Medical Physiology - General Organization of the CNS - Lesson 1 PDF

General organization of the Nervous System Dr. Hanadi Sourg, PhD. Assistant Professor of Physiology Objectives: By the end of this lecture, the student should be able to: List the basic structures and functions of the nervous system. Explain the anatomical and functional classifi...

General organization of the Nervous System Dr. Hanadi Sourg, PhD. Assistant Professor of Physiology Objectives: By the end of this lecture, the student should be able to: List the basic structures and functions of the nervous system. Explain the anatomical and functional classification of the nervous system. Define the central nervous system and peripheral nervous system and list the major parts of each. Injuries to NS cause: i.e why it is imp?  Death.  Weak or paralyzed movement.  Other symptoms, such as loss of sensation.  Others  Functions of nervous system: Monitoring changes: - Through sensory receptors to monitor changes occurring both inside and outside the body; which called stimuli, and the gathered information is called sensory input. Interpretation of sensory input. -It processes and interprets the sensory input and decides what should be done at each moment, a process called integration. Effects responses. -It then effects a response by activating muscles or glands (effectors) via motor output.  Functions of nervous system con….. Mental activity. -including consciousness, thinking, and memory. Homeostasis. -It can help stimulate or inhibit the activities of other systems to help maintain a constant internal environment. ORGANIZATION OF NERVOUS SYSTEM Nervous system, through signaling, acts as a control network within the body. Is about 2% of total body weight ( 1.4 kg) but it is the most complex of the 11 body systems. It consist of : Nervous Tissue, Central nervous system (CNS) Structural Classification Peripheral nervous system (PNS) ❖ Nervous Tissue: -Structure and Function Nervous tissue is made up of just two principal types of cells- ▪ Supporting cells and neurons. 1. Neurons: The specialized cells that constitute the functional units of the nervous system which are responsible for the reception and response to changes in the internal and external environment. (more than 100 billion). ❖ Functional Classification It is concerned only with PNS structures. Sensory division & Motor division. 1. Sensory division: - Or afferent division, nerves that convey impulses to the central nervous system from sensory receptors located in various parts of the body. ✓ Somatic sensory fibers. - Fibers that delivering impulses from the skin, skeletal muscles, and joints. ✓ Visceral sensory fibers. - Fibers that transmit impulses from the visceral organs. - Sensory Division: two components  Sensory Division: twocomponents 1. Somatic sensory components:  Special senses:  General somatic senses:  Taste  Touch  Vision  Pain  Hearing  Pressure  Balance  Vibration,  Smell  Temperature  Proprioception. Sensory Division: two components (Continued) 2. Visceral sensory component: -transmit nerve impulses from viscera to the CNS -visceral senses primarily include:  Temperature  Stretch (of the organ wall) ❖ 2. Motor division. - Or efferent division carries impulses from the CNS to effector organs, the muscles, and glands; - has two subdivisions: ❑ Somatic nervous system and the ❑ Autonomic nervous system. Somatic nervous system. - allows us to consciously, or voluntarily, control our skeletal muscles. Autonomic nervous system: - regulates events that are automatic, or involuntary; this subdivision, has two main parts:  Autonomic nervous system divitions ✓ Sympathetic division, also called thoracolumbar division, consists of thoracic and lumbar chains of sympathetic ganglia. ✓ Parasympathetic division, also called craniosacral division, consists of the ganglia associated with 3rd, 7th, 9th and 10th cranial nerves. ✓ Enteric nervous system (GIT)  Nervous Tissue: (neurons & supporting cells) Neurons may be classified functionally or structurally. ❖ 1.Neurons: (functional classification) - there are sensory, motor, and association neurons. ❑ Sensory neurons. carrying impulses from sensory receptors to the CNS. ❑ Motor neurons. carrying impulses from the CNS to the viscera and/or muscles and glands. ❑ Interneurons. known as association neurons; they connect the motor and sensory neurons in neural pathways. ❖ Neurons :Structural classification. is based on the number of processes extending from the cell body. Multipolar neuron: have several processes, because all motor and association neurons are multipolar, this is the most common structural type. Bipolar neurons. Neurons with two processes- an axon and a dendrite- these are rare in adults, found only in some special sense organs, where they act in sensory processing as receptor cells. A pseudounipolar : has one extension from its cell body. This type of neuron contains an axon that has split into two branches. A single process arises from the cell body and then divides into an axon and a dendrite.  