Nervous System Lecture Notes PDF

THE NERVOUS SYSTEM HES 029: Human Anatomy and Physiology with Pathophysiology Prepared by: Arwin Paul S. Avila, RMT Functions of the nervous system: ❖ Detects external and internal stimuli (Sensory Input) ❖ Processes and responds to sensory input (Integration) ❖ Controls muscles and...

THE NERVOUS SYSTEM HES 029: Human Anatomy and Physiology with Pathophysiology Prepared by: Arwin Paul S. Avila, RMT Functions of the nervous system: ❖ Detects external and internal stimuli (Sensory Input) ❖ Processes and responds to sensory input (Integration) ❖ Controls muscles and glands ❖ Maintains homeostasis by regulating other systems ❖ Center for mental activities NERVOUS SYSTEM: STRUCTURAL & FUNCTIONAL CLASSIFICATIONS HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM STRUCTURAL CLASSIFICATION ❖ Central Nervous System (CNS) ❖ Peripheral Nervous System (PNS) FUNCTIONAL CLASSIFICATION ❖ Sensory (Afferent) Division ❖ Motor (Efferent) Division Somatic Nervous System Autonomic Nervous System (ANS) STRUCTURAL CLASSIFICATION ❖ Central Nervous System (CNS) Brain and Spinal Cord Functions: Integration, Command Center Interprets sensory information and issues instructions. ❖ Peripheral Nervous System (PNS) Nerves outside CNS Includes Spinal Nerves and Cranial Nerves Links the body to the CNS through sensory and motor nerves. FUNCTIONAL CLASSIFICATION ❖ Sensory (Afferent) Division Sensory Receptors → CNS ❖ Motor (Efferent) Division CNS → Effectors Two subdivisions: Somatic Nervous System - Voluntary Control Autonomic Nervous System - Involuntary Control Motor (Efferent) Division Controls the following functions: Contraction of skeletal muscles throughout the body Contraction of smooth muscle in the internal organs Secretion of active chemical substances by both exocrine and endocrine glands in many parts of the body. Subdivided into the following divisions: ❖ Somatic (Voluntary) ❖ Visceral (Involuntary or Autonomic) Controlled by the Sympathetic and Parasympathetic NS ❖ Special MAJOR LEVELS OF CNS FUNCTIONS SPINAL CORD LEVEL Neuronal circuits in the cord can cause: Walking movement Reflexes that withdraw parts of the body from painful stimuli. Reflexes that stiffen the legs to support the body against gravity. Reflexes that control local blood vessels, gastrointestinal movements, and urinary excretion. MAJOR LEVELS OF CNS FUNCTIONS LOWER BRAIN OR SUBCORTICAL LEVEL Lower areas of the brain: Cerebellum ○ Coordinates voluntary movements such as posture, balance and coordination, resulting in smooth and balanced muscular activity. Thalamus (Lady Secretary) ○ Relay center for sensory and motor signals to the cerebral cortex. Hypothalamus ○ Regulates homeostasis, emotional responses, temperature, sleep and controls the endocrine system. ○ Produces hormones (e.g. ADH, Oxytocin) MAJOR LEVELS OF CNS FUNCTIONS LOWER BRAIN OR SUBCORTICAL LEVEL Lower areas of the brain: Basal ganglia ○ Regulates movement and motor control. Pons ○ Relays signals between cerebrum and cerebellum, regulates breathing. Mesencephalon (Midbrain) ○ Processes visual and auditory information, involved in motor control. MAJOR LEVELS OF CNS FUNCTIONS HIGHER BRAIN (CORTICAL LEVEL) Area: Cerebral Cortex The cortex never acts alone; it works in association with the lower brain centers. Integrates and refines the inputs from lower brain centers, playing a vital role in complex thought processes, memory, and precise execution of functions. Brodmann’s areas MAJOR LEVELS OF CNS FUNCTIONS HIGHER BRAIN (CORTICAL LEVEL) Lobes of the cerebral cortex: Frontal lobe Primary Motor Cortex (Area 4) ○ Location: Precentral gyrus ○ Initiates delicate isolated movements Premotor Cortex (Area 6) ○ Location: Anterior to the primary motor cortex ○ Produce automatic actions; initiates grasp reflex Brodmann’s areas MAJOR LEVELS OF CNS FUNCTIONS HIGHER BRAIN (CORTICAL LEVEL) Lobes of the cerebral cortex: Frontal lobe Frontal Eye Field (Area 8): ○ Location: Anterior to the premotor cortex. ○ Controls voluntary eye movements. Brodmann’s areas MAJOR LEVELS OF CNS