Nervous System PDF

Nervous system Nervous system The humannervous systemstarts toformvery early in the embryo\'sdevelopment, while final maturation is happening after a birth. Full functionality of the nervous system is established by myelination of nerve fibers and the formation of the blood-brain barrier. The ne...

Nervous system Nervous system The humannervous systemstarts toformvery early in the embryo\'sdevelopment, while final maturation is happening after a birth. Full functionality of the nervous system is established by myelination of nerve fibers and the formation of the blood-brain barrier. The nervous system is responsible for the reception and processing of sensory information from both the external and the internal environments. The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. A neuron can be compared to an electrical wire - it transmits a signal from one place to another. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Anatomy of a neuron Neurons vary in appearance, but all of them have three distinct structures: a cell body, dendrites, and an axon. The cell body contains the nucleus, as well as other organelles. Variousprocesses(appendages or protrusions) extend from the cell body. These include many short, branching processes, known asdendrites, and a separate process that is typically longer than the dendrites, known as theaxon. 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. Anatomy of a neuron The axon is the portion of a neuron that conducts nerve impulses. An axon can be quite long. The axon arises from the cell body at a specialized area called theaxon hillock. Many axons are covered with a special insulating substance calledmyelin, which helps them convey the nerve impulse rapidly. Myelin is never found on dendrites. Towards its end, the axon splits up into many branches and develops bulbous swellings known asaxon terminals(ornerve terminals). These axon terminals make connections on target cells. The gaps where there is no myelin sheath are called nodes of Ranvier. Anatomy of a neuron Unipolar neuronshave only one structure extending from the soma. Bipolar neuronshave one axon and one dendrite extending from the soma. Multipolar neuronscontain one axon and many dendrites. Pseudounipolar neurons are sensory neurons that have no dendrites, the branched axon serving both functions Glial cells They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. Types of glial cells are: astrocytes, oligodendrocytes, Schwann cells, microglial cells and ependymal cells. Astrocytes have several important jobs, including forming theblood-brain barrier (BBB). The BBB is like a strict security system, only letting in substances that are supposed to be in your brain while keeping out things that could be harmful. This filtering system is essential for keeping your brain healthy. Oligodendrocyte\'s main purpose is to help information move faster along axons. Microglial cells act as the brain\'s own dedicated immune system (phagocytosis). Schwann cells (PNS) function a lot like oligodendrocytes in that they provide myelin sheaths for axons. The resting membrane potential 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: Positively charged ions are stashed outside the cell, with negatively charged ions inside. In most neurons,K+ and organic anions (such as those found in proteins and amino acids)are present at higher concentrations inside the cell than outside. In contrast,Na+ and Cl- are usually present at higher concentrations outside the cell. This means there are stableconcentration gradientsacross the membrane for all of the most abundant ion types. Neurons have aresting membrane potential(or simply,resting potential) of about-30 mV to -90 mV. If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to bedepolarized. If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to behyperpolarized. Generation of Action Potentials All of the electrical signals that neurons use to communicate are either depolarizations or hyperpolarizations from the resting membrane potential. Anaction potential(AP) is the mode through which a neuron transports electrical signals. An action potential begins at the axon hillock as a result of depolarisation. Duringdepolarisationvoltage gated sodium ion channels open due to an electrical stimulus. As the sodium rushes back into the cell the positive sodium ions raise the charge inside the cell from negative to positive. Action potentials will only occur if a threshold is reached, as such they are described as all or nothing (then the maximum response will be elicited). Reflex The doctor tapping just below the knee activates a sensor that is located inside the muscle. Once the sensor is activated, the signal travels along the sensory neuron to the spinal cord. In the spinal cord, the signal crosses a synapse to a motor neuron and then travels back to the same muscle. This muscle contracts and causes the leg to kick forward. The brain is never involved in the case of a spinal reflex. The synapses Neurons communicate with one another at junctions calledsynapses. At a synapse, one neuron (thepresynaptic, or sending, neuron) sends a message to a target neuron (thepostsynaptic, or receiving, neuron) another cell (like a muscle or a gland). Most synapses arechemical - these synapses communicate using chemical messengers. Other synapses areelectrical - in these synapses, ions flow directly between cells. The synapses 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. Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synaptic vesicles in the axon terminals. The events at a synapse are: \(1) nerve signals traveling along an axon to reach an axon terminal; calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane; \(2) neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane there, neurotransmitter molecules bind with specific receptor proteins; \(3) Na diffuses into the receiving neuron and an action potential begins. https://youtu.be/cNaFnRKwpFk Division of human nervous system Central nervous system (CNS) The spinal cord and the brain make up the CNS, where sensory information is received, and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. The spinal cord and the brain are wrapped in protective membranes known as meninges. the outermost layer is the dura mater, the middle layer is the web-like arachnoid (arachnoidea) mater, and the inner layer is the pia mater, which directly contacts and covers the brain and spinal cord. The space between the arachnoid and pia maters is filled with cerebrospinal fluid (CSF). The brain floats in CSF, which acts as a cushion and shock absorber. The central nervous system (CNS) The central nervous system (CNS) consists of: The spinal cord (medulla spinalis), Medulla oblongata, Pons, Midbrain (mesencephalon), Cerebellum, Diencephalon, Cerebrum (telencephalon). Brain stem: The brain ventricles The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. The CNS is composed of two types of nervous tissue - gray matter and white matter. Gray matter (substantia grisea) contains cell bodies and dendrites. White matter (substantia alba) contains myelinated axons that run together in bundles called tracts. a\. Lateral view of ventricles seen through a transparent brain. b\. Anterior view of ventricles seen through a transparent brain. In the structure of the nervous system, in addition to the gray and white mass, there is also a reticular formation made up of numerous interconnected small nuclei. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle The spinal cord The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum and proceeds inferiorly in the vertebral canal. It is protected by vertebral column, cerebrospinal fluid and meninges. The spinal cord is segmentally organized. There are 31 segments, defined by 31 pairs of nerves exiting the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. The spinal cord The gray matter is centrally located and shaped like the letter H or a butterfly. The white matter of the spinal cord occurs in areas around the gray matter. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter (ascending tracts taking information to the brain). The ventral root of a spinal nerve contains motor fibers exiting the gray matter (descending tracts taking information from the brain). The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal, forming a mixed nerve. Many tracts cross just after they enter and exit the brain (in the medullary pyramids), so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body. The spinal cord As well as carrying motor and sensory signals between the brain and periphery, the spinal cord provides separate neural circuits for many of our reflexes - automatic, involuntary responses to sensory inputs. Some reflexes, such as the knee-jerk and the withdrawal reflex (for example, when touching something hot), are built into the nervous system and bypass the brain, while others can be learned over time. There are also vegetative centers in the spinal cord. The sympathetic centers are located in the thoracic lumbar region - they control the heart work, respiration and the digestive organs work. The parasympathetic centers are in the lumbar region and they control the colon, rectum and bladder emptying, as well as regulating some sexual functions (erection and ejaculation). The brain stem The three components of the brainstem are the medulla oblongata, midbrain, and pons. The medulla oblongata (myelencephalon) is the lower half of the brainstem continuous with the spinal cord. Its upper part is continuous with the pons. The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping and swallowing. The midbrain (mesencephalon) acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons lies between the medulla oblongata and the midbrain. It contains tracts that carry signals from the cerebrum to the medulla and to the cerebellum. It also has tracts that carry sensory signals to the thalamus. In addition, the pons function with the medulla oblongata is to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli. Midbrain Pons Medulla oblongata The cerebellum Thecerebellum(little brain) is a structure that is located at the back of the brain within the posterior cranial fossa. It lies at the same level and posterior to thepons, from which it is separated by the fourth ventricle. The cerebellum consists oftwo hemispheres which are connected by thevermis, a narrow midline area. Like other structures in the central nervous system, the cerebellum consists of grey matter and white matter: Grey matter located on the surface of the cerebellum. It is tightly folded, forming the cerebellar cortex. White matter located underneath the cerebellar cortex. Embedded in the white matter are the four cerebellar nuclei. The cerebellum The cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or football. Dysfunction of thecerebellumcan produce a wide range of symptoms and signs.The aetiology is varied; causes include stroke, physical