I took the liberty of creating a midterm review of all the concepts that I felt required deep understanding rather than memorization so that everyone can have a look at concepts they may have struggled comprehending during lecture.
More of an audutory learner? Click the link to listen to a podcast on this summary.
https://notebooklm.google.com/notebook/a11d09a0-30f6-4129-8775-80f383abda12/audio
The resting membrane potential is the voltage difference across the cell membrane when a neuron is at rest, typically around -70 mV. It's maintained by:
Na⁺/K⁺ ATPase pump, which pumps 3 Na⁺ out and 2 K⁺ into the cell.
More K⁺ leak channels, allowing K⁺ to exit the cell, making the inside more negative.
An action potential has three phases:
Depolarization: Na⁺ channels open, Na⁺ flows in, making the cell positive.
Repolarization: K⁺ channels open, K⁺ flows out, restoring negativity.
Hyperpolarization: K⁺ channels stay open briefly, making the cell more negative than resting.
Absolute: No new action potential can occur (Na⁺ channels inactive).
Relative: A stronger stimulus may trigger another action potential (some Na⁺ channels are reset).
Chemical gradient: Difference in ion concentration (e.g., Na⁺ high outside, K⁺ high inside).
Electrical gradient: Difference in charge across the membrane (inside is negative).
These gradients drive ion movement during action potentials.
Neurons communicate at synapses:
Action potential reaches the axon terminal.
Ca²⁺ channels open, causing neurotransmitter release into the synaptic cleft.
Neurotransmitters bind to receptors on the postsynaptic neuron.
This generates either an EPSP (excitatory) or IPSP (inhibitory), affecting whether the postsynaptic neuron fires an action potential.
A reflex arc is the neural pathway that controls a reflex, which is an automatic response to stimuli. It involves:
Sensory receptor: Detects stimulus (e.g., stretch).
Sensory neuron: Sends the signal to the spinal cord.
Integration center: Often in the spinal cord, processes the signal.
Motor neuron: Sends a command to the muscle.
Effector (muscle): Responds to the command (e.g., contraction).
The stretch reflex is a simple reflex that helps maintain muscle tone and posture. When a muscle is stretched, this pathway is activated:
Muscle spindle detects the stretch.
Sensory neurons send the signal to the spinal cord.
Motor neurons are activated, causing the stretched muscle to contract. This reflex prevents muscle overstretching and maintains balance.
Proprioceptors like muscle spindles and Golgi tendon organs detect changes in muscle length and tension. This sensory information is sent to the central nervous system, allowing the brain to monitor body position and movement.
CPGs are neural circuits that generate rhythmic, repetitive movements (e.g., walking, breathing) without sensory input.
Pacemaker-driven CPGs: Involve individual neurons that spontaneously generate rhythmic activity, driving the motor pattern.
Circuit-driven CPGs: Rhythm is produced by the interaction between neurons within a network, not relying on a single pacemaker neuron.
Muscle contraction occurs through the interaction between two key proteins: actin (thin filaments) and myosin (thick filaments). This process is known as the sliding filament theory.
Key Steps in Muscle Contraction:
Resting State:
In a relaxed muscle, tropomyosin blocks the myosin-binding sites on actin, preventing interaction.
Troponin holds tropomyosin in place.
Excitation:
When a muscle is stimulated by a nerve signal, calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum into the muscle cell.
Ca²⁺ binds to troponin, causing a conformational change.
This shifts tropomyosin away, exposing the myosin-binding sites on actin.
Cross-Bridge Formation:
Myosin heads, energized by ATP, attach to the exposed binding sites on actin, forming a cross-bridge.
Power Stroke:
Once bound, the myosin head pivots, pulling the actin filament toward the center of the sarcomere (the basic unit of muscle contraction).
This action shortens the muscle, resulting in contraction.
ADP and inorganic phosphate (Pi) are released during this process.
Cross-Bridge Detachment:
A new ATP molecule binds to the myosin head, causing it to detach from the actin filament.