FUNCTIONS HIGHER BRAIN (CORTICAL LEVEL) Lobes of the cerebral cortex: Frontal lobe Prefrontal area (Areas 9, 10 and 12) ○ Autonomic, Mental, Memory, Behaviour, Personality Broca’s Area (Area 44, 45) ○ Location: Inferior frontal gyrus ○ Speech production and language processing. Brodmann’s areas MAJOR LEVELS OF CNS FUNCTIONS HIGHER BRAIN (CORTICAL LEVEL) Lobes of the cerebral cortex: Parietal Lobe (Main sensory area) Primary Somatosensory Cortex (Area 3, 1, 2) ○ Location: Postcentral gyrus. ○ Processes sensory information from the body (touch, temperature, pain). Brodmann’s areas MAJOR LEVELS OF CNS FUNCTIONS HIGHER BRAIN (CORTICAL LEVEL) Lobes of the cerebral cortex: Temporal Lobe Primary Auditory Cortex (Area 41, 42) ○ Location: Superior temporal gyrus. ○ Processes auditory information. Wernicke’s Area (Area 22) ○ Location: Posterior superior temporal gyrus. ○ Function: Language comprehension. Brodmann’s areas Cerebrum Lobes of the cerebral hemispheres: Frontal Lobe: Involved in decision-making, problem-solving, planning, and voluntary motor activity. Parietal Lobe: Processes sensory information such as touch, temperature, & pain. Occipital Lobe: Primary center for visual processing. Temporal Lobe: Involved in hearing, memory, and speech. Cerebrum Primary Motor Cortex ○ Located in the precentral gyrus of the frontal lobe; controls voluntary movements. Primary Somatosensory Cortex ○ Located in the postcentral gyrus of the parietal lobe; processes sensory information from the body. NERVOUS TISSUE: STRUCTURE AND FUNCTION Nervous tissue is made up of just two principal types of cells—supporting cells and neurons. HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM SUPPORTING CELLS Non-neuronal cells that support, protect, and maintain neurons in the nervous system. Also known as neuroglia, glia or glial cells TYPES IN CNS TYPES IN PNS Astrocytes Ependymal cells Schwann cells Microglia Satellite cells Oligodendrocytes CNS SUPPORTING CELLS: ASTROCYTES Star-shaped Form a supportive framework for blood vessels and neurons. Promote the formation of tight junctions in the blood-brain barrier. Reactive astrocytosis Promote synapse development and regulate neurotransmitter activity. CNS SUPPORTING CELLS: EPENDYMAL CELLS Line the central cavities of the brain and the spinal cord. Produce and circulate cerebrospinal fluid (CSF). CNS SUPPORTING CELLS: MICROGLIA Spider-like phagocytes (CNS-specific immune cells). Increase in areas of CNS damage (e.g., due to infection, trauma, or stroke). CNS SUPPORTING CELLS: OLIGODENDROCYTES Form myelin sheaths around axons in the CNS. ○ One oligodendrocyte can myelinate multiple axons. PNS SUPPORTING CELLS: SCHWANN CELLS & SATELLITE CELLS Schwann Cells: Form myelin sheaths around axons in the PNS. ○ Each Schwann cell myelinates a segment of a single axon. Satellite Cells: Support and protect neuron cell bodies, absorb toxins. NEURONS Also called nerve cells. Electrically excitable cells. Highly specialized to transmit messages (nerve impulses) from one part of the body to another. Parts of the neuron: ○ Cell body ○ Dendrites ○ Axon Neuron Cell Body (Soma) Metabolic Center Contains: ○ Single, relatively large, and centrally located nucleus with a prominent nucleolus ○ Cytoplasm contains the usual organelles ○ Nissl bodies - rough ER, primary sites of protein synthesis in neurons. ○ Neurofibrils - maintain cell shape. Dendrites Short, often highly branched cytoplasmic extensions. Receiving portion of the neuron Dendritic spines - small extension where axons from other neurons form synapses with the dendrites. Axons Long, slender projections of neurons that conduct electrical impulses away from the neuron's cell body. Related Structures: ○ Axon hillock - cone-shaped area of the neuron cell body where axons arise. ○ As the axon hillock narrows, it transitions into the initial ligament, which is the actual beginning of the axon. Axons Synapse - junction where an impulse is transmitted from one neuron to another. Synaptic cleft - small gap between the presynaptic terminal and the postsynaptic membrane