2. Supporting Cells (Neuroglia) ▪ Neuroglia. “nerve glue”. - includes many types of cells that generally support, insulate, and protect the delicate neurons; in addition, each of the different types of neuroglia, has special functions. ▪ Astrocytes. -These are abundant, star-shaped cells, form a living barrier between the capillaries and neurons and play a role in making exchanges between the two so they protect neurons from harmful substances in the blood. ▪ Microglia. -These are spiderlike phagocytes that dispose of debris, including dead brain cells and bacteria. ▪ Ependymal cells: glial cells that line the central cavities of the brain and the spinal cord; the beating of their cilia helps to circulate the cerebrospinal fluid and forms a protective cushion around the CNS. ▪ Oligodendrocytes: wraps their flat extensions tightly around the nerve fibers, producing fatty insulating coverings called myelin sheaths. ▪ Schwann cells. Schwann cells form the myelin sheaths around nerve fibers that are found in the PNS. ▪ Satellite cells. Satellite cells act as protective, cushioning cells. Anatomical and functional divisions of the nervous system CNS Anatomically PNS Sensory Physiologically Motor CNS Nervous system PNS brain Cranial nerve (12 pairs) spinal cord Spinal nerves (31 pairs) ANATOMICAL DIVISIONS OF THE NERVOUS SYSTEM 1. Central nervous system (CNS), At the central axis of the body includes brain and spinal cord. ▪ Brain has three parts: ✓ Forebrain comprises telencephalon, i.e. central hemispheres (cerebrum) or anterior part of the forebrain, and ✓ diencephalon or posterior part of the forebrain. The upper 2/3rd of diencephalon is called thalamus and lower 1/3rd is called hypothalamus. ✓ Mid brain or mesencephalon and Hind brain comprises pons, medulla oblongata and cerebellum. - Discuss in four regions ✓ cerebral hemispheres, ✓ diencephalon, ✓ brain stem, and ✓ cerebellum. ▪ Cerebral Hemispheres -called the cerebrum, are the most superior part of the brain. Gyri: The entire surface of the cerebral hemispheres exhibits elevated ridges of tissue, separated by shallow grooves called sulci. Fissures. Less numerous are the deeper grooves of tissue called fissures, which separate large regions of the brain; the cerebral hemispheres are separated by a single deep fissure, the longitudinal fissure. Lobes. Other fissures or sulci divide each hemisphere into a number of lobes, named for the cranial bones that lie over them. Regions of the cerebral hemisphere. has three basic regions: o a superficial cortex of gray matter, o an internal white matter, and o the basal nuclei. ▪ Cerebral cortex -functions: Speech, memory, logical and emotional response, as well as consciousness, interpretation of sensation, and voluntary movement. ▪ Lobes of cerebral hemisphere Parietal lobe. Have the primary somatic sensory area and impulses traveling from the body’s sensory receptors are localized and interpreted in this area. Occipital lobe. The visual area is located in the posterior part. Temporal lobe. o The auditory area is bordering the lateral sulcus, o The olfactory area is found deep inside the temporal lobe. Frontal lobe. The primary motor area, which allows us to consciously move our skeletal muscles, is anterior to the central sulcus in the front lobe. ▪ Cortical functional areas:(Brodmann’s area) Motor areas include: Primary motor area (area 4 ), Premotor area (area 6), Frontal eye field (area 8), Supplementary motor area. Sensory areas include: Primary somaesthetic areas (area 3, 1 and 2). Secondary (supplementary) somaesthetic area, Somaesthetic association areas (area 5, 7 and higher association area 40). Auditory areas include: Primary auditory area (area 41) or auditory area I, Auditory association area (area 42) or auditory area II, Higher auditory association area (area 22) Visual areas include: Primary visual area (area 17) or visuostriate area of visual area I, Visual association area 18 (peristriate area) and Visual association area 19 (parastriate area). Speech areas include: Motor speech area comprises: Anterior area (Broca’s area) or areas 44, 45 and Superior area Sensory speech areas comprise: Area 39 (or reading centre), Area 40 and Area 22 (Wernicke’s area). Smell area is: Area 28. Gustatory area is: Area 43  Fiber tracts. Corpus callosum. One very large fiber tract, connects the cerebral hemispheres; such fiber tracts are called commissures. -Association fiber tracts connect areas within a hemisphere, and -projection fiber tracts connect the cerebrum with lower CNS centers. Basal nuclei. There are several islands of gray matter, called the basal nuclei, or basal ganglia, buried deep within the white matter of the cerebral hemispheres; it helps regulate voluntary motor activities. ❖ Cerebral cortex is divided into four lobes: frontal, parietal, temporal and occipital. ❑ Diencephalon (posterior) - acts as a primary relay and processing center for sensory information and autonomic control. -What are the parts of the diencephalon and what do they do? Thalamus: is a relay station for sensory impulses passing upward to the sensory cortex. Hypothalamus: makes up the floor of the diencephalon; it is an important: autonomic nervous system center (the regulation of body temperature), the center for many emotions, also regulates the pituitary gland.  