Nervous System Structure and Function PDF

-Body has two communication and control systems called nervous system and the endocrine system ​ The nervous system chemical messengers that it uses to help the body communicate with each other and they are made by neurons which are nerve cells called neurotransmitters -The neurotra...

-Body has two communication and control systems called nervous system and the endocrine system ​ The nervous system chemical messengers that it uses to help the body communicate with each other and they are made by neurons which are nerve cells called neurotransmitters -The neurotransmitter travels across spaces between nerve cells called synapses. -The neurotransmitters can react fast travling between the synapses, but there is a limited supply of them so they cannot sustain individual activity for a long time. -Another communication the body has is the endocrine system aka hormones. The hormones can secreate directly into the bloodstream where they can travel long distance to rach their targets. They do react more slowly to changes compare to the nerous sytstem, but hormons can be secreated for a long period of time, so they can substain individual activities for a long time -The nervous system has two main divisions which are the central nervous system(CNS) and the peripheral nervous system (PNS) -The central nervous system (CNS) is composed of the brain and spinal cord. -The peripheral nervous system (PNS) is made of cordlike nerves that link the centeral nervous system (CNS) with the rest of the body -The nervous system falls in three main categories which are 1. sensory functions, (2) integrating functions, and (3) motor functions. The nervous system can gather information from outside the body or in the body by sensing changes and it tells these information to the spinal cord and brain. When the brain and spinal cord get the information they analyzed it and make a response. -Motor response from the brain and spinal cord tells the body to do something, like contract a muscle or cause gland to secreate its products -Neurosns are nerve cells. They are the smallest part of the nervous system that shows basic nervous system functions. They are able to respond to stimuli and conduct impulses from one part of the cell to another. They also can’t live with out oxygen, so thats why a living being that has a cardiac arrest could dangmage the brain beacues neurons cant last long with out oxygen. Neurosns are not able to reporduce so thats why danmages in the cell is not reparable unless the cell body remains intact which allows the cell to regenerate. -Since Neurons are very important there is something called neuroglia or glial cells which means glue, but they are there to structurally and functionally support and protect the neurons. There is alot of more of neuroglia/ glial cells but they do not transport infromation or impulses through the the nervous system. Their jobs is just to support and protect the neurons A neuron is build by a cell body also called soma or perikaryson. The cell body has two different process extensions called dendrites and axons Dendrites they receive stimuli or impulses from other neurons and send that information to the cell body. They are also called afferent processess beacues their job is to send impulses to the cell body. Dendrites can also be modified into sensory receptors meaning that they can also receive, or sense stimuli, such as heat, cold, touch, pressure, stretch, or other physical changes from inside or outside the body. Dendrites look like branches of a tree they also tend to be short and there is numerous amount of them. -Axon conduct nerve impulses away from the cell body and instead they send impulse towards another neuron (nerve cell) or to an effector cell which is a cell that does something when it has been stimulated, like muscles and gland cell. Axon is also called efferent beacues they carry impulse alway from the cell body. Axon are very long. For example, a single axon is the horse may extend from the spinal cord all the way down the lower leg. Axons are also calld nerve fibers and that term is used when talking about them when they are in a buddle and make up a corelike structure in the body. -Axons somethimes are cover by sheath of a fat substance called myelin. Myelin looks white for that reason tissuse that has a lot of myelin is referred as white matter. There is also gray matter and that is tissue that has a lot of neuron cell bodies. -Myelin sheath are made of of the cell membranes of specialized glial cells called oligodendrocytes and schwann cells. Oligodendrocytes are in the spinal cord and brain and schwann cells are in the nerves outside of the brain and spinal cord. These special gliasl cells are wrap around the axon. In between the glial cells that are wrapped around the axon there is