within the synapse. Myelin Sheaths Multilayered lipid and protein (myelin) covering that wraps around the axons of many neurons. ○ Produced by oligodendrocytes in the CNS and Schwann cells in the PNS. Protects and insulates nerve fibers. Increases the speed of nerve impulse transmission by enabling saltatory conduction. Myelin Sheaths Nodes of Ranvier: Small gaps in the myelin sheath of myelinated axons, crucial for the rapid transmission of action potentials. NEURON TERMINOLOGIES HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM TERMINOLOGIES ❖ NUCLEI Clusters of neuron cell bodies found in the CNS. Located within the skull or vertebral column for protection. ❖ GANGLION Small collections of neuron cell bodies located in the PNS. Serve as relay points and intermediate connections. TERMINOLOGIES ❖ TRACTS Bundles of nerve fibers running through the CNS. Function as pathways for transmitting impulses. Myelinated tracts form white matter. ❖ NERVES Bundles of nerve fibers found in the PNS. Transmit sensory and motor information to and from the CNS. Encased in protective connective tissue. TERMINOLOGIES ❖ WHITE MATTER Regions of the CNS with primarily myelinated nerve fibers. Facilitates rapid impulse transmission. ❖ GRAY MATTER Regions of the CNS with mostly unmyelinated nerve fibers and neuron cell bodies. Involved in processing and integrating information. NEURON CLASSIFICATIONS HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM FUNCTIONAL CLASSIFICATION ❖ Sensory Neurons (Afferent Neurons) Carry nerve impulses from sensory receptors (in skin or internal organs) to the CNS. Specialized receptors: Cutaneous Sense Organs Proprioceptors Pain Receptors FUNCTIONAL CLASSIFICATION ❖ Motor Neurons (Efferent Neurons) Transmit nerve impulses from the CNS to muscles, glands, and viscera. ❖ Interneurons (Association Neurons) Connect sensory and motor neurons within neural pathways. STRUCTURAL CLASSIFICATION ❖ Multipolar Neurons Multiple processes extend from the cell body (one axon, several dendrites). Most common type; includes all motor and association neurons. STRUCTURAL CLASSIFICATION ❖ Bipolar Neurons Two processes extend from the cell body (one axon, one dendrite). Rare in adults; found in special senses like vision and smell. STRUCTURAL CLASSIFICATION ❖ Unipolar Neurons Has one primary process that can branch into dendrites and axons but does not split into two distinct branches. STRUCTURAL CLASSIFICATION ❖ Pseudo-Unipolar Neurons Single process from the cell body splits into two branches. Appears to have one process, which bifurcates into a central branch (axon) extending to the CNS and a peripheral branch with dendritic-like sensory receptors. PROPERTIES OF NEURONS ❖ Irritability The ability to respond to stimulus and convert it into a nerve impulse. ❖ Conductivity The ability to transmit impulses to other neurons, muscles or glands. SYNAPSE HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM MAJOR TYPES According to nature: ❖ Chemical Communication through neurotransmitters. Unidirectional, involves synaptic cleft. Presynaptic neuron → Postsynaptic neuron MAJOR TYPES According to nature: ❖ Electrical Direct communication via gap junctions. Characterized by open fluid channels that conduct electricity from one cell to the next. Bidirectional, fast, synchronized activity. Special characteristics of Synaptic Transmission ❖ POST-TETANIC FACILITATION Increased synaptic transmission after high-frequency stimulation. ❖ AFFECTED BY ALKALOSIS AND ACIDOSIS Alkalosis - Increases neuronal excitability. Acidosis - Decreases neuronal excitability. ❖ AFFECTED BY HYPOXIA: Reduces synaptic efficiency. Special characteristics of Synaptic Transmission ❖ AFFECTED BY DRUGS Modify neurotransmitter release or receptor activity. ○ Increase neuronal activity Caffeine in coffee, theophylline in tea, theobromine in cocoa Strychnine - inhibits the action of some normally inhibitory transmitter substances. ○ Decrease neuronal activity Anesthetics Special characteristics of Synaptic Transmission ❖ FATIGUE Progressive reduction in