Reactivation of Myosin:
ATP is hydrolyzed into ADP and Pi, re-cocking the myosin head into its high-energy position, ready for another cycle.
Relaxation:
When stimulation stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum.
Tropomyosin re-covers the myosin-binding sites on actin, and the muscle returns to its relaxed state.
Calcium binds to troponin, shifting tropomyosin and allowing myosin to bind to actin.
ATP is required for both myosin attachment and detachment during contraction.
This cycle of cross-bridge formation, power stroke, and reattachment allows muscles to contract efficiently.
The neuromuscular junction is where a motor neuron communicates with a muscle fiber to initiate contraction. Key steps:
Nerve impulse reaches the axon terminal, triggering the release of acetylcholine (ACh) into the synaptic cleft.
ACh binds to receptors on the muscle fiber membrane (sarcolemma), causing ion channels to open.
This generates an action potential in the muscle fiber, starting the contraction process.
This process links the muscle fiber’s electrical signal (excitation) to the contraction mechanism. Key steps:
The action potential travels along the sarcolemma and down the T-tubules.
This causes the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum.
Ca²⁺ binds to troponin, causing tropomyosin to move and expose the myosin-binding sites on actin, allowing contraction to begin.
The sliding filament theory explains how muscles contract by the sliding of actin (thin) filaments over myosin (thick) filaments. Key steps:
Myosin heads bind to actin, forming a cross-bridge.
Myosin heads perform a power stroke, pulling actin filaments toward the center of the sarcomere.
ATP is required to detach the myosin head and reset for the next cycle. This sliding action shortens the sarcomere, leading to muscle contraction.
Treppe refers to the phenomenon where muscle contractions become progressively stronger when a muscle is stimulated repeatedly after each relaxation phase. Each subsequent contraction generates slightly more force than the previous one due to increased availability of calcium ions (Ca²⁺) and warming of the muscle. This occurs when stimuli are spaced far enough apart to allow full relaxation between contractions.
Wave summation occurs when successive stimuli are delivered to a muscle before it has fully relaxed from the previous contraction. The second contraction starts from a partially contracted state, resulting in a stronger overall contraction. This leads to a gradual increase in muscle tension, as more calcium remains available in the cytoplasm for subsequent contractions.
Tetanus happens when a muscle is stimulated so rapidly that it has no chance to relax between stimuli. In incomplete (unfused) tetanus, the muscle exhibits some relaxation but still shows a smooth, sustained contraction. In complete (fused) tetanus, stimuli are so frequent that no relaxation occurs, leading to a sustained, maximal contraction.
The cardiac cycle consists of the events that occur during one heartbeat, including both contraction (systole) and relaxation (diastole) of the heart chambers. It has two main phases:
Atrial diastole: The atria are relaxed, filling with blood from the veins (vena cava and pulmonary veins).
Ventricular diastole: The ventricles relax, and the atrioventricular (AV) valves (tricuspid and mitral) open, allowing blood to flow from the atria into the ventricles.
Atrial systole: The atria contract, pushing the remaining blood into the ventricles.
Ventricular systole: The ventricles contract, forcing blood into the arteries. The right ventricle pumps blood into the pulmonary artery (pulmonary circulation), and the left ventricle pumps blood into the aorta (systemic circulation). The semilunar valves (pulmonary and aortic) open during this phase, while the AV valves close to prevent backflow.
The cycle repeats with each heartbeat, ensuring continuous blood circulation through the body.
Intercalated discs are specialized structures that connect cardiac muscle cells (cardiomyocytes) to each other. They contain:
Gap junctions, which allow electrical signals (ions) to pass between cells, enabling coordinated, synchronized contraction.
Desmosomes, which provide strong adhesion between cells, preventing them from pulling apart during contraction.
These structures are critical for the synchronized contraction of the heart muscle, allowing the heart to function as a unified pump.
The pacemaker of the heart is the sinoatrial (SA) node, located in the right atrium. It generates electrical impulses that initiate each heartbeat. These impulses:
Spread through the atria, causing them to contract.
