Nervous System Lecture PDF

Nervous System Presented by: Dr. Ienass Mohamed Bahaa 1 1. Outline classification of nervous system. 2. Recognize anatomical organization of nervous system. 3. Identify major levels of CNS functions. 4. Understand different types of sensations. 5....

Nervous System Presented by: Dr. Ienass Mohamed Bahaa 1 1. Outline classification of nervous system. 2. Recognize anatomical organization of nervous system. 3. Identify major levels of CNS functions. 4. Understand different types of sensations. 5. Recognize different parts of motor nervous system and identify their lesions. 6. Define "a synapse". Identify functional anatomy of synapses and mode of transmission at different synaptic junctions. 7. Recognize the sequence of events for neurotransmitter release at synaptic junctions. 2  The Nervous System The nervous system controls rapid activities of the body. It receives information from different sensory organs and integrates them to determine the response. ORGANIZATION OF Nervous System  ANATOMICAL ORGANIZATION OF NS Central Nervous System (CNS): Brain & spinal cord Housed in bony structures: skull & vertebral column Peripheral Nervous System (PNS): Transmits signals to & from CNS. Consists of nerves (= bundles of nerve fibers) that extend between CNS and peripheral tissues. The PNS consists of:  12 pairs of cranial nerves: They originate in the brain & pass through openings in skull.  31 pairs of spinal nerves: They originate in the spinal cord & pass through openings in the vertebral column. Individual fibers in PNS may be processes of:  Sensory neurons or  Motor neurons  CENTRAL NERVOUS SYSTEM: BRAIN - The brain is divided into 4 parts: A. Cerebrum B. Diencephalon: FOREBRAIN Thalamus Hypothalamus C. Brain Stem: Midbrain Pons Medulla oblongata D. Cerebellum - Cerebral Ventricles: They are 4 interconnected cavities filled with fluid – the cerebrospinal fluid (CSF). 6  CEREBRUM: 2 CEREBRAL HEMISPHERES The surface is highly convoluted: - Consists of folds (gyri) & grooves (sulci). - 2 main grooves: central & lateral sulcus. Cerebral cortex: - is the thin outer layer formed of gray matter. - contains cell bodies of many neurons. White matter: - Is a thick central core of white matter (beneath cerebral cortex) formed of bundles of myelinated axons. Right & left cerebral hemispheres are connected by a massive bundle of nerve fibers; the corpus callosum. Each cerebral hemisphere is formed of 4 lobes: frontal, parietal, occipital & temporal lobes. Each lobe contains areas responsible for specific functions, e.g., visual, auditory, motor & sensory areas. The activities of the cerebral cortex can be classified into 3 general categories: A. Motor: stimulate muscle activity B. Sensory: receive sensory stimuli C. Associative: integrate information → initiate coordinated responses. 7 8 Spinal cord  MENINGES CNS is surrounded by 3 layers of connective tissue: 1. Dura mater (“hard mother”): it is the outermost layer. 2. Arachnoid layer: it has a spider web appearance. 3. Pia mater (“tender mother”): delicate, vascular & adheres to brain and spinal cord. Cerebrospinal fluid (CSF) flows between pia mater & arachnoid layer. 12  CEREBROSPINAL FLUID (CSF) CSF is secreted by specialized cells called “choroid plexuses” in walls of cerebral ventricles. Total volume: 150 ml Turnover: 500 ml/day Protective function: The brain floats in CSF, which acts as a water cushion → prevents injury of delicate nervous tissue. Diagnostic value: Lumbar puncture to detect infections in CNS, e.g., meningitis. 13 1-15  Blood Supply to the Brain Brain – Requires a tremendous amount of blood – Receives 15-20% of blood pumped by heart – Interruption can cause unconsciousness and irreversible brain damage. – High metabolic rate; dependent upon constant supply of oxygen and glucose Blood-brain barrier – A highly selective semipermeable membrane barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). – To be considered when developing drugs. – Receives blood through arteries: Internal carotids and vertebral arteries. The vertebral arteries join to form the basilar artery. Carotids plus basilar form the cerebral arterial circle (Circle of Willis).  A. HISTOL. ORGANIZATION OF NS (cont.) STRUCTURE OF NEURON ◼ Neurons have variety of shapes and sizes, but they share common features. They usually possess 4 parts: 1. Cell Body = Soma: 2. Dendrites: ◼ Usually 5-7 processes (may be many more) ◼ Usually highly branched →  surface area. ◼ Dendrites receive most input and transmit electrical current toward the cell body only. 3. Axon = Nerve Fiber: ◼ Usually single & long (few μm to  1m). ◼ Transmits impulses away from soma toward target cell. ◼ Axon hillock or initial segment is the trigger zone where electric signals are generated in most neurons.  