Neuroscience PDF

1. Principles of Electricity in Membrane Potentials A. Basic Concepts of Electricity 1. Potential Difference (Voltage) Voltage is the difference in electric charge between two points, measured in volts (V) or millivolts (mV). In cells, voltage is measured across...

1. Principles of Electricity in Membrane Potentials A. Basic Concepts of Electricity 1. Potential Difference (Voltage) Voltage is the difference in electric charge between two points, measured in volts (V) or millivolts (mV). In cells, voltage is measured across the membrane, creating a potential difference. 2. Current and Resistance Current (I): Flow of electrical charge, measured in amperes. Resistance (R): Opposition to current, measured in ohms. Ohm’s Law: , where is electrical potential. 3. Conductors and Insulators in Cells Cells (aqueous solutions of ions) are good conductors. Lipid Membrane acts as an insulator due to high electrical resistance. Intracellular Fluid (ICF) and Extracellular Fluid (ECF) have low electrical resistance. 2. Membrane Potentials A. Definition and Key Characteristics Membrane Potential: Voltage difference between the inside and outside of a cell, typically ranging from -40 mV to -80 mV. Genesis: Created by differences in ion concentration across the cell membrane, particularly K+ and Na+ ions. B. Factors Determining Resting Membrane Potential (RMP) 1. Ion Gradients Inside the cell: High K+ concentration. Outside the cell: High Na+ concentration. 2. Selective Permeability The membrane is more permeable to K+ than Na+, which influences the RMP. Non-diffusible anions within the cell also contribute to the negativity inside. 3. Na+/K+ Pump Active transport mechanism that pumps 3 Na+ out and 2 K+ into the cell, helping maintain RMP. C. Equilibrium and Nernst Equation Equilibrium Potential: The voltage at which there’s no net ion movement across the membrane. Nernst Equation: Calculates the equilibrium potential for specific ions: D. Examples of Calculated Equilibrium Potentials 1. Na+: 2. K+: 3. Action Potential A. Phases of Action Potential 1. Threshold Minimum depolarization (-50 mV) needed to initiate an action potential. 2. Depolarization (Rising Phase) Voltage-gated Na+ channels open, allowing Na+ influx, further depolarizing the cell. Membrane potential approaches the Na+ equilibrium potential. 3. Peak of Action Potential Na+ channels begin to inactivate, and the potential reaches a maximum (close to Na+ equilibrium potential). 4. Repolarization (Falling Phase) Na+ channels inactivate. Voltage-gated K+ channels open, causing K+ efflux and returning the membrane towards RMP. 5. Hyperpolarization Excessive K+ outflow makes the inside temporarily more negative than RMP. B. All-or-Nothing Principle Once an action potential is initiated, it propagates along the membrane if threshold is reached. C. Direction of Propagation Action potential travels in one direction due to transient inactivation of Na+ channels, preventing reverse propagation. D. Influences on Action Potential Speed 1. Axon Diameter: Larger diameter allows faster conduction. 2. Myelination: Increases speed via saltatory conduction; loss of myelin (e.g., in multiple sclerosis) slows transmission. 4. Refractory Periods A. Absolute Refractory Period Na+ channels are inactivated, making it impossible to generate another action potential regardless of stimulus strength. B. Relative Refractory Period Na+ channels gradually recover; a strong stimulus can generate an action potential, but threshold is elevated, and amplitude may be reduced.  Physiology of Neurons and Nerve Fibers Objectives of Study 1. Understanding the function and mechanisms of neurotransmitters. 2. Exploring the processes following neurotransmitter release. 3. Examining the effects of neurotransmitters on target cells. 4. Identifying body functions regulated by nerves and neurotransmitters. Introduction to Neurotransmitters Neurotransmitters are essential chemical messengers in the body, responsible for carrying signals between neurons (nerve cells) and target cells, which can be other neurons, muscle cells, or glands. These signals are crucial for body and mind function, from muscle movement to cognitive processes. Body Functions Regulated by Nerves and Neurotransmitters The nervous system plays a central role in various physiological functions: Cardiovascular control: Heartbeat and blood pressure regulation. Respiratory control: Breathing rate and rhythm. Motor functions: Muscle movement and coordination. Cognitive functions: Thoughts, memory, learning, and emotions. Sleep and circadian rhythms: Sleep patterns, healing, and aging. Stress response: Reaction to external stressors. Hormonal regulation: Control of hormones for growth, metabolism, and reproductive functions. Digestive functions: Hunger, thirst, and digestion. Sensory functions: Response to sight, sound, touch, smell, and taste. Structure of Neurons Neurons, the basic units of the nervous system, consist of three main parts: Cell Body (Soma): Produces neurotransmitters and maintains cellular health. Axon: Conducts electrical impulses from the cell body to the axon terminal. Axon Terminal: Converts the electrical signal into a chemical one by releasing neurotransmitters to communicate with the target cell. Storage and Release of Neurotransmitters Neurotransmitters are stored in synaptic vesicles within the axon terminal of neurons. Each vesicle holds thousands of neurotransmitter molecules, ready to be released into the synapse for communication with target cells. Mechanisms of Neurotransmitter Actions Neurotransmitters deliver messages to target cells in one of three primary ways: 1. Excitatory Neurotransmitters: Stimulate the neuron, causing it to fire and pass along the message. Examples: Glutamate, Epinephrine, Norepinephrine. 