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The Nervous System The nervous system is the master controlling and communicating system of the body. Every thought, action, and emotion reflects its activity. Its cells communicate by electrical and chemical signals, which are rapid and specific, and usually cause almost immediate responses. Functions of the Nervous System Sensory input – gathering information To monitor changes occurring inside and outside the body Changes = stimuli Integration To process and interpret sensory input and decide if action is needed Motor output A response to integrated stimuli The response activates muscles or glands For example, when you are driving and see a red light ahead (sensory input), your nervous system integrates this information (red light means “stop”), and your foot goes for the brake (motor output). Structural Classification of the Nervous System Two divisions: Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Cranial nerves Spinal nerves Functional Classification of the PNS It divides them into TWO subdivisions: 1-Sensory (afferent) division Nerve fibers that carry information to the central nervous system from: - sensory receptors in the skin, skeletal muscles and joints (somatic sensory fibers). - Sensory receptors in the visceral organs (visceral sensory fibers) 2-Motor (efferent) division Nerve fibers that carry impulses away from the central nervous system ( to Muscles &Glands). It has two subdivisions 1-Somatic nervous system = voluntary, it controls skeletal muscles N. B. skeletal muscle reflexes are involuntary 2-Autonomic nervous system = involuntary, it controls smooth &cardiac muscles &glands This also is divided into: - sympathetic & - parasympathetic Organization of the Nervous System Histology of Nervous Tissue • Despite the complexity of the nervous system, there are only two functional cell types: • Neurons - excitable nerve cells that transmit electrical signals • Neuroglia (glial) cells (Astrocytes, Microglia, Ependymal cells, Oligodendrocytes ) are the supporting cells. Neuroglia cells - 2 types in the Peripheral NS Satellite cells - surround neuron cell bodies in the periphery - Protective , cushioning cells Schwann cells (neurolemmocytes) • surround axons/dendrites and form the myelin sheath around larger nerve fibers in the periphery • similar to oligodendrocytes in function – insulators Neuron (Nerve cell) Cells specialized to transmit messages Differ structurally but have common features, but all have : A Cell body with nucleus and the usual organelles Except centrioles Extensions (proceses) outside the cell body Dendrites – conduct impulses toward the cell body Axons – conduct impulses away from the cell body Neuron Anatomy Axons and Nerve Impulses Axons end in axonal terminals Axonal terminals contain vesicles which is full of chemical transmitter called neurotransmitters Axonal terminals are separated from the next neuron (neuroneural junction) by a gap called Synaptic cleft (Synapse) Nerve Fiber Coverings *Most long nerve fibers are covered with a whitish, fatty material called Myelin with waxy appearance. It : -insulates the fiber & -Increases transmission rate *Axons outside CNS are wrapped by Schwann Cells Figure 7.5 Neuron Cell Body Location Most are found in the central nervous system so CNS appears in two colors: Gray matter – cell bodies and unmyelinated fibers Nuclei – clusters of cell bodies within the white matter of the central nervous system Ganglia – collections of cell bodies outside the central nervous system White matter- collection of myelinated fibers. Functional Classification of Neurons 1-Sensory (afferent) neurons carry impulses from : Cutaneous sense organs (skin receptors) Proprioceptors – receptors that detect stretch or tension in muscles and tendons and joints to the CNS. Their cell bodies are in ganglia outside the CNS. 2-Motor (efferent) neurons carry impulses from the central nervous system to muscles and glands ,their cell bodies are always in CNS. 3-Interneurons (association neurons) Their cell bodies are always found in CNS. Connect sensory and motor neurons in neural pathways. Neuron Classification Figure 7.6 Functional Properties of Neurons Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse The plasma membrane at rest is polarized that is: Fewer positive ions are inside the cell than outside the cell. Starting a Nerve Impulse Depolarization – a stimulus depolarizes the neuron’s membrane A deploarized membrane allows sodium (Na+) to flow inside the membrane The exchange of ions initiates an action potential in the neuron The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane The sodium-potassium pump restores the original configuration This action requires ATP Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerve Neurotransmitter is released from a nerve’s axon terminal The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter So an action potential is started in the dendrite of the next neuron. How Neurons Communicate at Synapses Figure 7.10 The Reflex Arc Reflex – rapid, predictable, and involuntary responses to stimuli. Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector Ref. arc have a minimum 5 elements: Simple Reflex Arc Figure 7.11b, c Types of Reflexes Autonomic reflexes as: Salivary gland secretion Heart and blood pressure regulation Changes in size of the pupil Digestive system regulation Somatic reflexes as: Activation of skeletal muscles REFEXES MAY USE : • SPINAL CORD, as flexor reflex and knee jerk . • BRAIN ( for proper evaluation) as response of pupils to light • N.B.1, Exaggerated, Distorted or Absent reflexes indicate nervous system disorder. • N.B.2, Reflex changes often occur before the pathological condition become obvious Central Nervous System (CNS) CNS develops from the embryonic neural tube By the fourth week the anterior end begins to expand and brain formation begins, The rest of the tube becomes the spinal cord The central canal becomes enlarged in 4 regions of the brain to form the ventricles: -Four chambers within the brain -Filled with cerebrospinal fluid Regions of the Brain Cerebral hemispheres Diencephalon Brain stem Cerebellum Figure 7.12 Cerebral Hemispheres (Cerebrum) Paired (left and right) superior parts of the brain Include more than half of the brain mass Figure 7.13a Layers of the Cerebrum Gray matter(outside) Outer layer Composed mostly of neuron cell bodies White matter(inside) Axons(tracts) Example: corpus callosum connects the two hemispheres Basal nuclei – internal islands in the gray matter Figure 7.13a Cerebral Hemispheres (Cerebrum) The surface is made of ridges (gyri) and grooves (sulci) Figure 7.13a Specialized Area of the Cerebrum Cerebral areas involved in special senses Gustatory area (taste) Visual area Auditory area Olfactory area Interpretation areas of the cerebrum Speech/language region Language comprehension region General interpretation area Specialized Area of the Cerebrum Figure 7.13c Diencephalon Sits on top of the brain stem Enclosed by the cerebral heispheres Made of three parts Thalamus Hypothalamus Epithalamus Diencephalon Figure 7.15 Thalamus Surrounds the third ventricle The relay station for sensory impulses (except olfaction) Transfers impulses to the correct part of the cortex for localization and interpretation Hypothalamus (Under the thalamus) Important autonomic nervous system center Helps regulate body temperature Controls water balance Regulates metabolism An important part of the limbic system (emotions) The pituitary gland is attached to the hypothalamus Epithalamus Forms the roof of the third ventricle Houses the pineal body (an endocrine gland) Includes the choroid plexus which forms the cerebrospinal fluid Brain Stem Attaches to the spinal cord and is formed of: Midbrain Pons Medulla oblongata Brain Stem Figure 7.15a Midbrain Reflex centers for vision and hearing Pons The bulging middle part of the brain stem Mostly composed of fiber tracts Includes nuclei involved in the control of breathing Medulla Oblongata The lowest part of the brain stem Merges into the spinal cord Includes important fiber tracts Contains important control centers Heart rate control Blood pressure regulation Breathing Swallowing Vomiting Cerebellum Two hemispheres with convoluted surfaces Provides involuntary coordination of body movements Protection of the Central Nervous System Scalp and skin Skull and vertebral column Meninges Cerebrospinal fluid Blood brain barrier Figure 7.16a Meninges • Three connective tissue membranes lie external to the CNS – dura mater, -arachnoid mater, and - pia mater • Functions of the meninges • Cover and protect the CNS • Protect blood vessels and enclose venous sinuses • Contain cerebrospinal fluid (CSF) • Form partitions within the skull Meninges Figure 12.24a Dura Mater • Leathery, strong meninx composed of two fibrous connective tissue layers • The two layers separate in certain areas and form dural sinuses • Double-layered external covering • Periosteum – attached to surface of the skull • Meningeal layer – outer covering of the brain • Folds inward in several areas Arachnoid Mater • The middle meninx, which forms a loose brain covering • It is separated from the dura mater by the subdural space • Beneath the arachnoid is a wide subarachnoid space filled with CSF and large blood vessels • Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood Pia Mater Deep meninx composed of delicate connective tissue that clings tightly to the brain Cerebrospinal Fluid Similar to blood