A.HISTOL. ORGANIZATION OF NS (cont.) STRUCTURE OF NEURON (cont.) 4. Axon Terminal = Terminal Button: ◼ Each branch of the axon ends in an axon terminal. ◼ It is responsible for the release of neurotransmitters (NT) from axon to next neuron or to a target cell.  Synapses Definition: It is the junction between 2 neurons. Types of neurons in a synapse: 1. Presynaptic neuron 2. Postsynaptic neuron TYPES of synapses: 1- Chemical. Transmission across the majority of synapses in the nervous system is one-way and occurs through the release of chemical neurotransmitters from presynaptic axon endings. 2- Electrical. Adjacent cells that are electrically coupled are ❑ joined together by gap junctions. Gap junctions are present in cardiac muscle and smooth muscle ❑ 3- Conjoint. Both electrical and chemical (rare type). Chemical synapse Electrical synapse  Action of Neurotransmitter 1.Neurotransmitter molecules have been released from the presynaptic axon terminals, 2.They diffuse rapidly across the synaptic cleft and reach the membrane of the postsynaptic cell. 3.The neurotransmitters then bind to specific receptor proteins that are part of the postsynaptic membrane. Receptor proteins are ligand of the receptor protein. 4. Their action can be excitatory or inhibitory. Excitatory neurotransmitters Inhibitory neurotransmitters Causes excitatory post Causes inhibitory post synaptic synaptic potential. potential E.g: norepinephrine, epinephrine E.g: gamma aminobuteric acid and acetylcholine. (GABA), glycine, serotonin and endorphins.  Major levels of CNS function Cerebrum and cerebral cortex: The highest functional level of the CNS Processes, integrates, and analyzes information involved with highest levels of cognition Sensory perception and language Voluntary initiation of movement Brainstem: Receives sensory input and initiates motor output through cranial nerves from III to XII. Controls life-sustaining processes (e.g., respiration, circulation, digestion) Spinal cord: Its functions are at the lowest level of sophistication Processes reflexes Transmits nerve impulses to and from brain  Physiological Functional organization of the Nervous System The Afferent: Carries information to the CNS about changes in the external or internal environment. The Efferent: Orders are transmitted from CNS via efferent fibers to the effector organs (muscles, glands). - It is further subdivided into somatic & autonomic nervous system Sensory nervous system Motor nervous system Sensory nervous system faculty of nursing  Sensations The main function of the sensory system is to detect the changes that occur in the external and internal environment of the body. General sensations Special sensations Organic sensations Emotional sensations Somatic Visceral Thirst Hunger Vision Taste Hearing Smell Fear Sadness faculty of nursing 28 Sensations are carried to the cerebral cortex through sensory pathways. Each sensory pathway consists of 3 order neurons First order neuron: from receptor to spinal cord or brain stem. Second order neuron: from spinal cord to thalamus. Third order neuron: transmits signals to cerebral cortex faculty of nursing 29  5 4 1 (Receptors) 3 2 faculty of nursing 30  Pain sensation Definition: Unpleasant sensation, that may result in a reflex aiming at removal of the painful stimulus. Receptors: - Name: Specific pain sensitive F.N.E. (Nociceptors) - Type: 3 types 1. Mechanosensitive pain receptors: stimulated by strong pressure, sharp objects 2. Chemo-sensitive pain receptors: stimulated by pH extremes, environmental irritants and internal chemical substances 3. Thermosensitive pain receptors: stimulated by burning heat (>45ºC) and noxious cold faculty of nursing 31 - Distribution of nociceptors: Pain receptors are widely distributed. Absent nociceptors in: Skin, Arteries, Periosteum, Joint Surfaces. Brain Bone Deep Tissues (Muscles) Liver Parenchyma Lung alveoli Viscera Visceral layer of serous membranes faculty of nursing 32  Types of pain Cutaneous pain Deep pain Visceral pain Pain from the skin Pain from deep structures Pain from viscera. Causes: e.g. muscle pain Causes: - Injury of skin Causes: - Inflammation, Ischemia,& - Inflammation of skin. Injury, Inflammation, Spasm of viscera -Referred pain to skin from Ischemia of muscle. May lead to: other structures. Spasm of muscle. Parasympathetic autonomic Characters: May lead to effects. - Two types; fast & slow parasympathetic Rigidity of overlying skeletal autonomic effects e.g. muscles. hypotension, collapse Referred to other structures faculty of nursing 33  Headache It is a pain referred to the surface of the head from deep head structures. Causes: 1) Extra-cranial causes (90-95%) 2) Intra-cranial causes (5-10%) Tension headache Meningeal irritation Muscle spasm Brain tumor Eye diseases Loss of CSF Sinusitis Migraine Toothache and Anemia Hypertension faculty of nursing 34 Peripheral neuropathy Peripheral neuropathy: is a general term describing disease affecting the peripheral nerves. Neuropathy affects motor, sensory, or autonomic nerves result in different symptoms (impairs sensation, movement, gland, or organ function depending on which nerves are affected). Common causes include: - Systemic diseases (such as diabetes or leprosy). - Vitamin deficiency, medication. - Traumatic injury, ischemia, radiation therapy. - Excessive alcohol consumption and immune system disease. Motor nervous system faculty of nursing 37  Motor nervous system The main function of the motor system in the generation of voluntary movement and sending the motor plan via descending tracts to the effector muscles Types of voluntary movements : 1. Fine movements: Done by distal limb muscles. e.g.: Drawing & painting 2. Gross movements : Done by axial and proximal muscles. e.g.: Standing and walking 3. Skilled movements: It is mixed fine and gross movements. faculty of nursing 38  Past experience Plan or Idea program Association areas in the cortex (Prefrontal area) Motor - Responsible for generation of the idea of Hierarchy voluntary movement. The motor cortex, Basal ganglia and Cerebellum - Responsible for planning and programming of voluntary movement = convert idea into a plan. Feedback Execution The brain stem and the spinal cord. - Responsible for execution of voluntary movement faculty of nursing 39  Monosynaptic reflex MOTOR REFLEXES Polysynaptic reflex faculty of nursing 40  Muscle Stretch Reflex The stretch reflex is reflex contraction of skeletal muscle in response to passive stretch. It is the only MONOSYNAPTIC reflex. Stimulus: Stretch of the intrafusal fibers inside the receptor Receptor: Muscle spindle Afferent: Ia fibers Center: Spinal cord Efferent: Motor nerve to muscles Response: Muscle contraction faculty of nursing 41 Note that: It shows reciprocal innervation, i.e. if a muscle contracts the antagonistic muscle relaxes. faculty of nursing 42 Deep tendon reflexes (muscle stretch reflex) Flexor withdrawal Reflex & Crossed extensor reflex It is a protective reflex, Application of injurious stimulus to a limb produces flexion of ipsilateral limb and extension of the contralateral limb to support the body weight against gravity. It is Polysynaptic reflex. Stimulus: Painful stimulus Receptor: Nociceptors in skin Afferent: Aδ and C sensory fibers Center: Multiple interneurons in the spinal cord. Efferent: Motor nerve to muscles Response: Flexion withdrawal of the ipsilateral limb and extension of the contralateral limb. faculty of nursing 44  Upper and Lower Motor Neuron 1) Upper motor neurons (UMNs): neurons from the cerebral cortex and other subcortical (e.g. brain stem) centers that activate lower motor neurons. Descending tracts arise from UMN. 2) Lower motor neurons (LMNs): anterior horn cells (AHCs) , motor nuclei of cranial nerves, their axons and muscles supplied by them. faculty of nursing 45  Motor Lesions UMNL LMNL Cause -Damage of cortical - Damage of AHCs. motor areas. - Damage of motor nerves. -Damage of -Muscle disease (i.e. descending tracts. myopathies). Paralysis - Occurs in opposite - On the same side of the side of the body lesion. hemiplegia - Localized to muscles - Widespread. supplied by the affected segments only. Muscle tone - Hypertonia. - Hypotonia. - Spastic paralysis. - Flaccid paralysis. Muscle reflexes - Hyperreflexia - Hyporeflexia Muscle wasting - Absent. - Marked. 46  The Cerebellum It is important for the Control of voluntary movements, Control of muscle tone, & Control of equilibrium. Cerebellar Lesion: - Produces no paralysis. - Loss of its functions: - Incoordination of voluntary movements (= motor ataxia). e.g. abnormal incoordinated gait, speech, movements. faculty of nursing 47  The Basal ganglia It is important for Regulation of muscle tone, initiation of Voluntary movement, and Automatic associated movement. Basal Ganglia Lesion (Parkinsonism): - Produces no paralysis. - Loss of its functions: - The voluntary movements are difficulty to be initiated (in starting), slowly performed and of limited range. - Hypertonia (e.g. mask face) - loss of swinging of arms). Appearance of abnormal involuntary movement: sudden purposeless (arms, legs, face, head and trunk), during rest. faculty of nursing 48 Let’s think together The most common type of synapses in humans is: a) Conjoint b) Chemical c) Gap junctions Let’s think together Efferent nerves Carry information to the CNS about changes in the external or internal environment. A. True. B. False. faculty of nursing 51