CNS 1 PDF

NERVOUS SYSTEM (1) Dr. Sally Mohamed Lecturer of Physiology FOM/SCU  Nervous system Anatomical classification Functional classification Central Somatic Sensory Nervous Brain Nervous Integrati...

NERVOUS SYSTEM (1) Dr. Sally Mohamed Lecturer of Physiology FOM/SCU  Nervous system Anatomical classification Functional classification Central Somatic Sensory Nervous Brain Nervous Integrative System System Spinal Motor (CNS) (voluntary) Cord Spinal Autonomic Peripheral Sympathetic Nerves Nervous Nervous System System Cranial Para- (ANS) sympathetic (PNS) Nerves (Involuntary) Soma  On the basis of function, nerve cells are classified into two types: 1. Sensory or afferent neurons: carry the sensory impulses from periphery to central nervous system (CNS). 2. Motor or efferent neurons: carry the motor impulses from central nervous system (CNS)to peripheral effector organs like muscles, glands, blood vessels  Synapse Synapse is the junction between two neurons. It is not an anatomical continuation. But, it is only a physiological continuity between two nerve cells. Synapse is classified by two methods: A. Anatomical classification: 1. Axoaxonic synapse in which axon of presynaptic neuron ends on axon of postsynaptic neuron. 2. Axodendritic synapse in which the axon of presynaptic neuron ends on dendrite of postsynaptic neuron 3. Axosomatic synapse in which axon of presynaptic neuron ends on soma (cell body) of postsynaptic neuron. B. Functional classification: Based on mode of impulse transmission, synapse is classified into: 1. Electrical synapse: The physiological continuity between the presynaptic and the postsynaptic neurons is provided by gap junction between the two neurons, which allows direct exchange of ions between the two neurons Example is cardiac muscle fibers, smooth muscle fibers of intestine 2. Chemical synapse: Chemical synapse is the junction between a nerve fiber and a muscle fiber or between two nerve fibers, through which the signals are transmitted by the release of chemical transmitter. Mechanism of impulse transmission in chemical synapse: a. Arrival of impulse to the presynaptic neuron, stimulates the release of chemical neurotransmitter in the neurotransmitter synaptic cleft (= space between pre- and post- synaptic neurons). b. The neurotransmitter travel through the synaptic cleft to the post-synaptic neuron, where it binds to specific receptors stimulating generation of (=impulse) in the post-synaptic neuron.  Central Nervous System (CNS) ✓ CNS is formed by neurons and supporting cells called neuroglia. ✓ Structures of brain and spinal cord are arranged in two layers: 1) Gray matter is formed by nerve cell bodies and the proximal parts of nerve fibers, arising from nerve cell body. 2) White matter is formed by remaining parts of nerve fibers. ✓ In brain, white matter is placed in the inner part and gray matter is placed in the outer part. ✓ In spinal cord, white matter is in the outer part and gray matter is in the inner part. ✓Brain and spinal cord are surrounded by three layers of meninges called the outer dura mater, middle arachnoid mater and inner pia mater. ✓The space between arachnoid mater and pia mater is known as subarachnoid space. ✓Brain and spinal cord are actually suspended in the cerebrospinal fluid (CSF), which is filling the subarachnoid space.  Protection and Nourishment of the Brain Central nervous tissue is delicate. Because of this characteristic, and because damaged nerve cells cannot be replaced, this fragile, irreplaceable tissue must be well protected. Four major features help protect the CNS from injury: 1. It is enclosed by hard, bony structures. The cranium (skull) encases the brain, and the vertebral column surrounds the spinal cord. 2. Three protective and nourishing membranes, the meninges, lie between the bony covering and the nervous tissue. 3. The brain “floats” in a special cushioning fluid, the cerebrospinal fluid (CSF). 4. A highly selective blood–brain barrier limits access of blood-borne materials into the vulnerable brain tissue.  The Cerebrospinal fluid (CSF) Cerebrospinal fluid (CSF) surrounds the brain and spinal cord. The functions of CSF is to be a shock-absorbing fluid to prevent the brain from mechanical injury against the interior of the hard skull when the head is subjected to sudden, jarring movements. The CSF plays an important role in the exchange of materials between the neural cells and the interstitial fluid surrounding the brain. Only the brain interstitial fluid not the blood or CSF comes into direct contact with the neurons and glial cells. CSF is formed primarily by the choroid plexuses, The composition of CSF differs from that of blood. For example, CSF is lower in K and slightly higher in Na, making the brain interstitial fluid an ideal environment for movement of these ions down concentration gradients, a process essential for conduction of nerve impulses The biggest difference is the presence of plasma proteins in the blood but almost no proteins normally present in the CSF. Plasma proteins cannot exit the brain capillaries to leave the blood during formation of CSF.  