Chapter 2 and 3 Physio Nerve Impulses and Synapses PDF

Chapter 2: Nerve Cells, Nerve Impulses Chapter 3: Synapses Presentation prepared by: Ms. Glenda L. Sanchez, LPT College of Arts and Sciences PSY 304: Physiological/Biological Psychology  SUBTOPICS The Cells of the Nervous System The Nerve Impu...

Chapter 2: Nerve Cells, Nerve Impulses Chapter 3: Synapses Presentation prepared by: Ms. Glenda L. Sanchez, LPT College of Arts and Sciences PSY 304: Physiological/Biological Psychology  SUBTOPICS The Cells of the Nervous System The Nerve Impulses Synapses SUBTOPIC 1: THE CELLS OF THE NERVOUS SYSTEM Neurons and glia are two fundamental types of cells in the nervous system, each playing distinct yet complementary roles in brain function. Neurons Function: Neurons are the primary cells responsible for transmitting information throughout the nervous system. They communicate via electrical impulses and chemical signals. Structure: A typical neuron has three main parts: Cell Body (Soma): Contains the nucleus and other organelles, responsible for maintaining the cell's health. Dendrites: Branch-like structures that receive signals from other neurons. Axon: A long, thin projection that transmits signals away from the cell body to other neurons, muscles, or glands. The axon can be covered by a myelin sheath, which speeds up signal transmission. Types: Neurons are classified based on their function (sensory, motor, interneurons) or structure (unipolar, bipolar, multipolar). THE CELLS OF THE NERVOUS SYSTEM Types of Neurons According to Structure  THE CELLS OF THE NERVOUS SYSTEM Glia (Glial Cells) Function: Glial cells support and protect neurons. They do not transmit signals like neurons but are essential for overall nervous system health and function. Types of Glial Cells: Astrocytes: Star-shaped cells that provide structural support, regulate the blood-brain barrier, and maintain the chemical environment for neurons. Oligodendrocytes (in the CNS) and Schwann Cells (in the PNS): Produce the myelin sheath that insulates axons, enabling faster signal transmission. Microglia: Act as the brain's immune cells, removing waste and protecting against pathogens. Ependymal Cells: Line the ventricles of the brain and spinal cord, helping produce and circulate cerebrospinal fluid. The adult human brain contains approximately 86 billion neurons (Herculano-Houzel, Catania, Manger, & Kaas, 2015). The exact number varies from person to person. We now take it for granted that the brain is composed of individual cells, but the idea was doubtful as recently as the early 1900s. Until then, the best microscopic views revealed little detail about the brain. Observers noted long, thin fibers between one cell body and another, but they could not see whether a fiber merged into the next cell or stopped before it. In the late 1800s, Santiago Ramón y Cajal used newly developed staining techniques to show that a small gap separates the tip of a neuron’s fiber from the surface of the next neuron. The brain, like the rest of the body, consists of individual cells. SANTIAGO RAMÓN Y CAJAL, A PIONEER OF NEUROSCIENCE He was a Spanish physician and scientist considered to be the founder of modern neurobiology. He was the first to report with precision the fine anatomy of the nervous system. His findings were central in the elaboration of the neuron doctrine. Cajal demonstrated that the nervous system was made up of individual cells (neurons, term coined by Waldeyer) connected to each other by small contact zones (synapses, term coined by Sherrington). The 3 anatomical structures previously described as separate by Deiters — the cell body, the axis cylinder (the axon) and the protoplasmic processes (dendritic arborizations)— were actually all part of an individual nerve cell. (1852–1934)  THE STRUCTURE OF A NEURON A motor neuron, with its soma in the spinal cord, receives excitation through its dendrites and conducts impulses along its axon to a muscle. A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation, such as light, sound, or touch. Dendrites are branching fibers that get narrower near their ends. (The term dendrite comes from a Greek root word meaning “tree.” A dendrite branches like a tree.) The dendrite’s surface is lined with specialized synaptic receptors, at which the dendrite receives information from other neurons.  THE STRUCTURE OF A NEURON The greater the surface area of a dendrite, the more information it can receive. Many dendrites contain dendritic spines, short outgrowths that increase the surface area available for synapses The cell body, or soma (Greek for “body”; plural: somata), contains the nucleus, ribosomes, and mitochondria. Most of a neuron’s metabolic work occurs here. Cell bodies of neurons range in diameter from 0.005 millimeter (mm) to 0.1 mm in mammals and up to a millimeter in certain invertebrates. In many neurons, the cell body is like the dendrites— covered with synapses on its surface.  