Mammillary bodies. -a pair of small round bodies, form part of the limbic system. -The mammillary bodies, reflex centers involved in olfaction. Epithalamus. - It is function to connect the limbic system to other parts of the brain. - secretion of melatonin by the pineal gland and - regulation of motor pathways and emotions. and - the choroid plexus, which forms the cerebrospinal fluid. - assists with the sense of smell and regulating sleep. ❑Brain Stem - Structures. 3 part the midbrain, pons, and medulla oblongata. Midbrain: extends from the mammillary bodies to the pons inferiorly; it is composed of two fiber tracts: Cerebral peduncles, which convey descending and ascending impulses. & Corpora quadrigemina. Which a reflex centers involved in vision and hearing. Pons: is just below the midbrain, and this area is mostly fiber tracts; however, it does have important nuclei involved in the control of breathing.  Medulla oblongata: is the most inferior part of the brain stem; it contains nuclei that regulate vital visceral activities; it contains centers that control heart rate, blood pressure and others. Reticular formation. (the reticular activating system (RAS)), Extending the entire length of the brain stem is a diffuse mass of gray matter, are involved in motor control of the visceral organs; plays a role in consciousness and the awake/sleep cycles. ❑ Cerebellum - The large projects dorsally from under the occipital lobe of the cerebrum. - Structure. it has an outer cortex made up of gray matter and an inner region of white matter. - Function: provides precise timing for skeletal muscle activity and controls our balance and equilibrium. ❖ Spinal Cord - The cylindrical spinal cord is white continuation of the brain stem. (17 inches (42 cm) long). is cushioned and protected by the meninges. - Major function. provides a two-way conduction pathway to and from the brain, and it is a major reflex center. - Spinal nerves. In humans, 31 pairs of spinal nerves. Gray Matter of the Spinal Cord & Spinal Roots looks like a butterfly or a letter H. Projections. The two posterior projections are the dorsal, or posterior, horns; the two anterior projections are the ventral, or anterior, horns. Central canal. The gray matter surrounds the central canal of the cord, which contains CSF. Dorsal root ganglion. The cell bodies of sensory neurons, damage of it cause lost of the sensation. Dorsal horns. contain interneurons. Ventral horns: contain cell bodies of motor neurons of somatic nervous system. Spinal nerves. The dorsal and ventral roots fuse to form the spinal nerves. White Matter of the Spinal Cord - is composed of myelinated fiber tracts. Regions. divided into three regions: The dorsal, lateral, and ventral columns; each of the columns contains a number of fiber tracts made up of axon with the same destination and function. Sensory tracts. conducting sensory impulses to the brain. Motor tracts. carrying impulses from the brain to skeletal muscles. Protection of the Central Nervous System -Nervous tissue is very soft and delicate, so to protect the brain and the spinal cord that by enclosing them within the bone (the skull and vertebral column), membranes (the meninges), and a watery cushion (cerebrospinal fluid). The Blood-Brain Barrier What is the blood-brain barrier and its function? -Is a tightly locked layer of cells that defend your brain from harmful substances, germs and other things that could cause damage. -Function. The neurons are kept separated from blood substances by the BBB. Substances allowed: water-soluble substances, only water, glucose, and essential amino acids pass easily. Prohibited substances: Metabolic wastes, such as toxins, urea, proteins, and most drugs are prevented from entering the brain tissue. Fat-soluble substances:The BBB is useless against fats, respiratory gases, and other fat-soluble molecules. 