2. Inhibitory Neurotransmitters: Inhibit the neuron, blocking the message from being transmitted. Examples: GABA, Glycine, Serotonin. 3. Modulatory Neurotransmitters: Adjust the strength and efficiency of synaptic communication, often affecting multiple neurons simultaneously. Examples: Dopamine, Serotonin. Fate of Neurotransmitters After Signal Transmission After a neurotransmitter delivers its message, it must be cleared from the synaptic cleft to prevent continuous signaling. This can happen in one of three ways: Diffusion: Neurotransmitter molecules fade away from the synaptic cleft. Reuptake: The neurotransmitter is reabsorbed by the releasing neuron and recycled. Degradation: Enzymes break down the neurotransmitter in the synapse, preventing further action. Types of Neurotransmitters Scientists have identified over 100 types of neurotransmitters, with more likely to be discovered. Key categories and examples include: 1. Amino Acid Neurotransmitters Glutamate: Primary excitatory neurotransmitter, important for learning and memory. Imbalances are linked to conditions like Alzheimer’s and Parkinson’s. Gamma-Aminobutyric Acid (GABA): Main inhibitory neurotransmitter in the brain, regulates mood, anxiety, and seizure activity. Glycine: Inhibitory neurotransmitter in the spinal cord, involved in pain transmission and hearing. 2. Monoamine Neurotransmitters Serotonin: Regulates mood, sleep, appetite, and pain. Imbalance can lead to depression and anxiety. Histamine: Affects wakefulness, feeding behavior, and motivation; also linked to allergic responses. Dopamine: Involved in pleasure, motivation, and focus. Dysfunctions can lead to Parkinson’s disease, schizophrenia, and ADHD. Epinephrine and Norepinephrine: Central to the “fight-or-flight” response, increasing heart rate and blood pressure under stress. 3. Peptide Neurotransmitters Endorphins: Natural pain relievers that produce “feel-good” sensations and can be reduced in conditions like fibromyalgia. 4. Acetylcholine An excitatory neurotransmitter crucial for muscle contraction, memory, and learning. Imbalances are associated with Alzheimer’s disease and muscle spasms. Dysfunctional Neurotransmitter Activity When neurotransmitters do not function properly, it can lead to various health issues: Underproduction or overproduction: Too much or too little of a neurotransmitter can disrupt normal functions. Receptor issues: Damaged or inflamed receptors may fail to recognize neurotransmitters. Premature reuptake or enzyme activity: Excessive reabsorption or enzyme breakdown limits neurotransmitter availability, affecting signaling. Examples of neurotransmitter-related diseases: Alzheimer’s Disease: Linked to low acetylcholine levels. Autism Spectrum Disorders: Possibly associated with excessive serotonin. Seizures: Caused by overactive glutamate or underactive GABA. Bipolar Disorder: Manic episodes are linked to imbalanced dopamine and norepinephrine levels. Impact of Medications on Neurotransmitters Mechanism of Neurotransmission Neurons communicate with target tissues through synapses where they release chemical substances called neurotransmitters (ligands). This chemical communication process, called chemical neurotransmission, occurs within chemical synapses. Neurotransmission involves a sequence of events that enables neurons to transmit signals across the synaptic cleft to the postsynaptic cell. Steps in Neurotransmission: 1. Arrival of Nerve Impulse: When an action potential (nerve impulse) reaches the presynaptic terminal, it depolarizes the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open, allowing calcium ions to enter the presynaptic terminal. 2. Release of Neurotransmitters: Calcium influx triggers the fusion of synaptic vesicles (membrane-bound sacs) containing neurotransmitters with the presynaptic membrane. This process, called exocytosis, releases neurotransmitters into the synaptic cleft. 3. Binding to Receptors on Postsynaptic Membrane: Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane. This binding can open or close ligand-gated ion channels in the postsynaptic membrane, altering its permeability to ions such as sodium, potassium, calcium, or chloride. 4. Response Generation: The opening or closing of ion channels generates either an excitatory or inhibitory postsynaptic potential, depending on the ions involved and the type of receptor activated. 5. Termination of Signal: After neurotransmitters bind to receptors, they are either degraded by enzymes in the synaptic cleft or taken up by the presynaptic cell for recycling. This rapid termination of neurotransmitter activity ensures a brief and precise signal transmission, typically occurring within 0.5 to 4 milliseconds. Types of Neurons Neurons are the basic functional units of the brain and nervous system. They send and receive signals that control everything from muscle movement to sensory perception and thought. Neurons can be categorized based on function and location: 1. Sensory Neurons Function: Activated by external stimuli such as touch, sound, or light. Pathway: Send information from sensory receptors to the central nervous system (CNS). Structure: Most sensory neurons are pseudounipolar, with one axon split into two branches to transmit signals. 