THE STRUCTURE OF A NEURON The axon is a thin fiber of constant diameter. (The term axon comes from a Greek word meaning “axis.”) The axon conveys an impulse toward other neurons, an organ, or a muscle. Axons can be more than a meter in length, as in the case of axons from your spinal cord to your feet. The length of an axon is enormous in comparison to its width, and in comparison to the length of dendrites.  OTHER TERMS ASSOCIATED WITH NEURONS Other terms associated with neurons are afferent, efferent, and intrinsic. An afferent axon brings information into a structure. An efferent axon carries information away from a structure. If a cell’s dendrites and axon are entirely contained within a single structure, the cell is an interneuron or intrinsic neuron of that structure. For example, an intrinsic neuron of the thalamus has its axon and all its dendrites within the thalamus  Did You Know? Every sensory neuron is an afferent to the rest of the nervous system, and every motor neuron is an efferent from the nervous system. Within the nervous system, a given neuron is an efferent from one structure and an afferent to another.  THE BLOOD–BRAIN BARRIER Although the brain, like any other organ, needs to receive nutrients from the blood, many chemicals cannot cross from the blood to the brain (Hagenbuch, Gao, & Meier, 2002). The mechanism that excludes most chemicals from the vertebrate brain is known as the blood–brain barrier.  WHY WE NEED A BLOOD– BRAIN BARRIER The blood-brain barrier (BBB) is essential for several reasons, primarily related to the protection and maintenance of the brain's highly sensitive environment. Here's why the BBB is crucial: 1. Protection from Harmful Substances Toxins and Pathogens: The BBB prevents potentially harmful substances in the blood, such as toxins, bacteria, and viruses, from entering the brain tissue. The brain is particularly vulnerable to damage because neurons, once damaged, often cannot regenerate. The BBB acts as a defensive shield, ensuring that only certain molecules can pass through. Blood-Borne Chemicals: Many chemicals circulating in the blood could disrupt brain function if they reached the brain. The BBB restricts these chemicals, maintaining the brain's chemical stability.  WHY WE NEED A BLOOD– BRAIN BARRIER 2. Regulation of the Brain's Microenvironment Homeostasis: Neurons in the brain require a very stable environment to function correctly. Fluctuations in ion concentrations, pH levels, and other factors can impair neuronal activity. The BBB tightly regulates the passage of ions, nutrients, and other substances to maintain this stability, ensuring optimal conditions for nerve signal transmission. Controlled Nutrient Supply: The BBB allows essential nutrients like glucose and amino acids to enter the brain through specialized transport mechanisms, while preventing potentially harmful substances from crossing. This ensures that the brain gets the nutrients it needs without being exposed to harmful substances. WHY WE NEED A BLOOD– BRAIN BARRIER 3. Prevention of Neurotoxicity Blood-Borne Neurotoxins: Certain substances in the blood, even those that are harmless elsewhere in the body, can be toxic to the brain. The BBB helps prevent these substances from entering the brain, protecting neurons from damage. Immune Response Modulation: While the immune system protects the body from infections, an uncontrolled immune response can be harmful. The BBB regulates the entry of immune cells and antibodies into the brain, preventing excessive inflammation that could damage neural tissue.  WHY WE NEED A BLOOD– BRAIN BARRIER 4. Isolation of Neurotransmitter Systems Separation from Peripheral Neurotransmitters: Neurotransmitters like adrenaline and serotonin are present in both the brain and the peripheral nervous system. The BBB prevents peripheral neurotransmitters from affecting brain function, ensuring that neurotransmitter signaling within the brain remains precisely regulated. 5. Facilitation of Brain Function and Cognitive Processes Neuronal Sensitivity: Neurons are extremely sensitive to changes in their environment. The BBB ensures that these cells are not exposed to fluctuations in the blood's composition, which could disrupt cognitive processes such as memory, learning, and decision-making. WHY WE NEED A BLOOD– BRAIN BARRIER 6. Minimization of Immune System Interaction Protection from Autoimmunity: The brain is considered an "immune-privileged" site, meaning that the immune system's access to the brain is limited. The BBB helps maintain this status, reducing the risk of autoimmune attacks on brain tissue. In summary, the blood-brain barrier is vital for protecting the brain from external threats, maintaining a stable environment for neural function, and ensuring that the brain operates efficiently and safely. Without the BBB, the brain would be vulnerable to a wide range of dangers that could impair its ability to function properly.  