plasma composition Formed by the choroid plexus Forms a watery cushion to protect the brain Circulated in subarachnoid space, ventricles, and central canal of the spinal cord Ventricles and Location of the Cerebrospinal Fluid Figure 7.17a Blood Brain Barrier Includes the least permeable capillaries of the body to exclude many potentially harmful substances It can not prevent some substances Fats and fat soluble molecules Respiratory gases Alcohol Nicotine Anesthesia Traumatic Brain Injuries Concussion Slight brain injury No permanent brain damage Contusion Nervous tissue destruction occurs Nervous tissue does not regenerate Cerebral edema Swelling due to inflammation of the brain May compress brain tissue causing death Cerebrovascular Accident (CVA) Commonly called a stroke Caused by ruptured or obstructed blood vessel supplying a region of the brain Brain tissue supplied with oxygen from that blood source will die. Loss of some functions or death may result Alzheimer’s Disease • Progressive degenerative brain disease Mostly seen in the elderly, but may begin in middle age leading to memory loss, irritability, confusion and ultimately, hallucinations and death Spinal Cord Extends from the medulla oblongata to below T12 Below T12 is the cauda equina (a collection of spinal nerves) Enlargements present in the cervical and lumbar regions Figure 7.18 Spinal Cord Anatomy Exterior white mater – conduction tracts Internal gray matter - mostly cell bodies Dorsal (posterior) horns Anterior (ventral) horns Central canal filled with cerebrospinal fluid Figure 7.19 Peripheral Nervous System Nerves and ganglia outside the CNS Nerve = bundle of neuron fibers outside CNS bundled by connective tissue, may be: -Afferent (sensory) nerves – carry impulses toward the CNS -Efferent (motor) nerves – carry impulses away from the CNS -Mixed nerves – both sensory and motor fibers Cranial Nerves 12 pairs of nerves that mostly serve the head and neck Numbered in order, front to back Most are mixed nerves, but three are sensory only. Distribution of Cranial Nerves Figure 7.21 Cranial Nerves I Olfactory nerve – sensory for smell II Optic nerve – sensory for vision III Oculomotor nerve – motor fibers to eye muscles IV Trochlear – motor fiber to eye muscles Cranial Nerves V Trigeminal nerve – sensory for the face; motor fibers to chewing muscles VI Abducens nerve – motor fibers to eye muscles VII Facial nerve – sensory for taste; motor fibers to the face VIII Vestibulocochlear nerve – sensory for balance and hearing Cranial Nerves IX Glossopharyngeal nerve – sensory for taste; motor fibers to the pharynx X Vagus nerves – sensory and motor fibers for pharynx, larynx, and abdominal viscera XI Accessory nerve – motor fibers to neck and upper back XII Hypoglossal nerve – motor fibers to tongue Spinal Nerves There is a pair of spinal nerves at the level of each vertebrae for a total of 31 pairs Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord Spinal nerves are named for the region from which they arise. Spinal Nerves Figure 7.22a Examples of Nerve Distribution Figure 7.23 Autonomic Nervous System The involuntary part of the peripheral nervous system Consists of motor nerves only Divided into two divisions Sympathetic division Parasympathetic division Differences Between Somatic and Autonomic Nervous Systems Nerves Somatic – one motor neuron Autonomic – two neurons ,preganglionic and postganglionic nerves Effector organs Somatic – skeletal muscle Autonomic – smooth muscle, cardiac muscle,and glands Nerurotransmitters Somatic – always use acetylcholine Autominic – use acetylcholine, epinephrine, or norepinephrine Figure 7.24 Sympathetic Division (thoracolumber) Originates from T1 through L2 Norepinephrine and epinephrine are neurotransmitters to the effector organs Parasympathetic Division(craniosacral) Originates from the brain stem and S1 through S4 Always uses acetylcholine as a neurotransmitter Slide 7.70 Sympathetic Pathways Figure 7.26 Slide 7.71 Anatomy of the Autonomic Nervous System Figure 7.25 Autonomic Functioning Sympathetic – “fight-or-flight” Increase activities Remember the “E” division = exercise, excitement, emergency, and embarrassment Parasympathetic – housekeeping activities Conserves energy Maintains daily necessary body functions Remember as the “D” division - digestion, defecation, and diuresis Development Aspects of the Nervous System The nervous system is formed during the first month of embryonic development so any early maternal infection can have extremely harmful effects on the baby. The hypothalamus is one of the last areas of the brain to develop. No more neurons are formed after birth, but growth and maturation continues for several years. The brain reaches maximum weight in young adults (1300-1400gr.)