small gaps in the myelin sheath called nodes of ranvier The myelin sheath and nodes of ranvier together they speed up the nerve impulses along the axon. Axons thats are myelinaed are faster than unmyelinaed axons. -The centeral nervous system (CNS) and the peripheral nervous system (PNS) are the two components of the nervous system. CNS is composed of the brain and spinal cord which are found along the central axis of the body. PNS is made of components of the nervous system that extend away from the central axis outward to the periphery of the body. The only PNS nerves that originate from the brain are called cranial nerves, but most PNS nerves are spinal nerves that emerge from the spinal cord -Some nerve fibers (budddle of axon) conduct impulses from the peripheral toward the CNS, and other nerve fibers conduct impulses in the opposite direction, from the CNS toward the periphery. These two functional type of nerve fiber are called afferent nerve fibers and efferent nerve fibers. Afferent nerve fibers conduct nerve impulses towards the CNS, wheras efferent nerve fibers conduct nerve impulses away from the CNS. -Afferent nerve fibers are able to conduct sensations from the senory receptors in the skin and other locations in the body to the centeral nervous system (CNS). For that reason afferent nerve fibers are usually called sensory nerve fibers. -Efferent nerve fibers conduct impulses from the centeral nervous system (CNS) out toward muscles and other organs. Efferent nerves impulses cause the skeletal muscle contraction and movement, so thats why efferent nerve fibers are usually called motor nerve fibers. -There is also mixed nerves which have both motor nerve fibers and sensory nerve fibers. Most nerves in the PNS are mixed nerves. -When an animal turns its head in response to it name being called by its owner, efferent (outgoing) motor impulses from the brain are sent to the muscle in the neck to turn the head toward the sound. Becuse this action was a conscious or voluntary this means that the control of skeletal muscle is referred to as somatic nervous system function. Impulses being sent to the central nerves system (CNS) from receptors in the muscles, skin, eyes, or ears are somatic sensory functions, beacuse they are consciously perceived by the brain. -There also autnomic nervous system which is the part of the nervous system that controls and coordinates automatic functions. For example, animals do not consciously have to think to contract their intestines, increase their heart rate in response to a threat, or simulate release of digestive juices in response to ingestion of a meal. Blood pressure receptors informing the body that the blood pressure is too low or of stretch receptors indicating that the lungs have inflated. -Somatic sensory and autonomic system both have motor nerves and sensory nerves. However, instead of the motor nerves and senory nerves going to the skeletal muscles to cause voluntary limb or body movment like somatic senory, the autonomic motor nerves send impulses to smooth muscle, cardiac muscle, glands to regulate different anutomatic body functions -when a neuron is not being stimulated it is called resting state, however it is still working to keep it self at the resting state. The cell membranes of the neurons are electrically polarized when its in rest state, which are like tiny charged up batteries. -specialized molecules located in the neuron’s cell membrane pumps sodium ion (Na+) from inside of the neuron to the outside of the neuron and pumps potassium ion (K+) from the outsite to the inside of the neuron. Those specialized molecules are called sodium-potassium pump -A higher accumulate of sodium will occur outside the cell beacues sodium (Na+) cannot diffuse or leak through the cell membrane on its own. On the inside of the cell there is a higher concentration of potassium (K+) beacuse of the actions of the sodium-potassium pump which casues Na+ to go out of the cell and K+ to go from out of the cell to in and also the negative charges from inside of the cell also cause K+ to be higher inside of the cell. Keeping the sodium in the outside and potassium in the inside the cellular membrane is said to be polarized beacues it has two distinct poles of ions on either side of the membrane. This electrical difference in charges across the membrane is called the resting membrane potential. It keeps the membrane batteries charge. -when a neuron is stimulated by a signal from another neuron or an external tigger like heat, touch, or taste, it “fires” or depolarizing. This happens because a special structure on the neuron’s surface, called a sodium channel, opens where the stimulus occurs. -The sodium channel allows only sodium ions (Na+) to pass through it. Because there is a higher a amonut of sodium ions that live outside compare to