the firing rate of postsynaptic neurons when excitatory synapses are rapidly and repetitively stimulated. Acts as a protective mechanism. Mechanism: Exhaustion of neurotransmitters. Special characteristics of Synaptic Transmission ❖ SYNAPTIC DELAY The time required for signal transmission from a presynaptic neuron to a postsynaptic neuron. Minimal synaptic delay is about 0.5 milliseconds. Chemical substances that function as neurotransmitters: ❖ Small-Molecule, Rapidly Acting Transmitters CLASS I ○ Acetylcholine - Excitatory and inhibitory. Excitatory - Increase level of wakefulness Inhibitory - Vagus nerve CLASS II ○ Norepinephrine - Excitatory and inhibitory. ○ Dopamine - Inhibitory ○ Serotonin - Inhibitory (causes sleep) Chemical substances that function as neurotransmitters: ❖ Small-Molecule, Rapidly Acting Transmitters CLASS III ○ GABA (Gamma Aminobytyric Acid) - Inhibitory ○ Glycine - Inhibitory ○ Glutamate - Excitatory TYPES OF NEURONAL INHIBITION 1. Presynaptic Inhibition (Indirect) ○ Reduces neurotransmitter release: caused by presynaptic synapse that prevents opening of Ca channels. 2. Postsynaptic Inhibition (Direct) ○ Decreases postsynaptic neuron's response: inhibitory neurotransmitter (e.g., GABA, glycine) binds to receptors on the postsynaptic neuron. 3. Renshaw Cell Inhibition ○ Specific form of feedback inhibition that regulates motor neuron activity: Motor neurons synapse with Renshaw cells in the spinal cord. PHYSIOLOGY: NERVE IMPULSE HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM Electrical Conditions of a Resting Neuron RESTING MEMBRANE POTENTIAL The membrane is polarized. ○ Fewer positive ions sitting on the inner face (slightly negative) of the neuron’s plasma membrane than there are on its outer face (slightly positive). Resting membrane potential: ~ -90 mV Ion Concentration ○ K⁺: High concentration inside the cell. ○ Na⁺: High concentration outside the cell. Action Potential Initiation and Generation STIMULUS ACTIVATION Different stimuli excite neurons to generate an impulse. ○ Types of Stimuli: Light, sound, pressure, or neurotransmitters. Stimuli open sodium channels, allowing Na⁺ to rush in and cause depolarization. Action Potential Initiation and Generation DEPOLARIZATION The membrane potential becomes more positive inside the neuron due to Na⁺ influx. Creates a graded potential that, if strong enough, triggers an action potential. ○ Summation: Graded potentials can add up to reach the threshold potential. ○ Threshold potential: The critical membrane potential (~ -55 mV) that must be reached to trigger an action potential. Action Potential Initiation and Generation PROPAGATION OF ACTION POTENTIAL Depolarization of the first membrane patch causes permeability changes in the adjacent membrane, and the events are repeated. Action potential propagates rapidly along the entire length of the membrane like dominoes falling one after another, or a crowd doing the "wave". Action Potential Initiation and Generation REPOLARIZATION The membrane potential is restored to its resting state after depolarization. Sodium channels close and potassium channels open, allowing K⁺ to exit the cell. The inside of the neuron becomes more negative again. This process restores the membrane potential back to -90 mV. Action Potential Initiation and Generation RESTORATION The ionic conditions of the resting state are restored later by the activity of the sodium-potassium pump. Three sodium ions are ejected for every two potassium ions carried back into the cell. The neuron is now ready to "fire" again. NERVE IMPULSE PROPAGATION ❖ Unmyelinated Fibers Action potential travels continuously along the axon membrane. Sequential opening and closing of voltage-gated Na⁺ and K⁺ channels. ❖ Myelinated Fibers Saltatory Conduction: Action potential jumps from one Node of Ranvier to the next. Faster transmission of the nerve impulse compared to unmyelinated fibers. SYNAPTIC TRANSMISSION Action Potential Arrival: Action potential reaches the axon terminal. Calcium Influx: Voltage-gated Ca²⁺ channels open, Ca²⁺ enters the