked Potential (SSEP) to find out how her nervous system was processing electrical impulses Sarah was diagnosed with Multiple Sclerosis (MS), an inflammatory disease This conditions affect the myelin sheath around neurons, thus impusles are not conducted normally. MS affects firstly the optic nerves (for unknown reasons) and proceeds with other areas of the brain. There is no cure but immunosuppresive drugs are prescribed in such cases. Dr. Ermira Hodo  Sarah case as an example to understand the nervous system functioning Think about the following questions: 1.Why would the deterioration of the myelin sheaths cause a nerve cell to function incorrectly? 2.How would an MRI and SSEP test indicate that there was a problem with Sarah’s neurological functions? 3.Why are many individuals who contract MS eventually confined to a wheelchair? Overview of the nervous system: Nervous system is responsible for the: 1) Reception & 2) Processing of sensory information from both internal and external environment Dr. Ermira Hodo The Nervous system is divided into: 1. Central Nervous System (CNS) (brain and spinal cord) 2. Peripheral Nervous System (PNS) (consist of nerves) Dr. Ermira Hodo Three main functions of Nervous System: CNS receives nerve impulses from sensory receptors like those of skin, tongue and sight. 1. Nerve signals are carried from PNS to CNS (ex: olfactory receptors transmit the smell of biscuits through PNS to CNS). 2. CNS processes and integrates the information (reviews, stores the info as memory and creates the motor response). 3. CNS generates motor output via the nerves in CNS which go through PNS nerves to muscles, glands and organs, (ex: all in response to the smell of the cookie). Signals to the salivary gland induce the production of saliva, stomach produces acid and enzymes needed to digest the biscuit, even before having the cookie. CNS coordinates the movement of your muscles and body as you reach the biscuit. Dr. Ermira Hodo  Nervous tissue contains two types of cells: 1) neurons: cells that transmit nerve impulses between parts of the nervous system 2) neuroglia (sometimes referred to as glial cells): support and nourish neurons ex: Microglia are phagocytic cells that help remove bacteria and debris ex: Astrocytes provide metabolic and structural support directly to the neurons ex: Oligodendrocytes form the myelin sheath in CNS ex: Schwann cells form the myelin sheath in PNS Dr. Ermira Hodo Anatomy of the neuron Classified according to function, the three types of neurons are: 1) sensory neurons: takes nerve signals from a sensory receptor to the CNS 2) İnterneurons: lies entirely within the CNS. Interneurons can receive input from sensory neurons and from other interneurons in the CNS 3) motor neurons: takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland) - Effectors carry out our responses to environmental changes, whether these are external or internal. Dr. Ermira Hodo Neurons vary in appearance, but all of them have three distinct structures: 1) A cell body: contains the nucleus, as well as other organelles 2) Dendrites: are short extensions that receive signals from sensory receptors or other neurons. Incoming signals from dendrites can result in nerve signals that are then conducted by an axon. 3) An axon: is the portion of a neuron that conducts nerve impulses. An axon can be quite long. Individual axons are termed nerve fibers, and collectively they form a nerve. In sensory neurons, a very long axon carries nerve signals from the dendrites associated with a sensory receptor, to the CNS, and this axon is interrupted by the cell body. In interneurons and motor neurons, on the other hand, multiple dendrites take signals to the cell body, and then an axon conducts nerve signals away from the cell body. - Many axons are covered by a protective myelin sheath. - Myelin sheath covers only a portion of the axon, which means that it is interrupted by meylin-absent portions named as ‘’nodes of Ranvier’’ - Long axons tend to have myelin sheath while short ones do not. The gray matter of the CNS is gray because it contains no myelinated axons; the white matter of the CNS is white because it does. - Function: it serves as an insulator - When myelin sheath is damaged/absent (as in the case of MS) it is difficult for the nerve cell to transmit information. - When the axon get damaged, myelin sheath protects the axon by serving as a passageway for new fiber growth (regeneration) - A resting neuron also has potential energy, much like a fully charged battery. This energy, called the resting potential, exists because the plasma membrane is polarized: More positively charged ions are concenntrated outside the cell, and more negatively charged environment inside. - The outside of the cell is positive because positively charged sodium ions (Na+) gather around the outside of the plasma membrane. At rest, the neuron’s plasma membrane is permeable to potassium, but not to sodium. Thus, positively charged potassium ions (K+) contribute to the positive charge by diffusing out of the cell to join the sodium ions. The inside of the cell is negative in relation to the exterior of the cell because of the presence of large, negatively charged proteins and other molecules that remain inside the cell because of their size. - A protein carrier in the membrane, called the sodium-potassium pump, pumps sodium ions (Na+) out of the neuron and potassium ions (K+) into the neuron. The sodium-potassium pump returns the ions to their resting positions, after an action potential is generated. Dr. Ermira Hodo Action potential - The process of conduction of the signal is termed an action potential, and it occurs in the axons of neurons. - A stimulus activates the neuron and begins the action potential. (Ex: a stimulus for pain neurons in the skin would be the prick of a sharp pin.) However, the stimulus must be strong enough to cause the cell to reach threshold, the voltage that will result in an action potential, around −55 mV. An action potential is an all-or-nothing event. Once threshold is reached, the action potential happens automatically and completely. - On the other hand, if the threshold voltage is never reached, the action potential does not occur. Increasing the strength of a stimulus (such as pressing harder with the pin) does not change the strength of an action potential. However, it may cause more action potentials to occur in a given period; as a result, the person may perceive that pain has increased. Dr. Ermira Hodo 1. Sodium Gates Open Protein channels specific for sodium ions are located in the plasma membrane of the axon. When an action potential begins in response to a threshold stimulus, these protein channels open and sodium ions rush into the cell. Adding positively charged sodium ions causes the inside of the axon to become positive compared to the outside (Fig. 14.3c). This change is called depolarization, because the charge (polarity) inside the axon changes from negative to positive. 