the inside of the cell. The sodium will flow inside of the cell using the open sodium channel by passive diffusion. -Depolarization means the opening of the sodium channels and the sudden movement of many sodium ions into the cell. -During depolarization we lose the two distinct poles of sodium and potassium in either side of the membrane. Sodium will move inside the cell making it more positive and the outside more negative. The change of charges from negative and positive is also called action potential. -The sodium channel will come to a hold and close up. At the same time the potassium channel will open up and the potassium will move outside of the cell. This is called repolarization beacues it is going back to sodium and potassiums being oposite sides (oposite poles). Which means it goes back to resting state -For the molecules to complete a depolarization-repolarization cycle the inital stimulus it must be strong enough to make the neuron respond. when the stimulus is strong enough to complete depolarization it is said to have reached the threshold and that causes the cell to depolarize or fire. A strong stimulus is called a threshold stimulus -The waves of sodium channels opeing when threshold has been reach its called wave of depolarization. The wave of depolarization can also be called conduction of the action potential and in clinical terms, nerve impulse. -all-or-nothing-principle says either the neuron depolarized to its maximum strength or does not depolarize at all. -refractory period is for a very brief period during and after a neuron has generated a nerve impulse it cant generate another impulse. During the exceution and learing phase of depolarization or repolarization a second one cant not occure. It needs to recover before another one can happen. However the cell can depolarize again if a very strong stimulus comes during last minute the membrane is repolarizing and restoring the resting memebrane potential, it maybe able to do another depolarization. -The period of absoulte refractory period tells us that no matter how strong, it can cause the cell to depolarize again -The myelin sheath that is wrapped around the axon prevents sodium ions from flowing across the neuronal cell membrane. So that means that sodium ions can only travel at the gaps in the myelin known as nodes of ranvier. When enough sodium enters at one node of ranvier, it triggers the opeing of sodium channels at the next node. In myelinated axon this causes the electricle signal to jump from one node to the next, making it travel much faster down the axon. This quick way of transmitting signals is callled saltatory conduction -When the impulse or the action potential has been successfully conducted to the end of the axon it then transmite it to the next neurin or cell of the target organ or tissue. The two adjecent neurons do not touch each other the neurons release a chemical that stimulates the next neuron or cell. the action of this is called synaptic transmission -A synapse is where two neurons or a neuron and another cell connect. There is a small gap between them called the synaptic cleft. The neuron that sends the signal and releases chemicals to activate the next cell is called the presynaptic neuron. -the chemical released by the preynaptic neuron is called the neurotransmitter, and the neuron that has the receptors that are given then neurotransmitter is the postsynaptic neuron -If we look at end of the axon on the presynaptic neuron, there is a bracnched struture called telodendorn. Each telodenron ends in a slightly enlarge bulb called terminal bouton or symaptic end bulb or synaptic knob. They provide energy and when the axon’s wave of depolarization reaches the synapic knob, calcium channels open in the knob’s cellular membrane which result in a influx of calcium in the synaptic knob. This causes the vesicles that have the neurotransmitters to fus with the knob’s cellular membrane and dumb its contents into the synaptic cleft. The neurotransmitters diffuse fast across the synaptic cleft and towards the postsynaptic membrane. -The poststynaptic membrane has specialized proteins called receptors. The neurotransmitter molecules that are released from the synaptic knob bind to the receptors and trigger a change in the postsynaptic cell. However the postsynaptic cells are very specific about which neurtotransmitters they will bind. If the neurotransmitter and receptor are not a match nothing will happen in the postsynaptic cell. -many diffrent types of neurotransmitter are associated with synapses in both the CNS and PNS. We can classify them into two categories: excitatory neuritransmitters and inhibitory neurotransmitters. -Exciatory neurotransmitters have an excitatory effect on the postsynaptic membrane when they combine with their specific receptors. This helps beacues it