2. Motor Neurons Function: Transmit impulses from the CNS to muscles and glands, controlling muscle movements. Types: Upper Motor Neurons: Connect the brain to the spinal cord. Lower Motor Neurons: Connect the spinal cord to muscles. Structure: Typically multipolar, with one axon and multiple dendrites. 3. Interneurons Function: Act as connectors, relaying signals between sensory and motor neurons within the CNS. Location: Found mainly in the spinal cord and brain, forming complex networks and circuits. Structure: Multipolar, similar to motor neurons. Synapses: Sites of Communication A synapse is the site where neurons communicate with other neurons, muscles, or glands. There are two primary types of synapses: chemical and electrical, but the majority in humans are chemical synapses. Components of a Chemical Synapse Presynaptic Membrane: The axon terminal of the sending (presynaptic) neuron. Postsynaptic Membrane: The membrane of the receiving cell (postsynaptic neuron or effector). Synaptic Cleft: A narrow gap, approximately 20-40 nm wide, that separates the presynaptic and postsynaptic membranes. Process of Chemical Synaptic Transmission 1. Neurotransmitter Release: An action potential triggers calcium influx into the presynaptic terminal, initiating neurotransmitter release. 2. Diffusion Across the Synaptic Cleft: Neurotransmitters diffuse across the cleft to the postsynaptic membrane. 3. Activation of Postsynaptic Receptors: Binding to receptors opens ion channels, causing excitatory or inhibitory postsynaptic potentials. 4. Deactivation of Neurotransmitters: Neurotransmitters are quickly deactivated by enzymes or reabsorbed into the presynaptic neuron, ensuring precise signal timing. Functional Organization of Neurons The functional organization of neurons within the CNS is essential for processing sensory information, initiating motor output, and creating complex behaviors. Neurons in the Brain In the brain, neurons are classified based on their role in sensory, motor, or associative (integrative) functions. Sensory Neurons in the Brain: Process inputs in areas like the visual cortex or auditory cortex. Motor Neurons in the Brain: Involved in motor control regions such as the cerebellum and motor cortex. Brain neurons exhibit a wide variety of shapes and functions, making categorization more complex compared to spinal cord neurons. Neurotransmitters Neurotransmitters are chemical messengers that facilitate communication between neurons and their target cells. They are categorized into several groups based on their chemical structure and function: 1. Amino Acids: Includes glutamate (excitatory) and GABA (inhibitory). 2. Monoamines: Includes dopamine, serotonin, and norepinephrine, involved in mood, arousal, and cognitive functions. 3. Peptides: Such as endorphins, which modulate pain and reward responses. 4. Others: Acetylcholine, involved in muscle activation and memory; nitric oxide, involved in vasodilation and synaptic plasticity. Summary of Neurotransmission Process Trigger: Action potential arrives at presynaptic terminal. Calcium Influx: Depolarization opens calcium channels. Exocytosis: Synaptic vesicles release neurotransmitters. Synaptic Cleft: Neurotransmitters cross to bind receptors on postsynaptic membrane. Response: Postsynaptic ion channels open, causing excitation or inhibition. Termination: Enzymatic degradation or reuptake of neurotransmitters. This framework of neurotransmission and neuron types provides an organized view of how the nervous system processes, transmits, and responds to information, which is essential for all nervous system functions. Neurotransmission and Synaptic Mechanisms Neurotransmission in the nervous system enables communication between neurons and other cells. This transmission can occur through chemical or electrical synapses, each with unique mechanisms and effects. Types of Synaptic Transmission 1. Chemical Transmission Chemical transmission likely evolved in large, complex vertebrate nervous systems where long-distance, multi-signal communication is necessary. When a nerve impulse arrives at a synapse, a neurotransmitter is released from the presynaptic neuron into the synaptic cleft, where it binds to receptors on the postsynaptic neuron. Binding causes ion channels to open, shifting the electric polarization of the postsynaptic membrane and creating a postsynaptic potential (PSP). This can lead to the generation of a new action potential if it reaches the threshold. 2. Electric Synapses Electric synapses enable direct, rapid communication between neurons by allowing ions to flow between cells through gap junctions. Found in invertebrates and lower vertebrates, electric synapses also occur between glial cells in the human body. They allow for fast transmission and the synchronization of neuron groups. Postsynaptic Potentials (PSP) 1. Excitatory Postsynaptic Potential (EPSP) EPSPs result from the inflow of positively charged ions, which depolarize the postsynaptic membrane. Depolarization moves the membrane potential closer to the threshold, increasing the likelihood of firing an action potential. Example: Acetylcholine acts as an excitatory neurotransmitter, stimulating muscle contraction. 