2. Potassium Gates Open Almost immediately after depolarization, the channels for sodium close and a separate set of potassium channels opens. Potassium flows rapidly out of the cell. The inside of the cell becomes negative again because of the presence of large, negatively charged ions trapped inside the cell. This change in polarity is called repolarization, because the inside of the axon resumes a negative charge as potassium exits the axon (Fig. 14.3d). 3. Finally, the sodium-potassium pump completes the action potential. Potassium ions are returned to the inside of the cell and sodium ions to the outside, and resting potential is restored. Dr. Ermira Hodo Propagation of an Action Potential - If an axon is unmyelinated, an action potential at one locale stimulates an adjacent part of the axon membrane to produce an action potential. Conduction along the entire axon in this fashion can be rather slow because each section of the axon must be stimulated. - In myelinated fibers, an action potential at one node of Ranvier causes an action potential at the next node, jumping over the entire myelin-coated portion of the axon. This type of conduction is called saltatory conduction (saltatio is a Latin word that means “to jump”) and is much faster. In thick, myelinated fibers, the rate of transmission is very high (more than 100 m/s). Regardless of whether an axon is myelinated or not, its action potentials are self-propagating. Each action potential generates another, along the entire length of the axon. - It is interesting to note that all functions of the nervous system, from our deepest emotions to our highest reasoning abilities, are dependent on the conduction of nerve signals. Dr. Ermira Hodo The Synapse - Every axon branches into many fine endings, each tipped by a small swelling called an axon terminal. Each terminal lies very close to either the dendrite or the cell body of another neuron. This region of close proximity is called a synapse. - At a synapse, a small gap called the synaptic cleft separates the sending neuron from the receiving neuron. The nerve signal is unable to jump the cleft. Therefore, another means is needed to pass the nerve signal from the sending neuron to the receiving neuron. - Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synaptic vesicles in the axon terminals.  Signal Transmission - (1) nerve signals traveling along an axon to reach an axon terminal; - (2) calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane; and - (3) neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane; - neurotransmitter molecules bind with specific receptor proteins. - Depending on the type of neurotransmitter, the response of the receiving neuron can be toward excitation or toward inhibition. In Figure 14.5, excitation occurs because the neurotransmitter, such as acetylcholine (ACh), has caused the sodium gate to open. Sodium ions diffuse into the receiving neuron. - Inhibition would occur if a neurotransmitter caused potassium ions to exit the receiving neuron.  Signal Transmission - Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft. In some synapses, the receiving membrane contains enzymes that rapidly inactivate the neurotransmitter. (Ex: the enzyme acetylcholinesterase (AChE) breaks down the neurotransmitter acetylcholine.) - In other synapses, the sending membrane rapidly reabsorbs the neurotransmitter, possibly for repackaging in synaptic vesicles or for molecular breakdown. - The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of receiving membranes. The receiving cell needs to be able to respond quickly to changing conditions. If the neurotransmitter were to linger in the cleft, the receiving cell would be unable to respond to a new signal from a sending cell. Drug Therapy Although neurological drugs are quite varied, each type has been found to: 1. promote, 2. prevent, or 3. replace the action of a particular neurotransmitter at a synapse. Dr. Ermira Hodo - Neurons do usually have synapses with multiple other neurons. Its is the combinatorial effect of the signals which induces or not the Action Potential. - Integration is the summing up of excitatory and inhibitory signals. - Inhibitory neurotransmitter drives the neuron farther from an action potential (red line in Fig. 14.5b) by opening the gates for potassium. - If a neuron receives more inhibitory than excitatory signals, summing these signals may prohibit the axon from reaching threshold and then depolarizing Functions of the Spinal Cord - The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. - Ex: When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (red arrows). - Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. - The spinal cord creates reflex arcs for the internal organs. Ex: when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the spinal cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises. Dr. Ermira Hodo The Peripheral Nervous System The peripheral nervous system (PNS), which lies outside the central nervous system, contains the nerves. Nerves are designated as cranial nerves when they arise from the brain and spinal nerves when they arise from the spinal cord. In any case, all nerves carry signals to and from the CNS. So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text. Humans have 12 pairs of cranial nerves attached to the brain. Some cranial nerves are sensory nerves— they contain only sensory fibers; some are motor nerves that contain only motor fibers; others are mixed nerves that contain both sensory and motor fibers. Cranial nerves are largely concerned with the head, neck, and facial regions of the body. However, the vagus nerve (X) has branches not only to the pharynx and larynx but also to most of the internal organs.