causes an influx of sodium so that the postsynaptic membrane moves toward threshold. If there is enough excitaory neurotransmitters the threshold will reach and depolarization of the postsynaptic membrane will happen. -inhibitory neurotransmitters tend to hyperpolarize the postsynaptic cell farther away from the threshold. This makes it harder for the postynaptic membrane to begin a new impuse -Acetylcholine can either be excitaory or inhibitory depending where it is in the body. If the acetylcholine is in the muscle it is excitatory and it stimulates muscle fibers to contract. -in the site where parasympathetic nerves synapse with the heart, the acetycholine has an inhibiory effect that slows the heart rate -Norepinephrine, dopamine, and epinephrine are neurotransmitters that belong to a group called neutrotransmitters that belongs to a group called catecholamines. -Norepineprine is associated with fight and flight in the sympathetic nerves system -Epinephrine, is a hormone in the fight and flight in the sympathetic nerves system -Dopamine inovlved with autonomic functions and muscle control -Acetycholine is a neurotransmitter broken down quickly by an enzyme called acetylcholinesterase found on the postsynaptic membrane. The broken-down components of acetylcholine are reabsorbed by the synaptic knob, ressembled into new acetylcholine molecules, and repackaged into vesicles for release with the next wave of depolarization. Gamma-aminobutyric acid (GABA) and gycine are two neurotransmitters that are inhibitory. GABA are found in the brain, and glycine is found in the spinal cord -the brain is divided as the cerebrum, cerebellum, diencephalon and brainstem -diencephalon and brainstem are the more primitive part of the brain -cerebellum coordinates motor control. Higher learning and intelligence are found in the cerebellum -cerebrum is made of gray matter in the cerebral cortex which means the outermost superficial part of the brain and white matter beneath the cortex, including corpus callosum which is a set of fibers that connects the two halves of the cerebral cortex. Its the larget part of the brain and is associated with higher-order behaviors like learing and reasoning and intelligence. -gyri or gyrus are the wrinkles in the brain that help make room for the gray matter by increassing the size of the cerebral cortex -deep grooves in the brain called fissure and shallow grooves called sulci or sulcus. The biggest groove is the longitudinal fissure that divides the cerebrum from left and right -Cerebellum is the part of the brain that helps with coordinated movement, balance, posture and complex reflexes. The cerebellum also has gray matter in the cortex and white matter beneath it. It also has wrinkles. -Cerebellum. The cerebellum compares the movement the body intends with the actual position of muscles and joints to determine if the intentions of the cerebral cortex are actually being carried out. If the movements are not being carried out accurately, the cerebellum will stimulate or inhibit muscles to fine-tune the movments. -Diencephalon does not have white and gray matter. it serves as a nervous system passageway between the primitive brainstem and the cerebrum. Which gets its name the between brain. -thalamus part acts as a relay station for regulating sensory inputs to the cerebrum -hypothalams is an interface between the nervous system and the endocrine system -pituitary is the endocrine “ master gland” that regulates productions and release of hormones throughout the body -the brainstem is composed of the medulla obongata, the pons and midbrain. The brainstem job is to maintain basic support functions of the body. It is involved in autonomic control functions related to the hear, blood vessel, swallowing, and vomiting. Crainal nerves originate from this part of the brain. -meninges are a set of connective tissue layers that surround the brain and spinal cord. -three layers are dura natter, arachnoid, pia matter -they provide cushioning and protection and nurtients to the CNS The brain and spinal cord are bathed and protected from the hard inner surfaces of the skull and spinal column by a fluid called cerebrospinal fluid (CSF) -Blood-brain barrier is a functional barrier separating the capillaries in the brain from the nervous tissue itself. The capillary wall in the brain is aligned tightly together with out opeings. This stops drugs, proteins, ions and other molecules from readily passing from the blood into the brain -Cranial nerve I (CN I) has olfactory nerve. job: impulses from receptors for the nose to the brain for the sense of smell -cranial nerve II (CN II) has optic nerves, Job: perception of light and vision -crainal nerve III (CN III) is the oculomotor nerve: it is the motor crainal nerve that controls the eye and other cranial movments. -crainal nerve V (CN V) is the trigeminal nerve: it controls the muscles in the jaw sor chewing and convey sensations from the nose, mouth and part of the throat. ​ What are the 12 crainal nerves? Which nerves are motor, which are sensory, and which are both? 1.