NOURISHMENT OF VERTEBRATE NEURONS Most cells use a For certain other chemicals, the brain uses active transport, a protein-mediated process that expends variety of energy to pump chemicals from the blood into the brain. carbohydrates and Chemicals that are actively transported into the brain include glucose (the brain’s main fuel), amino acids (the fats for nutrition, but building blocks of proteins), purines, choline, a few vertebrate neurons vitamins, and iron (Abbott, Rönnback, & Hansson, depend almost 2006; Jones & Shusta, 2007). Insulin and probably certain other hormones also cross the blood–brain entirely on glucose, a barrier, at least in small amounts, although the sugar. mechanism is not yet known (Gray, Meijer, & Barrett, 2014; McNay, 2014)  SUBTOPIC 2: The Nerve Impulse A nerve impulse, also known as an action potential, is an electrical signal that travels along the axon of a neuron, enabling communication between neurons and other cells, such as muscles or glands. A nerve impulse is like a quick electrical message that moves along a neuron’s long, skinny part (called the axon). This message helps neurons talk to each other and also to muscles and glands.  All parts of a neuron are covered by a membrane about 8 nanometers (nm) thick. That is about one ten-thousandth the width of an average human hair. The membrane is composed of two layers (free to float relative to each other) of phospholipid molecules (containing chains of fatty acids and a phosphate group). Embedded among the phospholipids are cylindrical protein molecules through which certain chemicals can pass. When at rest, the membrane maintains an electrical gradient, also known as polarization—a difference in electrical charge between the inside and outside of The cell membrane keeps a difference in electrical the cell. The electrical potential inside the membrane charge between the inside and outside of the cell. This is slightly negative with respect to the outside, mainly difference is called polarization. Inside the cell, it's a bit more negative compared to the outside. This because of negatively charged proteins inside the happens because there are negatively charged cell. This difference in voltage is called the resting proteins inside. This voltage difference is known as the resting potential. potential Here is the illustration showing the series of events involved in the transmission of a nerve impulse along a neuron. The image depicts the stages of resting membrane potential, depolarization, propagation of the action potential, repolarization, return to resting state, and saltatory conduction in a myelinated axon. 1. Resting Membrane Potential Polarization: At rest, a neuron has a negative electrical charge inside its membrane compared to the outside, typically around -70 millivolts (mV). This difference in charge is called the resting membrane potential. Ion Distribution: The resting membrane potential is maintained by the distribution of ions across the neuron's membrane, particularly sodium (Na⁺) and potassium (K⁺) ions. There are more Na⁺ ions outside the cell and more K⁺ ions inside the cell. Sodium-Potassium Pump: This pump actively transports 3 Na⁺ ions out of the neuron and 2 K⁺ ions into the neuron, using energy from ATP. This action maintains the concentration gradients of Na⁺ and K⁺ and contributes to the negative charge inside the cell. The Sodium and Potassium Gradient for a resting membrane 2. Generation of an Action Potential Depolarization: When a neuron is stimulated by a signal (e.g., from another neuron or a sensory input), voltage-gated Na⁺ channels open, allowing Na⁺ ions to rush into the cell. This influx of positive ions causes the membrane potential to become less negative, or depolarize. If the depolarization reaches a certain threshold (around -55 mV), an action potential is triggered. Rapid Depolarization: Once the threshold is reached, more Na⁺ channels open, leading to a rapid influx of Na⁺ and a further increase in the membrane potential, typically reaching around +30 to +40 mV. 3. Propagation of the Action Potential All-or-None Principle: The action potential follows an all-or-none principle, meaning that once the threshold is reached, the action potential will occur fully and propagate along the entire length of the axon. Local Current Flow: As the action potential occurs at one point on the axon, it generates a local current that depolarizes the adjacent section of the membrane, triggering an action potential in that section. This process continues along the length of the axon, propagating the nerve impulse. Propagation of an Action Potential 4. Repolarization Closing of Na⁺ Channels: After the peak of the action potential, Na⁺ channels close, and voltage-gated K⁺ channels open. K⁺ Efflux: K⁺ ions flow out of the neuron, causing the membrane potential to return to a negative value, a process called repolarization. Hyperpolarization: Sometimes, the membrane potential temporarily becomes more negative than the resting potential, known as hyperpolarization, because K⁺ channels close slowly, allowing more K⁺ to leave the cell. 5. Return to Resting State Restoration of Ion Balance: After the action potential, the sodium-potassium pump and other ion channels restore the original distribution of Na⁺ and K⁺, returning the neuron to its resting membrane potential. Refractory Period: During this period, the neuron is temporarily unable to generate another action potential. There are two phases: Absolute Refractory Period: The neuron cannot fire another action potential, no matter how strong the stimulus. Relative Refractory Period: A stronger-than-normal stimulus is required to generate another action potential. 