trauma, tumors. Acute cerebellar ataxia (ACA) is a disorder that occurs when the cerebellum becomes inflamed or damaged. The termataxiarefers to a lack of fine control of voluntary movements, uncontrolled or repetitive eye movements, slurred speech, headaches, dizziness. ACA is also known as cerebellitis. CT scan of a left sided cerebellar stroke The Diencephalon Thediencephalon (interbrain)connects the midbrain to the forebrain. It is located deep within the brain and comprises the thalamus, hypothalamus, subthalamus, epithalamus and metathalamus. These parts are in a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, heart work, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems. The Diencephalon The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus participates in higher mental functions, such as memory and emotions. The epithalamus is the most dorsal of the structures of the diencephalon. Within the epithalamus are several important structures including the habenular nuclei and the pineal gland. The Cerebrum (Telencephalon) The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain. It consists of two cerebral hemispheres (left and right), separated by a deep groove called the longitudinal fissure. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts. The Cerebrum (Telencephalon) The cerebrum is comprised of two different types of tissue grey matter and white matter: Grey matterforms the surface of each cerebral hemisphere (known as the cerebral cortex) and is associated withprocessing and cognition. White matterforms the bulk of the deeper parts of the brain. It consists of glial cells and myelinated axons that connect the various grey matter areas. The characteristic appearance of the cerebrum is the result of thick folds, called gyri separated by shallow grooves called sulci. Central sulcus groove separating the frontal and parietal lobes. Lateral sulcus groove separating the frontal and parietal lobes from the temporal lobe. The Cerebrum (Telencephalon) The sulci divide each cerebral hemisphere into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe - control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe. The Cerebrum (Telencephalon) The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernickes area is located in the posterior part of the left temporal lobe. Wernickes area helps us understand both the written and the spoken word and sends the information to Brocas area. Brocas area is located in the left frontal lobe. Brocas area is primary motor area for speech musculature (lips, tongue, larynx, and so forth). Brocas area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing. The primary motor and primary somatosensory areas of the brain the primary motor area (blue) is located in the frontal lobe, the primary somatosensory area in the parietal lobe. the primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region. The Cerebrum (Telencephalon) The gray matter formations are also basal nuclei (basal ganglia). The basal nuclei are motor structures that help the motor cortex to perform movements, so that they become coordinated and smooth. Parkinson disease is believed to be caused by degeneration of specific neurons in the basal nuclei. Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body? Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side. The Cerebrum Left hemisphere performs tasks that have to do with logic, such as in science and mathematics. On the other hand, theright hemisphere performs tasks that have do with creativity and the arts. The peripheral nervous system (PNS) The peripheral nervous system (PNS), which lies outside CNS consists the cranial nerves and the spinal nerves. A nerve is composed of bundles of axons separated from one another by connective tissue. The nerves, which also contain connective tissue and blood vessels, reach out to the muscles, glands and organs in the entire body. Nerves of the peripheral nervous system are classified based on thetypes of neuronsthey contain - sensory, motor or mixed nerves (if they contain both sensory and motor neurons), as well as the direction of information flow towards or away from the brain. Ganglia are collections of nerve cell bodies outside the CNS. Cranial nerves Cranial nerves are pairs of nerves that connect yourbrainto different parts of your head, neck, and trunk. There are 12 of them, each named for their function or structure. Each nerve also has a corresponding Roman numeral between I and XII. This is based off their location from front to back. For example, your olfactory nerve is closest to the front of your head, so its designated as I. Cranial nerves can be sensory, motor and mixed nerves. I - Theolfactory nerve (n.olfactorius)transmits sensory information to your brain regarding smells that you encounter. II - Theoptic nerve (n. opticus)is the sensory nerve that involves vision. III - Theoculomotor nerve (n. oculomotorius) IV - Thetrochlear nerve (n. trochlearis) VI The abducens nerve (n. abducens) motor nerves that innervate the eyeball and eyelid muscles Cranial nerves V - Thetrigeminal nerve ( n. trigeminus)is the largest of all cranial nerves and has both sensory and motor functions. It innervates facial skin, mucous membranes of the lips and nasal cavity, teeth, eyeball parts , lacrimal glands, masticatory muscles. VII The facial nerve (n. facialis) is a mixed nerve, innervates mimic muscles, sensitive fibers transmit stimuli from tongue receptors; innervates both the lacrimal glands