​ Olfactory: sensory  2.​ optic: sensory 3.​ Oculomotor: motor 4.​ Trochlear: motor 5.​ trigeminal : both 6.​ abducent: motor 7.​ facial: both sensory and motor 8.​ vestibulocochlear: sensory 9.​ Glossopharyngeal: both sensory and motor 10.​vagus: both sensory and motor 11.​Accessory: motor 12.​Hypoglossal: motor -The spinal cord has gary matter. It conducts sensory information to the brain and descending nerve fibers conducting motor information to muscles and other organs -the automatic function are performed by two divisions by sympathetic nervous system: the sympathetic nervpus system and the parasympathetic nervouse system -sympathetic nervous system is often called the fight-or-flight system. -parasympathetic nervous system is called the rest-and-restore system -In the parasympathetic and sympathetic nervous sytem the ganglia lie longer for the parasympathetic compare to the sympathetic which are shorter in preganglionic and the oposite during postganglionic -The neurons associated with the parasympathetic nervous system secrete acetylcholine as their neurotransmitter. Thats why those neurons are called cholinergic neurons. -cholinergic receptors come in different types. The different types of acetylcholine receptors are called nicotinic and muscarinic. -nicotinic receptors are found in the postganglionic neurons of the parasympathetic and sympathetic nervous systems as also found in the motor neurons and muscle in the somatic (voluntary) motor system. -muscarinic receptors are found on the target organs and tissue supplied by the postganglionic neurons of the parasympathetic nervous system. Reflexes are rapid, automatic responses to stimuli designed to protect and maintain homeostasis -Reflexes can be somatic reflexes which involve contraction of the skeletal muscle -or autonomic reflexes which regulate smooth muscle, cardiac muscle, and endocrine glands A sensory receptor detects a change either in the external environments or within the body itself. Once stimulated to threshold, the sensory receptor sends an action potential (nerve impulse ) along the sensory neuron to the gay matter of the spinal cord or brain stem. In the CNS gray matter, the sensory neuron synapses with other interneurons, which serve to integrate the incoming sensory impulse with other impulses from other sensory neurons. Finally, the integrated response of the reflex is sent out from the spinal cord or brain stem by the motor neuron, which ends at the target organ ( muscle or endocrine gland. The sensory receptor in the stretch reflex arc is a specialized structure within the muscle called the muscle spindles. Muscle contraction results from the stretch arc stimulation. Ipsilateral in a withdrawal reflex A strong stimulus to a receptor causes the sensory somatic neuron to send impulses to the spinal cord. This reflex involves synapsing with several interneurons. Some of these interneurons will synapse with motor neurons that will cause contraction of a specific set of muscles responsible for pulling the limb away from the painful stimulus. Other interneurons will inhibit those opposing muscle groups so that the withdrawal of the limb is rapid and complete. This reflex is more complex than a stretch reflex. -Under normal conditions, the patellar ligament stretching usually only produces a relatively small kick because of the damping effect of the upper CNS. With spinal cord trauma, this damping effect from the upper CNS is blocked and any intact reflex arcs caudal to the spinal cord trauma are now exaggerated, or hyperreflexive -If spinal cord trauma occurs where the reflex arc enters or leves the spinal; cord, or if the senosry nerve or motor nerve of the ref;ex arc id damaged, the reflex will be less then normal (hyproreflexive) The palpebral reflex arc originates from receptors on the eyelid margins, travels via sensory neurons in CN V to the pons (brain stem), synapses with neuronsin the pons, and then travels via CN VII to the muscles that blink the eyelids. If the reflex is active, a light tap on the medial canthus of eye (the medial corner of the eye where the top and bottom eyelids meet) produces a blink of the eyelids. When an animal is anesthetized, the neurons in the pons become less responsive. Therefore, as anesthesia deepens, the palpebral reflex ( also called the palpebral blink reflex) becomes less responsive and provides an indication of the animal’s depth of anesthesia. A normal response to shining a light into the eye od an animal (pupillary light reflex [PLR]) is for the iris in both eyes to constrict, thus making both pupils smaller