6. Saltatory Conduction (in Myelinated Neurons) Myelin Sheath: In myelinated neurons, the axon is covered by a myelin sheath, which acts as an insulator and speeds up the conduction of the nerve impulse. Nodes of Ranvier: The myelin sheath is interrupted at intervals by gaps called nodes of Ranvier. The action potential "jumps" from node to node, a process called saltatory conduction, which is much faster than the continuous conduction in unmyelinated axons. 7. Synaptic Transmission Reaching the Axon Terminal: When the action potential reaches the end of the axon (axon terminal), it triggers the release of neurotransmitters from synaptic vesicles into the synaptic cleft (the gap between neurons). Communication with the Next Cell: These neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic cell (another neuron, muscle cell, or gland), initiating a response in the next cell and continuing the communication process.  SUBTOPIC 3: SYNAPSES Synapses connect neurons in the brain to neurons in the rest of the body and from those neurons to the muscles. Synapses are the crucial junctions where communication occurs between neurons or between a neuron and another type of cell, such as a muscle or gland cell. This communication is essential for all nervous system functions, including movement, sensation, thought, and emotion. 1. Type of Synapse A. Chemical synapse: Properties of Synapses 1. A chemical synapse involves chemical neurotransmitters (chemical messengers) such as acetylcholine by the axon terminal of a presynaptic neuron. 2. The chemical messengers released in a synaptic cleft move towards the receptor located at the dendrite of a postsynaptic neuron. B. Electrical synapse: 1. Electrical synapse involves the transfer of nerve impulses via ions and small metabolites. 2. Electrical synapses are composed of gap junctions (intracellular aggregates permitting the direct cell to cell transfer). 3. It generates an action potential to So, in summary: be transferred further. Chemical synapses use chemical messengers to send signals, with a little delay. Electrical synapses pass signals directly using tiny electric charges, which is much faster A. Chemical Synapse: Imagine two neurons (nerve cells) trying to talk to each other. They don't actually touch but are separated by a tiny gap called a synaptic cleft. When the first neuron (presynaptic neuron) wants to send a message, it releases special chemicals called neurotransmitters into this gap. These neurotransmitters float across the gap and attach to specific spots (receptors) on the second neuron (postsynaptic neuron). Once they attach, they pass the message along, and the second neuron knows what to do, like sending the message further along. B. Electrical Synapse: In an electrical synapse, neurons are so close that they are almost directly connected through special channels called gap junctions. These gap junctions allow tiny electric signals (like small ions) to pass straight from one neuron to the next, kind of like flipping a switch. This way, the message gets passed on really fast because it doesn't need to wait for chemicals to do the job. Found in specialized locations: Heart Smooth muscle Pulp of the tooth Retina of the eye 2. Synaptic Strength Properties of Synapses Excitatory Synapses: Increase the likelihood of the post-synaptic neuron firing an action potential (e.g., glutamate as a neurotransmitter). Inhibitory Synapses: Decrease the likelihood of the post-synaptic neuron firing (e.g., GABA as a neurotransmitter). Excitatory Synapses: These are like a "go" signal. They make it more likely for the next neuron to send a message forward. Inhibitory Synapses: These are like a "stop" signal. They make it less likely for the next neuron to send a message. In short: Excitatory = Go Inhibitory = Stop  Properties of Synapses 3. Plasticity Short-Term Plasticity: Changes in synaptic strength over short periods (milliseconds to minutes), such as facilitation or depression. Long-Term Plasticity: Persistent changes in synaptic strength, such as Long-Term Potentiation (LTP) or Long-Term Depression (LTD), which are key for learning and memory. Short-Term Plasticity: This is like a quick change in how strong a signal is between neurons. It lasts just a short time, like when you quickly get better at something but only for a little while. Long-Term Plasticity: This is a lasting change in how strong the signal is. It’s like when you learn something new and remember it for a long time. Short-term plasticity is a temporary change, and long-term plasticity is a lasting change in how neurons talk to each other. How Do Synapses Change Their Strength? When a neuron sends a signal, it goes through these three steps: 1. Neurotransmitter Release: The first neuron releases chemical messengers (neurotransmitters) into the gap between neurons. 2. Binding to Receptors: These neurotransmitters then attach to special spots (receptors) on the next neuron. 