2. Inhibitory Postsynaptic Potential (IPSP) IPSPs are created by hyperpolarization, increasing the negative charge inside the cell and moving the membrane potential further from the threshold. Hyperpolarization reduces the likelihood of generating an action potential. Example: GABA is an inhibitory neurotransmitter that prevents involuntary muscle movements. Types of Neurotransmitters 1. Excitatory Neurotransmitters These neurotransmitters depolarize the postsynaptic membrane, moving it closer to threshold. Examples include acetylcholine, norepinephrine, and epinephrine, which facilitate rapid, short-lived responses. 2. Inhibitory Neurotransmitters These neurotransmitters hyperpolarize the postsynaptic membrane, making it more challenging to reach the threshold. GABA is a major inhibitory neurotransmitter in the CNS. Termination of Neurotransmitter Action The action of neurotransmitters is brief, typically lasting only seconds. This rapid inactivation ensures precise control over synaptic responses. Mechanisms include: Enzymatic Breakdown: Enzymes like acetylcholine esterase degrade neurotransmitters. Reuptake Mechanisms: Neurotransmitters are reabsorbed into the presynaptic neuron for recycling. Synaptic Plasticity and Neuromodulation 1. Repeated Synaptic Activity Repeated stimulation can induce long-lasting changes in synaptic strength, structure, and connections. This process underlies learning and memory, as it enhances synaptic efficiency and the formation of new synapses. 2. Neuromodulators Neuromodulators like dopamine, serotonin, and acetylcholine differ from neurotransmitters by their longer-lasting effects on multiple neurons. Unlike neurotransmitters, they are not reabsorbed quickly but persist in cerebrospinal fluid, influencing neuron activity over extended periods. 3. Neurohormones Neurohormones, such as oxytocin and vasopressin, are synthesized by neurons and secreted into the bloodstream to reach distant target tissues. Functional Organization of Neurons The nervous system consists of sensory, interneuron, and motor neurons arranged in circuits that allow diverse responses and coordination. 1. Diverging Circuits In a diverging circuit, an impulse spreads widely to various neurons. For example, sensory input from touching a hot surface can signal pain to the brain and trigger a reflex. 2. Converging Circuits In a converging circuit, multiple inputs affect a single postsynaptic neuron. For example, carrying a hot plate while feeling pain may involve input from sensory, motor, and brain neurons to produce a controlled response. 3. Reverberating Circuits Reverberating circuits create feedback loops that sustain repetitive actions, such as breathing. The signal continues in a loop until inhibited, making it useful for repetitive motor activities. Summary of Key Concepts Neurotransmission occurs via chemical or electric synapses, creating excitatory or inhibitory effects. Postsynaptic Potentials (PSP) can lead to depolarization (EPSP) or hyperpolarization (IPSP) in the target cell. Neurotransmitters and Neuromodulators have distinct roles, with neurotransmitters acting briefly and neuromodulators having extended effects on brain activity. Neural Circuits (divergent, convergent, reverberating) organize neurons in functional pathways for coordinated responses, such as reflexes or repeated movements. This framework of neurotransmission and neuronal organization provides an essential understanding of how the nervous system controls behavior, movement, and cognitive functions. Overview of Neurotransmitters and Associated Disorders Neurotransmitters are essential chemicals that facilitate communication between neurons in the nervous system. They can either be excitatory or inhibitory, influencing whether an action potential will occur in the postsynaptic neuron. Additionally, neurotransmitters can be classified based on their chemical structure, each playing distinct roles in neural function and associated with specific neurological disorders. Types of Neurotransmitters Classification by Function Excitatory Neurotransmitters: Activate receptors on the postsynaptic membrane to enhance action potential generation. Inhibitory Neurotransmitters: Function to prevent an action potential by hyperpolarizing the postsynaptic membrane. Classification by Chemical Structure Amino Acids: Includes GABA and glutamate. Monoamines: Includes serotonin and histamine. Catecholamines (a subtype of monoamines): Includes dopamine, norepinephrine, and epinephrine. Common Neurotransmitters and Their Functions 1. Acetylcholine (ACh) Type: Primarily excitatory. Function: Secreted by motor neurons to stimulate muscle contraction; also involved in autonomic functions, sensory processing, and REM sleep. Special Role: Inhibitory at parasympathetic endings of the vagus nerve to slow heart rate. Clinical Notes: Blockage of acetylcholine receptors by toxins (e.g., curare, hemlock) causes muscle paralysis; botulinum toxin inhibits acetylcholine release, also leading to paralysis. 2. Norepinephrine (NE) / Noradrenaline (NAd) Type: Excitatory. Function: Produced by the brainstem, hypothalamus, and adrenal glands; increases alertness, wakefulness, and prepares the body for stress. Clinical Notes: Low levels are linked to depression and anxiety, while high levels can disrupt sleep. 3. Epinephrine (Adrenaline) Type: Excitatory. Function: Produced by the adrenal glands; triggers the fight-or-flight response by increasing heart rate, blood pressure, and energy availability. Clinical Notes: Excess release associated with stress responses; plays a critical role in emergency responses. 4. Dopamine (DA) Type: Both excitatory and inhibitory. Function: Involved in motor control, reward, and mood; regulates prolactin secretion in the pituitary gland. Clinical Notes: Deficiency is linked to Parkinson’s disease; excessive dopamine activity is associated with psychosis and schizophrenia. Drug use can increase dopamine, affecting focus and motivation. 