2. Peripheral nervous system (PNS) (lies outside the CNS) - Consists of nerves and scattered groups of neuronal cell bodies (ganglia) found outside the CNS. -Peripheral nerves attached to the brain are called Cranial nerves (12 pairs) primarily serve the head and the neck. and those attached with spinal cord are called Spinal nerves (31 pairs). Spinal Nerves and Nerve Plexuses ❖ Spinal Nerves The 31 pairs are formed by the combination of the ventral and dorsal roots. Rami. each spinal nerve divides into dorsal and ventral rami contains both sensory and motor fibers. Dorsal rami. The smaller dorsal rami serve the skin and muscles of the posterior body trunk. Ventral rami. The ventral rami of spinal nerves T1 through T12 form the intercostal nerves, which supply the muscles between the ribs and the skin and muscles of the anterior and lateral trunk. ❖ Nerve Plexuses (anatomy lec) Cervical plexus. - it serves the diaphragm, skin, and muscles of the shoulder and neck. Brachial plexus. - The axillary nerve serves the deltoid muscles and skin of the shoulder, muscles, and skin of the superior thorax; the radial nerve serves the triceps and extensor muscles of the forearm, and the skin of the posterior upper limb; - The median nerve serves the flexor muscles and skin of the forearm and some muscles of the hand; the musculocutaneous nerve serves the flexor muscles of the arm and the skin of the lateral forearm; and the ulnar nerve serves some flexor muscles of forearm; wrist and many hand muscles, and the skin of the hand.  Lumbar plexus. - The femoral nerve serves the lower abdomen, anterior and medial thigh muscles, and the skin of the anteromedial leg and thigh; the - Obturator nerve serves the adductor muscles of the medial thigh and small hip muscles, and the skin of the medial thigh and hip joint. Sacral plexus. - The sciatic nerve (the largest nerve in the body) serves the lower trunk and posterior surface of the thigh, and it splits into the common Fibular and Tibial nerves; the common fibular nerve serves the lateral aspect of the leg and foot, while the tibial nerve serves the posterior aspect of leg and foot; the superior and inferior gluteal nerves serve the gluteal muscles of the hip. Functional or physiological divisions of the nervous system 1. Somatic nervous system (Sensory division & Motor division) Sensory division - Collects the information about the changes that take place in the external environment and interprets the meaning of these changes and consists of: ✓ Sensory receptors that receive stimulus from the external environment. - A stimulus is a change of environment of sufficient intensity to cause a response in an organism. (mechanical, chemical, thermal, auditory or visual). ✓ Afferent neurons that carry impulses from the receptors to the brain and spinal cord. ✓ Parts of the brain that primarily deal with the processing of information.  Motor division - consists of neurons that carry signals away from the brain and spinal cord to the skeletal muscles. -It also co-ordinates the actions of different skeletal muscles of the body. -Thus skeletal muscles are the effector organs of somatic nervous system. -A single motor neuron arising in the CNS traverses directly to the skeletal muscle without the mediation of ganglia. -Somatic nervous system is under voluntary control. Physiology of the Nervous System - It involves a complex journey of impulses: Nerve Impulse - Neurons have two major functional properties: irritability, the ability to respond to a stimulus and convert it into a nerve impulse, and - conductivity, the ability to transmit the impulse to other neurons, muscles, or glands. If the stimulus is strong enough, the local depolarization activates the neuron to initiate and transmit a long- distance signal called an action potential, also called a nerve impulse; the nerve impulse is an all-or-none response; it is either propagated over the entire axon, or it doesn’t happen at all.  Electrical conditions of a resting neuron’s membrane. - The plasma membrane of a resting, or inactive, neuron is polarized. Action potential initiation and generation. Most neurons in the body are excited by neurotransmitters released by other neurons. Depolarization. - The inward rush of sodium ions changes the polarity of the neuron’s membrane at that site. Graded potential. - Locally, the inside is now more positive, and the outside is less positive, a situation called graded potential.  Repolarization. - The outflow of positive ions from the cell restores the electrical conditions at the membrane to the polarized or resting, state. Saltatory conduction. - Fibers that have myelin sheaths conduct impulses much faster because the nerve impulse literally jumps from node to node along the length of the fiber; this occurs because no electrical current can flow across the axon membrane where there is fatty myelin insulation. Communication of Neurons at Synapses The events occurring at the synapse are arranged below. o Arrival. The action potential arrives at the axon terminal. o Fusion. The vesicle fuses with the plasma membrane. o Release. Neurotransmitter is released into the synaptic cleft. o Binding. The neurotransmitter binds to a receptor on receiving neuron’s end. o Opening. The ion channel opens. o Closing. Once the neurotransmitter is broken down and released, the ion channel close. Thank you