and the glands of the lips and nasal cavity. VIII - Thevestibulocochlear nerve (n. vestibulocochlearis)has sensory functions involving hearing and balance. IX - Theglossopharyngeal nerve (n. glossopharyngeus) is a mixed nerve that innervates the muscles of the pharynx, the sensitive part innervates the pharynx and tongue mucous membrane. Cranial nerves X - The vagus nerve (n. vagus) is a mixed cerebral nerve involved in the innervation of the most important organs and their functions. It participates in the innervation of the organs of the head and neck, chest and abdomen. XI - The accessory nerve(n. accessories) is a motor nerve that controls the muscles in your neck. XII - Thehypoglossal nerve (n. hypoglossus)is the 12th cranial nerve which is responsible for the movement of most of the muscles in your tongue. Spinal nerves Spinal nerves are mixed nerves that interact directly with the spinal cord to modulate motor and sensory information from the bodys periphery. Spinal nerves are 31 pairs of nerves formed by the union of the dorsal and ventral roots of the spinal cord on each side. The ventral root is motor, the dorsal root is sensitive. A sensitive ganglion is attached to it. The roots fuse to form a mixed nerve. This nerve passes through the opening (intervertebral foramen) between the two vertebrae and divides into its branches (anterior, posterior, meningeal, and rami communicantes). Each pair of spinal nerves corresponds to one segment of the spinal cord. The segments represent the center of the elementary reflex arc. Spinal nerves Spinal nerves form plexuses from which nerves are extracted to innervate certain body parts. The cervical plexus (plexus cervicalis)innerves the head, neck and shoulders. The most important motor branch is nerve for diaphragm. The brachial plexus (plexus brachialis)is responsible for cutaneous and muscularinnervationof the entire upper limb. Theintercostal nerves (nn. intercostales) supplyinnervation to the external andinternal intercostal muscles. Lumbal and sacral plexus (plexus lumbalis and plexus sacralis) innervate the muscles and skin of the lower extremities. The pudendal nerve (n. pudendus) and the coccygealplexus (plexus coccygeus) innervate the pelvic muscles and the genital skin. The autonomic nervous system (ANS) The autonomic nervous system (ANS) is also called thevegetative nervous system. It controls theinvoluntary functionsand influences the activity of internal organs. The autonomic nervous system is required for cardiac function, respiration, excretion, and otherreflexes, including vomiting, coughing, and sneezing, by influencing the reproductive organs enables species survival. The ANS can be divided into sympathetic and parasympathetic nervous systems. The sympathetic and parasympathetic nervous systems have opposite actions. The sympathetic nervous system carries out certain functions to prepare the body to make a decision within that moment. The symptoms normally experienced in a fight-or-flight situation are dilated pupils, accelerated heart rate, rapid breathing, and sweating. The autonomic nervous system (ANS) With these symptoms, it helps one to see and process information more clearly, transfer more blood to your muscles, and helps fight off whatever it is encountered. Sympathetic gangliacan be divided intotwomajorgroups, paravertebral and prevertebral, on the basis of their location within the body. Paravertebralgangliagenerally are located on each side of the vertebrae and are connected to form thesympatheticchain, or trunk. There are usually 21 or 22 pairs of these ganglia: \- 3 in the cervical region, 10 or 11 in the thoracic region, 4 in the lumbar region, 4 in the sacral region and a single unpaired ganglion lying in front of thecoccyx, called the ganglion impar. The autonomic nervous system (ANS) The parasympathetic nervous system has almost the exact opposite effect and relaxes the body and inhibits or slows many high energy functions. The effects of the parasympathetic nervous system can be summarized by the phrase \'rest and digest. The parasympathetic division, sometimes called the housekeeper division, promotes all the internal responses we associate with a relaxed state. For example, it causes the pupil of the eye to contract, promotes digestion of food, and slows heart rate. The centers of the parasympathetic nervous system are located in the brainstem and through III, VII, IX and X cranial nerve, it effects the head and neck organs, as well as the organs of the thoracic and abdominal cavities. Special parasympathetic center in the lumbal part of the spinal cord is starting point of fibers for the pelvis and the innervation of the pelvic organs. Damage and diseases of the nervous system The most common causes of damage and diseases of the nervous system are fractures of the spine or skull bones, bone and nerve tissue tumors, rupture or blockage of blood vessels, infections caused by viruses or bacteria. Paraplegiais the loss of sensation and movement of the lower extremities resulting from an injury to the spinal cord. Quadriplegia(also called tetraplegia) is the paralysis of all four limbs (from the neck down). Hemiplegia is paralysis of the muscles of the lower face, arm, and leg on one side of the body. Causes ofhemiplegiainclude stroke, trauma, brain tumors. Poliomyelitis (polio), is a highly contagious disease that is caused by a virus that attacks the body\'s nervous system. Polio largely affects children under 5 years old. QUESTIONS?