5. Gamma-Aminobutyric Acid (GABA) Type: Inhibitory. Function: Primary inhibitory neurotransmitter in the CNS; reduces neuronal excitability and is linked to mood regulation. Clinical Notes: Low levels of GABA can lead to anxiety; involved in epilepsy and Huntington’s disease. 6. Glutamate (Glu) Type: Excitatory. Function: Most powerful excitatory neurotransmitter; critical for learning, memory, and overall excitability in the CNS. Clinical Notes: Excess glutamate is linked to epilepsy and certain cognitive disorders. 7. Serotonin (5-HT) Type: Inhibitory. Function: Involved in mood, emotion, sleep cycle, and pain perception. Clinical Notes: Low serotonin levels are associated with depression, anxiety, OCD, and immune dysfunction. 8. Histamine Type: Excitatory. Function: Involved in wakefulness, pain perception, immune response, and gastric acid secretion. Clinical Notes: Released in allergic reactions, causing itching and inflammation. Disorders Associated with Neurotransmitter Imbalances 1. Epilepsy Cause: Often linked to a lack of inhibitory neurotransmitters like GABA or excessive excitatory neurotransmitters like glutamate. Treatment: Aimed at increasing GABA or decreasing glutamate activity. 2. Huntington’s Disease Cause: A genetic disorder with reduced GABA uptake in neurons, leading to motor dysfunction. Treatment: Symptom management through medications that increase inhibitory neurotransmitters. 3. Myasthenia Gravis Cause: Autoimmune condition blocking acetylcholine receptors at neuromuscular junctions, leading to muscle weakness. Treatment: Medications to improve acetylcholine transmission or suppress immune response. 4. Alzheimer’s Disease Cause: Linked to decreased acetylcholine in the brain, impacting memory and cognition. Treatment: Medications to increase acetylcholine levels or slow its breakdown. 5. Depression Cause: Depletion of norepinephrine, serotonin, and dopamine in the CNS. Treatment: Antidepressants that increase neurotransmitter concentrations (SSRIs, SNRIs, etc.). 6. Schizophrenia Cause: Excess dopamine in the frontal lobes. Treatment: Antipsychotic drugs to reduce dopamine activity. 7. Parkinson’s Disease Cause: Degeneration of dopamine-producing neurons in the substantia nigra. Treatment: Medications to replace or mimic dopamine. Here’s a structured and organized outline for the content you provided, with some additional contextual information for easier studying: Nervous System Overview The nervous system is the master control and communication system of the body, responsible for monitoring stimuli (sensory input), processing information (integration), and generating responses (motor output). Weight: 2 kg, about 3% of total body weight Primary Function: Control and coordination of essential functions in all body systems to maintain homeostasis Functions of the Nervous System 1. Sensory Input: Monitors changes inside and outside the body 2. Integration: Processes and interprets sensory input 3. Motor Output: Responds to stimuli by initiating actions 4. Complex Functions: Perceptions, emotions, behaviors, and memories Composition of the Brain Neurons: 85 billion Neuroglia: 10-50 trillion Synapses: Each neuron forms ~1000 synapses, creating about a thousand trillion (10^15) synapses in total. Organization of the Nervous System Central Nervous System (CNS) Components: Brain and spinal cord Function: Integration and command center Role of the Brain: Control center for registering sensations, intellect, emotions, behavior, and memory. Peripheral Nervous System (PNS) Components: Paired spinal and cranial nerves Function: Transmits messages between the CNS and the body Sensory (Afferent) Division: Transmits impulses from sensory receptors to CNS Motor (Efferent) Division: Transmits impulses from CNS to effectors (muscles, glands) Motor Division of PNS 1. Somatic Nervous System: Conscious control over skeletal muscles 2. Autonomic Nervous System (ANS): Regulates involuntary functions (smooth/cardiac muscle, glands) Sympathetic Division: Activates fight-or-flight response Parasympathetic Division: Promotes rest and digest Nervous Tissue Types of Nervous Tissue 1. Excitatory (Neurons): Cells that respond to stimuli 2. Non-Excitatory (Neuroglia): Support and protect neurons Neurons Components: Dendrite: Receives stimuli and carries impulses to the cell body Cell Body: Contains the nucleus Axon: Carries impulses away from the cell body Myelin Sheath: Insulates axons to speed up impulse transmission Nodes of Ranvier: Gaps in myelin sheath for faster impulse propagation Types of Glial Cells and Their Functions Central Nervous System (CNS) Glial Cells 1. Astrocytes: Star-shaped, form blood-brain barrier, maintain brain’s chemical environment 2. Microglia: Act as macrophages, clear debris via phagocytosis 3. Ependymal Cells: Line brain ventricles, produce and circulate cerebrospinal fluid (CSF) 4. Oligodendrocytes: Form myelin sheaths around CNS neurons Peripheral Nervous System (PNS) Glial Cells 1. Schwann Cells: Form myelin sheath around PNS neurons 2. Satellite Cells: Support neurons, regulate material exchange Cerebrospinal Fluid (CSF) Composition: Water, glucose, proteins, electrolytes, few cells Functions: 1. Supports and cushions the brain and spinal cord 2. Maintains pressure and moisture around the CNS 3. Delivers nutrients and removes waste Major Parts of the