3. Opening Ion Channels: This attachment opens tiny doors (ion channels) in the second neuron, letting electrical signals pass through. Changing Synaptic Strength: Synapses can become stronger or weaker by adjusting two things: 1. Amount of Neurotransmitter Released: The signal becomes stronger if more neurotransmitters are released. If less is released, the signal becomes weaker. Both of these changes affect how much electrical current flows through the neuron, which in turn changes the 2. Number of Receptors: If there are more receptors on the strength of the signal between the two neurons. This second neuron, it can catch more neurotransmitters, process is how synapses adjust and adapt, making the making the signal stronger. Fewer receptors mean a brain more flexible and capable of learning. weaker signal.  Properties of Synapses 4. Synaptic Transmission Presynaptic Mechanisms: Involves neurotransmitter release from vesicles, modulated by factors like calcium concentration and vesicle availability. Postsynaptic Mechanisms: Involves receptor binding and subsequent cellular responses, such as ion channel opening or second messenger cascades. Synaptic Transmission: Presynaptic Mechanisms: This is what happens in the first neuron. It releases tiny chemicals (neurotransmitters) from little storage bubbles (vesicles) into the gap between neurons. The release is controlled by things like how much calcium is around and how many of those storage bubbles are available. Postsynaptic Mechanisms: This is what happens in the second neuron. The released chemicals (neurotransmitters) stick to specific spots (receptors) on the second neuron. When they stick, it causes changes in the cell, like opening tiny doors (ion channels) or starting other processes inside the cell. So, the first neuron sends the message, and the second neuron receives it and reacts.  1. A Nerve Signal Arrives: The neuron gets a message and In a clearer view: gets ready to pass it on. 2. Calcium Channels Open: Little doors in the neuron open, letting calcium ions rush in. 3. Neurotransmitters Are Released: The calcium causes tiny bubbles (vesicles) full of neurotransmitters to move to the edge of the neuron and pop, releasing the neurotransmitters into the gap between neurons (the synaptic cleft). 4. Neurotransmitters Bind to Receptors: These neurotransmitters float across the gap and attach to special spots (receptors) on the next neuron. 5. Ion Channels Open: When the neurotransmitters bind, they open more tiny doors (ion channels) on the next neuron, allowing ions to flow in. 6. New Signal Is Generated: If enough ions flow in, the next neuron gets excited and sends the signal forward. In short, the signal jumps from one neuron to the next by releasing chemicals and opening doors to pass the message along.  Properties of Synapses 5. Integration Temporal Summation: Integration of multiple signals arriving at a synapse in rapid succession. This is like when one neuron sends several quick messages in a row to another neuron, and the signals add up. Spatial Summation: Integration of signals arriving simultaneously from multiple synapses on the same neuron. This is when signals from different neurons arrive at the same time on one neuron, and those signals add up. In short: Temporal Summation = Signals adding up over time. Spatial Summation = Signals adding up from different places.  Properties of Synapses 6. Synaptic Delay Time taken for a signal to be transmitted across a synapse. This is the short amount of time it takes for a signal to pass from one neuron to another across a tiny gap called a synapse. It usually takes between 0.5 and 2 milliseconds. 7. Specificity Synaptic Specificity: Synapses form between specific neurons and at specific sites, ensuring precise communication networks within the brain. It means that neurons connect with each other in very particular ways. This ensures that messages are sent to the right places in the brain, creating precise communication networks. 1. The neuron synthesizes chemicals that serve as neurotransmitters. It synthesizes the smaller The Sequence of Chemical Events at a Synapse neurotransmitters in the axon terminals and synthesizes neuropeptides in the cell body. 2. Action potentials travel down the axon. At the presynaptic terminal, an action potential enables calcium to enter the cell. Calcium releases neurotransmitters from the terminals and into the synaptic cleft, the space between the presynaptic and postsynaptic neurons. 3. The released molecules diffuse across the narrow cleft, attach to receptors, and alter the activity of the postsynaptic neuron. Mechanisms vary for altering that activity. 4. The neurotransmitter molecules separate from their receptors. 5. The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling or they may diffuse away. 6. Some postsynaptic cells send reverse messages to control the further release of neurotransmitter by presynaptic cells.  END OF THE PRESENTATION Thank you for listening! Presentation prepared by: Ms. Glenda L. Sanchez, LPT College of Arts and Sciences PSY 304: Physiological/Biological Psychology