The blood–brain barrier (BBB) Blood-brain barrier (BBB) is a neuroprotective structure that prevents the entry of many substances and pathogens into the brain tissues from blood. It was observed more than 50 years ago, that when trypan blue, the acidic dye was injected into living animals, all the tissues of body were stained by it, except the brain and spinal cord. This observation suggested that there was a hypothetical barrier, which prevented the diffusion of trypan blue into the brain tissues from the capillaries. This barrier was named as blood-brain barrier (BBB). It exists in the capillary membrane of all parts of the brain, except in some areas of hypothalamus.  The function of the blood– brain barrier (BBB) The BBB acts as both a mechanical barrier and transport mechanisms. It prevents potentially harmful chemical substances and permits metabolic and essential materials into the brain tissues. By preventing injurious materials and organisms, BBB provides healthy environment for the nerve cells of brain. Substances which can Pass through Blood-Brain Barrier 1. Oxygen 2. Carbon dioxide 3. Water 4. Glucose 5. Amino acids 6. Electrolytes 7. Drugs such as L-dopa, 5-hydroxytryptamine sulfonamides, tetracycline and many lipid- soluble drugs 8. Lipid-soluble anesthetic gases such as ether and nitrous oxide 9. Other lipid-soluble substances. Substances which cannot Pass through Blood-Brain Barrier 1. Injurious chemical agents 2. Pathogens such as bacteria 3. Drugs such as Penicillin and the catecholamines. Dopamine also cannot pass through BBB. So, parkinsonism is treated with L-dopa, instead of dopamine.  Brain Cerebral cerebellum Brain Stem Others Cortex Midbrain Thalamus Pons Hypo- thalamus Medulla Basal Oblongata Ganglia  Spinal Cord Spinal cord lies loosely in the vertebral canal. It extends from foramen magnum, as a continuation of medulla oblongata  Functions of CNS Three major levels of the CNS have specific functional characteristics: (1) the spinal cord level, (2) the lower brain or subcortical level (it includes medulla, pons, hypothalamus, thalamus, cerebellum, and basal ganglia), (3) the higher brain or cortical level.  (1) The spinal cord level: Spinal cord has neuronal circuits which can cause: (1) Walking movements, (2) Reflexes that withdraw portions of the body from painful objects, (3) Reflexes that stiffen the legs to support the body against gravity, and (4) Reflexes that control local blood vessels, gastrointestinal movements, or urinary excretion. (2) The lower brain or subcortical level: It is responsible for control of subconscious activities, including: 1) subconscious control of arterial pressure and respiration is achieved mainly in the medulla and pons. 2) Control of equilibrium is a combined function of the cerebellum and the medulla, and pons. 3) Feeding reflexes, such as salivation and licking of the lips in response to the taste of food, are controlled by areas in the medulla, pons, and hypothalamus. 4) Many emotional patterns such as anger, reaction to pain, and reaction to pleasure.  (3) The higher brain or cortical level. : The cerebral cortex is an extremely large memory storehouse. The cortex never functions alone but always in association with lower centers of the nervous system. Without the cerebral cortex, the functions of the lower brain centers are often imprecise. The storehouse of cortical information converts the functions of lower brain centers into determinative and precise operations. Memory- thnkiing- intelligence – higher function  Determining intelligence Determining personality Motor function Planning and organization Touch sensation Processing sensory information Language processing For example, if door is nocking, the cerebral cortex receive auditory signal and interpret it as “someone is out door”, so sending commands to lower centers to start moving, and opening door.  Peripheral nervous system (PNS) PNS is formed by neurons and their processes present in all regions of the body. It consists of cranial nerves (arising from brain) and spinal nerves (arising from the spinal cord). It is again divided into two subdivisions: 1. Somatic nervous system: concerned with somatic functions (somatic sensation& motor functions). 2. Autonomic nervous system: concerned with regulation of visceral functions (involuntary), it consists of Sympathetic and Parasympathetic nervous systems. Reflex Activity Reflex activity is the response of the peripheral nervous system stimulation that occurs without our consciousness. For example, 1. when hand is placed on a hot object, it is withdrawn immediately. 2. When a bright light is thrown into the eyes, eyelids are closed and pupil is constricted to prevent the damage of retina by entrance of excessive light into the eyes.  REFLEX ARC Reflex arc is the anatomical nervous pathway for a reflex action. 4 3 5 Stimulus 2 1 6 1. Stimulus thing or event that evokes a specific functional reaction in an organ or tissue. 2. Receptor Receptor is the end organ, which receives the stimulus. When receptor is stimulated, impulses are generated in afferent nerve. 3. Afferent Nerve Afferent or sensory nerve transmits sensory impulses from the receptor to center. 4. Center Center receives the sensory impulses via afferent nerve fibers and in turn, it generates appropriate motor impulses. Center is located in the brain or spinal cord. 5. Efferent Nerve Efferent or motor nerve transmits motor impulses from the center to the effector organ. 6. Effector Organ Effector organ is the structure such as muscle or gland where the activity occurs in response to stimulus.