Brain 1. Telencephalon: Cerebrum (cerebral hemispheres) 2. Diencephalon: Thalamus, hypothalamus, and epithalamus 3. Brainstem: Medulla oblongata, pons, midbrain 4. Cerebellum: Coordination of movement Cerebral Hemispheres (Cerebrum) Divisions: Sulci: Larger folds that divide the cerebrum into lobes Gyri: Smaller folds that increase surface area Lobes of the Cerebrum 1. Frontal Lobe 2. Parietal Lobe 3. Temporal Lobe 4. Occipital Lobe Layers of the Cerebrum 1. Gray Matter: Outer layer, mostly neuron cell bodies 2. White Matter: Inner nerve fiber tracts connecting hemispheres Functional Areas of the Cerebral Cortex 1. Sensory Areas: Somatic Sensory: Receives skin sensations Visual: Processes visual stimuli in the occipital lobe Auditory: Processes sounds near lateral sulcus Olfactory: Processes smells in the temporal lobe Taste: Processes taste near the parietal lobe 2. Motor Areas: Primary Motor: Controls voluntary movements Broca’s Area: Involved in speech production 3. Interpretation Areas: Wernicke’s Area: Interprets speech and language Prefrontal Cortex: Responsible for personality, intellect, judgment, and complex reasoning General Interpretation Area: Integrates sensory inputs for comprehensive perception Classification of Nerve Fibers Basis of Classification: Nerve fibers are classified by diameter, degree of myelination, and conduction speed. Group A Fibers: Serve joints, skeletal muscles, and skin. Somatic sensory and motor fibers. Large diameter, thick myelin sheaths. Conduct impulses up to 300 mph. Group B Fibers: Intermediate diameter, light myelination. Conduct impulses around 30 mph. Group C Fibers: Nonmyelinated, smallest diameter. Conduct impulses at 2 mph or less. Includes ANS motor fibers and sensory fibers for smaller somatic and visceral functions. Neuron Classification 1. Structural Classification of Neurons Multipolar Neurons: Most common, found in CNS (brain, spinal cord). Three or more processes (one axon, others dendrites). Bipolar Neurons: Found in specialized parts of the eye and nose. Two processes (one axon, one dendrite). Unipolar Neurons (Pseudounipolar): Found in ganglia outside CNS. Single process that splits into two branches. 2. Functional Classification of Neurons Sensory (Afferent) Neurons: Carry impulses to CNS from body parts. Types of sensory receptors: Interoceptors: Sense internal environment. Exteroceptors: Sense external stimuli (temperature, touch). Proprioceptors: Sense position and movement. Interneurons: Connect neurons within CNS. Facilitate communication between sensory and motor pathways. Motor (Efferent) Neurons: Conduct impulses from CNS to muscles or glands. Somatic motor neurons control skeletal muscles, while visceral motor neurons control smooth/cardiac muscles and glands. Mechanisms of Neuronal Inhibition 1. Postsynaptic (Direct) Inhibition: Reduces excitability of the postsynaptic neuron. Caused by inhibitory neurotransmitters (e.g., GABA, glycine). Results in hyperpolarization, as seen in Reciprocal Inhibition (in stretch reflex). Antagonists like strychnine can block this inhibition. 2. Presynaptic (Indirect) Inhibition: Inhibition occurs without an IPSP in the postsynaptic neuron. Inhibitory neuron acts on excitatory neuron’s axon, reducing neurotransmitter release (e.g., GABA increases Cl⁻/K⁺ conductance). Result: Reduced calcium influx and lesser neurotransmitter release. Picrotoxin blocks this type of inhibition. 3. Renshaw Cell (Feedback) Inhibition: A special postsynaptic inhibition. Renshaw cells (interneurons) in the spinal cord inhibit their own motor neuron. Activated by the collateral of a motor neuron. Secretes glycine, preventing repetitive action potentials. Types of Inhibition in Neural Circuits Recurrent Inhibition: Inhibition loop where the post-neuron connects back to inhibit pre-neurons. Lateral Inhibition: Neurons inhibit their neighbors to sharpen signal resolution. Reciprocal Inhibition: Activation of one neuron inhibits the antagonistic pathway (e.g., stretch reflex). Feedforward Inhibition: Pre-neuron activates an inhibitory neuron, preventing over-excitation in target neurons. Reflex Responses and Arc Components Reflex: Automatic, rapid response to a stimulus, integral to survival instincts. Reflex Arc Components: 1. Receptor: Senses stimulus. 2. Sensory Neuron: Sends impulse to CNS. 3. Integration Center: Processes information (in CNS). 4. Motor Neuron: Sends response signal. 5. Effector: Muscle or gland executes response. Types of Reflexes: Stretch Reflex: Sudden muscle stretch causes contraction (monosynaptic). Golgi Tendon Reflex: Inhibits muscle contraction if tension is excessive, preventing damage. Flexor Reflex (Withdrawal Reflex): Painful stimulus causes withdrawal of limb, involving multiple synapses for coordinated action. Nerve Centers and Neural Networks Transmission and Processing in Neural Networks: Neural networks consist of neurons with multiple interconnections, allowing signal integration and processing. Networks are structured in layers, with deep networks used in complex processing tasks. Propagation Characteristics in Nerve Centers: One-Way Conduction: Signal travels in a single direction in synapses. Convergence: Multiple neurons converge on a single neuron, allowing for summation of inputs. Divergence: A single neuron projects to multiple neurons, spreading the signal. Afterdischarge: Continued neuron firing even after stimulus ends. Facilitation: Previous activity makes neurons more responsive to new stimuli. Fatigue: Continuous stimulation