terminal. Neurotransmitter Release: Ca²⁺ triggers vesicles to release neurotransmitters into the synaptic cleft. SYNAPTIC TRANSMISSION Neurotransmitter Binding: Neurotransmitters bind to receptors on the postsynaptic neuron. Generation of Graded Potential: If enough neurotransmitters are released, it can generate a graded potential in the postsynaptic neuron. Neurotransmitter Removal: Neurotransmitters are broken down or reabsorbed. NEUROMUSCULAR JUNCTION HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM Neuromuscular junction is formed by the junction between a large myelinated nerve fiber and a skeletal muscle fiber. ❖ END-PLATE (BRANCHING NERVE TERMINAL) Invaginates into the muscle fiber but lives outside the muscle fiber, this invagination is called synaptic gutter or synaptic trough. ❖ SYNAPTIC CLEFT (20-30 nm wide) Space between the terminal end and the fiber membrane ❖ SUBNEURAL CLEFTS Numerous smaller folds of muscle membrane at the bottom of the gutter. ❖ MITOCHONDRIA (AXON TERMINAL) Supplies energy for the synthesis of the transmitter acetylcholine. ❖ ACETYLCHOLINESTERASE (SYNAPTIC CLEFT) Capable of destroying acetylcholine. SEQUENCE OF EVENTS AT THE NEUROMUSCULAR JUNCTION 1. Action Potential Initiation 2. Neurotransmitter Release 3. ACh Binding 4. Ion Channel Opening 5. End Plate Potential Generation 6. Action Potential in Muscle Fiber 7. ACh Degradation 8. Recycling of Choline Drugs that affect transmission at the neuromuscular junction ❖ Methacholine, Carbachol, Nicotine Drugs that stimulate muscle fiber by acetylcholine-like action. Same effects as acetylcholine (spasm) but are not destroyed by acetylcholinesterase. ❖ Neostigmine, Botulinus toxin, Curare, Hemicholinium Drugs that stimulate the neuromuscular junction by inactivating acetylcholinesterase. Production of muscle spasm. ❖ Curariform, D-tubocurarine Drugs that block transmission at the neuromuscular junction. Prevents passage of impulses from end-plate to muscle. FORMATION AND FUNCTION OF CEREBROSPINAL FLUID & THE BLOOD-BRAIN BARRIER HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM Cerebrospinal Fluid (CSF) Formation: ○ Choroid Plexuses: Specialized capillary networks located in the ventricles (primarily the lateral and fourth ventricles) where blood plasma is filtered through ependymal cells to form CSF. ○ Ependymal Cells: Line the ventricles and central canal of the spinal cord, involved in CSF production and circulation. Composition: Clear, colorless fluid containing glucose, proteins, lactate, urea, electrolytes (e.g., Na+, K+, Cl-), and very few cells. DISORDERS OF THE NERVOUS SYSTEM HES 029: Human Anatomy and Physiology with Pathophysiology THE NERVOUS SYSTEM ❖ MYASTHENIA GRAVIS Autoimmune disorder affecting acetylcholine receptors, leading to muscle weakness. ❖ ANALGESIA Reduction or absence of pain without the loss of consciousness. ❖ HYPOTHALAMIC OBESITY Excessive weight gain due to hypothalamic dysfunction. ❖ HYPERKINETIC DISEASE Characterized by excessive and involuntary movements. ○ Chorea - irregular, unpredictable, and involuntary muscle movements. Huntington’s disease ○ Athetosis - characterized by slow, writhing, and continuous involuntary movements, often affecting the hands, feet, and face. Cerebral palsy ○ Wilson’s Disease - involuntary laughing or crying ❖ HYPOKINETIC DISEASE Characterized by a reduction in movement. ○ Parkinson’s disease (Paralysis Agitans) characterized by: Tremors Rigidity Bradykinesia (slowness of movement) Postural instability ❖ PROSOPAGNOSIA Inability to recognize faces. Dysfunction in the fusiform gyrus, which is involved in facial recognition. ❖ LANGUAGE DISORDERS Aphasia - Broca’s area or Wernicke’s area damage affecting speech or comprehension. Dysphasia - Mild form of aphasia affecting language use. Agraphia - Inability to write or draw. ❖ ATAXIA Lack of coordination. Damage to the cerebellum or its connections affects motor control. ❖ EPILEPSY Recurrent seizures due to abnormal brain activity. Hyperexcitability of neurons leading to uncontrolled electrical activity in the brain.