Then travel to the atrioventricular (AV) node, and further down the Bundle of His and Purkinje fibers to stimulate ventricular contraction.
The SA node sets the natural rhythm of the heart (heart rate), and this electrical activity ensures the regular pumping of blood.
Pressure gradients are differences in pressure between two regions, driving the movement of fluids in the body. These gradients are crucial for fluid movement across capillary walls and influence blood circulation, filtration, and reabsorption.
Hydrostatic pressure is the force exerted by a fluid on the walls of its container or vessel.
In the circulatory system, blood hydrostatic pressure (BHP) is the pressure exerted by blood against the walls of blood vessels, primarily in the capillaries.
It pushes fluid out of capillaries into surrounding tissues, facilitating nutrient and oxygen delivery.
Osmotic pressure is the pressure exerted by solutes (like proteins and ions) drawing water across a semi-permeable membrane.
In the blood, blood colloid osmotic pressure (BCOP) is generated by plasma proteins (mainly albumin) that draw water into the capillaries from the surrounding tissue.
This helps to balance the outward force of hydrostatic pressure by pulling fluid back into the bloodstream.
Arterial end of the capillaries: Blood hydrostatic pressure is higher than osmotic pressure, resulting in filtration—fluid moves out of the capillaries into the tissues.
Venous end of the capillaries: Osmotic pressure becomes higher than blood hydrostatic pressure, causing reabsorption—fluid moves back into the capillaries from the tissues.
These pressure gradients regulate fluid exchange between the blood and tissues:
Proper balance between hydrostatic and osmotic pressures ensures fluid homeostasis.
If hydrostatic pressure is too high (e.g., in hypertension), excess fluid leaves the capillaries, potentially leading to edema.
If osmotic pressure is too low (e.g., due to low plasma protein levels), less fluid is reabsorbed, which can also contribute to edema.
Overall, these gradients are essential for maintaining blood volume, pressure, and tissue fluid balance in the circulatory system.
Respiration involves the exchange of gases—oxygen and carbon dioxide—between the atmosphere and the body's cells. It consists of two main processes: ventilation (movement of air in and out of the lungs) and gas exchange (between the alveoli and blood).
Inhalation (Inspiration):
The diaphragm contracts and moves downward, and the external intercostal muscles contract, lifting the ribcage.
This increases the volume of the thoracic cavity, decreasing the intrapulmonary pressure (pressure inside the lungs) below atmospheric pressure.
Air flows into the lungs from an area of higher pressure (outside) to lower pressure (inside).
Exhalation (Expiration):
The diaphragm and intercostal muscles relax, causing the thoracic cavity to decrease in volume.
The intrapulmonary pressure increases above atmospheric pressure, forcing air out of the lungs.
This is typically a passive process, but during forced exhalation, the internal intercostal muscles and abdominal muscles contract to expel more air.
Intrapulmonary Pressure (Alveolar Pressure):
This is the air pressure inside the alveoli of the lungs.
During inhalation, intrapulmonary pressure decreases (to below atmospheric pressure), allowing air to flow in.
During exhalation, intrapulmonary pressure increases (above atmospheric pressure), driving air out of the lungs.
Intrapleural Pressure:
This is the pressure within the pleural cavity, the space between the lungs and the chest wall.
Intrapleural pressure is always negative relative to intrapulmonary pressure (around -4 mmHg at rest), which keeps the lungs expanded and prevents them from collapsing.
During inhalation, intrapleural pressure becomes even more negative as the chest cavity expands, creating a stronger suction that helps expand the lungs.
During exhalation, the pressure becomes less negative as the chest cavity contracts.
The lungs are able to overcome pressure differences through the mechanics of breathing:
Diaphragm and intercostal muscle contraction changes the volume of the thoracic cavity, leading to changes in pressure.
The negative intrapleural pressure keeps the lungs "stuck" to the thoracic wall, preventing collapse.
Elastic recoil of lung tissue helps return the lungs to their original shape during exhalation.
This constant balance of pressure changes ensures efficient airflow into and out of the lungs, allowing for continuous gas exchange necessary for respiration.