leads to reduced responsiveness in neurons. Motor Neuron Types and Functions 1. Alpha Motor Neurons: Large neurons in the spinal cord. Innervate large skeletal muscle fibers, forming the motor unit. 2. Gamma Motor Neurons: Smaller neurons that innervate intrafusal fibers in muscle spindles. Adjust the sensitivity of the spindle to detect muscle stretch. Sensory Receptor Mechanisms in Muscle and Tendon 1. Muscle Spindle: Detects changes in muscle length. Contains intrafusal fibers that are sensitive to stretch. Stretch Reflex: Quick response to muscle stretching, helping maintain muscle tone. 2. Golgi Tendon Organ: Detects tension in tendons. Prevents muscle damage by inhibiting contraction under high tension. Characteristics of the Propagation of Excitation in Nerve Centers In nerve centers, excitation propagation is a complex process, influenced by various synaptic properties. Understanding the characteristics of excitation in nerve centers provides insight into how the nervous system processes and transmits signals. 1. One-Way Conduction Definition: Synapses generally permit impulse conduction in a single direction—from the pre-synaptic neuron to the post- synaptic neuron. Synaptic Delay: The minimum time required for transmission across a synapse is approximately 0.5 milliseconds. This delay is due to the following processes: Release of neurotransmitters from the pre-synaptic terminal. Diffusion of neurotransmitters across the synaptic cleft. Binding of neurotransmitters to receptors on the post-synaptic membrane. Induction of changes in membrane permeability, leading to an influx of Na+ ions and the generation of post-synaptic potential. 2. Synaptic Inhibition Synaptic inhibition is essential for regulating nervous system function and preventing excessive neuronal firing. Types of inhibition include: A. Direct Inhibition Mechanism: Occurs when an inhibitory neuron releases neurotransmitters that act on a post-synaptic neuron. This leads to hyperpolarization by opening Cl⁻ (IPSP) or K⁺ channels, making the neuron less likely to fire. Example: Glycine is an inhibitory neurotransmitter in the spinal cord that helps block pain signals. B. Indirect Inhibition (Pre-Synaptic Inhibition) Mechanism: This form of inhibition occurs when an inhibitory synaptic knob is located directly on the termination of a pre- synaptic excitatory fiber, releasing a neurotransmitter that inhibits the release of excitatory transmitters. Example: GABA is an inhibitory neurotransmitter involved in pain modulation by reducing the release of excitatory neurotransmitters. C. Reciprocal Inhibition Mechanism: This occurs when the inhibition of antagonist muscles is initiated by the contraction of agonist muscles. The motor neurons of the agonist muscles send impulses to inhibitory interneurons, which inhibit the antagonist motor neurons. Function: Facilitates coordinated movement and prevents muscle opposition during contraction. D. Inhibitory Interneuron (Renshaw Cells) Description: Renshaw cells are inhibitory interneurons in the spinal cord that provide negative feedback inhibition to motor neurons. Function: Control the strength of muscle contraction by limiting motor neuron activity, preventing excessive contraction. 3. Summation Summation is a process by which multiple inputs combine to produce a stronger post-synaptic response, critical for reaching the threshold for an action potential. A. Spatial Summation Definition: Occurs when multiple pre-synaptic neurons synapse on a single post-synaptic neuron, collectively contributing to the generation of an action potential. B. Temporal Summation Definition: Happens when a single pre-synaptic neuron fires at a high frequency, releasing neurotransmitters rapidly enough to add up and produce a post-synaptic potential strong enough to reach the threshold. 4. Convergence and Divergence in Neural Circuits A. Convergence Definition: A process where multiple pre-synaptic neurons converge on a single post-synaptic neuron. Function: Enables integration of information from different sources, allowing the post-synaptic neuron to respond based on multiple inputs. B. Divergence Definition: A single pre-synaptic neuron’s axon branches out to connect with multiple post-synaptic neurons. Function: Allows a single neuron to send signals to multiple neurons, spreading the signal and coordinating responses across different regions. 5. Fatigue Definition: Fatigue occurs when there is a temporary exhaustion of neurotransmitter supplies in a continuously stimulated pre-synaptic neuron, resulting in a reduction or cessation of signal transmission. Implication: Synaptic fatigue protects neurons from overstimulation, which could lead to cell damage or death. Additional Synaptic Properties Afterdischarge Definition: Continued firing of post-synaptic neurons after the original stimulus has stopped, often due to persistent excitatory post-synaptic potentials (EPSPs) or complex feedback within a neural circuit. Function: Allows sustained response even after the removal of an initial stimulus, aiding in processes that require prolonged action. Facilitation Definition: Occurs when previous activation of a neuron enhances its responsiveness to subsequent stimuli, increasing the likelihood of action potential generation. Mechanism: Increased neurotransmitter release or receptor sensitivity can lower the threshold for action potential initiation. These synaptic properties contribute to the intricate functioning of neural circuits, enabling both simple reflexive actions and complex Hereâs a structured, detailed breakdown of the content on neuromuscular junction and muscle contraction, with headings and subheadings that make it easier to study and understand: Neuromuscular Junction & Muscle Contraction Objectives 1. Understand the functional anatomy of the neuromuscular junction (NMJ) and the mechanism of transmission of excitation through the NMJ. 2. Learn about the functional anatomy of striated muscle, focusing on the structural components involved in muscle contraction. 3. Comprehend the molecular mechanism of muscle contraction, including the sliding filament mechanism, the impact of muscle length on contraction force, and the process of muscle relaxation. 1. Transmission of Impulses at the Neuromuscular Junction (NMJ) A. Overview of the Neuromuscular Junction The NMJ is the synapse or connection point between a motor neuron and a skeletal muscle fiber, where impulses from nerves are transmitted to muscle fibers, initiating contraction. B. Components of the Motor End Plate 1. Axon Terminal (Nerve Terminal): ⢠Contains approximately 300,000 synaptic vesicles, each with 10,000 molecules of acetylcholine (Ach), the neurotransmitter responsible for NMJ communication. 2. Synaptic Cleft: ⢠A gap of about 20â30 nm between the axon terminal and the muscle cell membrane, containing extracellular fluid (ECF) and acetylcholinesterase, which breaks down Ach to regulate muscle stimulation. 3. Synaptic Gutter (Synaptic Trough): ⢠The part of the muscle membrane that contacts the nerve terminal. It contains numerous folds (subneural clefts) that increase surface area and accommodate a large number of Ach receptors. C. Function of Acetylcholine (Ach) 1. Synthesis: Ach is synthesized in the cytoplasm of the nerve terminal from acetyl-CoA and choline. 2. Storage and Release: Ach is stored in synaptic vesicles within the nerve terminal and released upon stimulation. 3. Role in Transmission: Ach binds to receptors on the muscle membrane, initiating depolarization and muscle contraction. 4. Transduction: Electrical impulses trigger the release of Ach, which converts the electrical signal into a chemical one, a process known as transduction. 2. Spread of Action Potential and Excitation-Contraction Coupling A. Components Involved 1. Transverse Tubules (T-Tubules): ⢠Membranous channels that run transversely through muscle fibers and are filled with ECF, carrying the action potential into the muscle. 2. Sarcoplasmic Reticulum (SR): ⢠Stores calcium ions (Ca++) necessary for muscle contraction. Composed of terminal cisternae (large chambers) and longitudinal tubules. B. Mechanism of Excitation-Contraction Coupling 1. Depolarization: ⢠The action potential in T-tubules causes a voltage change that activates dihydropyridine (DHP) receptors. 2. Calcium Release: ⢠DHP receptors interact with ryanodine receptors on the SR, releasing Ca++ into the sarcoplasm, initiating contraction. 3. Calcium Reuptake: ⢠A calcium pump removes Ca++ after contraction, allowing the muscle to relax by binding to calsequestrin in the SR. 3. Muscle Fiber Anatomy and the Sliding Filament Mechanism A. Organization of Skeletal Muscle 1. Sarcomere: ⢠The basic contractile unit, defined by the distance between two Z-discs, typically 2 μm in length. 2. Myofilaments: ⢠Actin (Thin Filaments): Attached to Z-discs, these filaments are pulled toward the sarcomere center during contraction. ⢠Myosin (Thick Filaments): Contains myosin heads, which form cross-bridges with actin, powered by ATP. B. The Sliding Filament Theory ⢠Myosin heads use ATP to âwalkâ along actin filaments, pulling them closer together. This movement shortens the sarcomere and leads to muscle contraction. C. Changes in Band Length During Contraction 1. H Band: Decreases. 2. I Band: Decreases and may disappear. 3. A Band: Remains constant in length. 4. Overall Fiber Length: Remains unchanged. 4. Muscle Relaxation Mechanism A. Calcium Reuptake ⢠After contraction, Ca++ is pumped back into the SR, allowing tropomyosin to block actin binding sites, which stops cross-bridge formation and leads to muscle relaxation. B. ATP Depletion and Fatigue ⢠ATP is necessary for muscle contraction. When depleted, muscles may enter a state of fatigue, unable to contract until ATP is replenished. 5. Drugs Affecting Neuromuscular Transmission A. Neuromuscular Blockers (NMBs) 1. Competitive (Non-Depolarizing) NMBs: ⢠Compete with Ach for nicotinic (Nm) receptors at the motor end plate, causing muscle paralysis (e.g., d-tubocurarine). 2. Non-Competitive (Depolarizing) NMBs: ⢠Cause sustained depolarization of the motor end plate, leading to muscle paralysis after initial stimulation (fasciculations). B. Therapeutic Uses ⢠Muscle relaxation in surgery, intubation, mechanical ventilation, and managing convulsions during electroconvulsive therapy (ECT). 6. Muscle Length-Tension Relationship A. Length-Tension Relationship and the Frank-Starling Mechanism 1. Optimal Sarcomere Length: ⢠The muscle generates maximum tension at a specific sarcomere length due to optimal overlap of actin and myosin. 2. Active and Passive Tension: ⢠Active Tension: Generated by actin-myosin interaction. ⢠Passive Tension: Generated by stretching non-contractile components like titin. 3. Cardiac Muscle: Follows a similar length-tension relationship, critical for heart muscle contraction. Summary The neuromuscular junction (NMJ) facilitates communication between nerves and muscles through Ach. This initiates an action potential that propagates through T-tubules,