Physiology of cells and neurons part 2

Synapses

  • Overview: Synapses are the areas where neurons communicate with each other or with other types of cells. This communication is crucial for all functions of the nervous system, enabling everything from sensory perception to muscle movement and glandular secretions.

  • Definition:

    • Junctions that allow neurons to communicate.

    • Involve the transmission of signals between a neuron and another cell.

    • Can be electrical or chemical.

  • Electrical vs. Chemical:

    • Electrical Synapses:

      • Direct physical connections between neurons (gap junctions).

      • Faster transmission.

      • Bidirectional signaling.

    • Chemical Synapses:

      • Involve neurotransmitters.

      • Slower transmission.

      • Unidirectional signaling.

  • Neurone-Neurone:

    • Communication between two neurons.

    • Involves presynaptic and postsynaptic neurons.

    • Neurotransmitters released from presynaptic neuron affect the postsynaptic neuron.

  • Neurone-Muscle (Neuromuscular Junction):

    • Communication between a motor neuron and a muscle cell.

    • Neurotransmitter (acetylcholine) released to stimulate muscle contraction.

    • Specialized structure: motor end plate on the muscle cell.

  • Neurone-Gland:

    • Communication between a neuron and a gland cell.

    • Neurotransmitters or neuromodulators released to stimulate or inhibit gland secretion.

    • Affects hormone release or other glandular products.

Action Potential (AP) Phases

  • Overview: An action potential is a rapid sequence of changes in the voltage across a membrane. These phases are crucial for nerve and muscle cell communication, involving ion movement and membrane potential shifts to transmit signals.

  • Stimulus:

    • Initial trigger that begins the action potential.

    • Must be strong enough to reach the threshold potential.

  • Depolarization (Na+ influx):

    • Rapid influx of Na+ into the cell.

    • Membrane potential becomes more positive.

  • Repolarization (K+ efflux):

    • K+ flows out of the cell.

    • Membrane potential returns to its resting state.

  • Sodium-Potassium Pump:

    • Restores ion gradients.

    • Maintains proper Na+ and K+ concentrations.

  • Threshold:

    • Minimum membrane potential that must be reached.

    • Triggers opening of voltage-gated Na+ channels.

Sodium-Potassium Pump

  • Overview: The sodium-potassium pump is a transmembrane protein that actively transports sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. This process maintains the electrochemical gradient essential for nerve impulse transmission, muscle contraction, and cell volume regulation.

  • Function (Na+ out, K+ in):

    • Pumps 3 Na+ ions out of the cell for every 2 K+ ions pumped in.

    • Creates a concentration gradient with higher Na+ outside and higher K+ inside the cell.

    • Maintains cell volume and resting membrane potential.

  • Active Transport (ATP):

    • Requires energy in the form of ATP to move ions against their concentration gradients.

    • ATP hydrolysis provides the energy for conformational changes in the pump protein.

    • Classified as primary active transport.

  • Mitochondria:

    • Provides ATP.

    • Generates the ATP required for the sodium-potassium pump to function.

  • Oxygen:

    • Oxygen is required for ATP production within the mitochondria.

    • Without oxygen, ATP production decreases, impairing the function of the sodium-potassium pump.

    • Oxygen is indirectly essential for maintaining the ion gradients.

Neuromuscular Junction (NMJ)

  • Overview: The Neuromuscular Junction (NMJ) is a specialized chemical synapse between a motor neuron and a muscle fiber. It is responsible for transmitting signals from the nervous system to muscles, enabling muscle contraction and movement. The NMJ involves the release of acetylcholine (ACh) from the motor neuron, which binds to receptors on the muscle fiber, triggering depolarization and subsequent muscle contraction.

  • Chemical Synapse:

    • NMJ functions as a chemical synapse

    • Utilizes neurotransmitters to transmit signals.

  • Somatic Nervous System:

    • NMJ is a part of the somatic nervous system.

    • Controls voluntary movements through skeletal muscles.

  • Alpha (α) Motor Neurones:

    • Innervate extrafusal muscle fibers.

    • Responsible for generating muscle contraction.

  • Neurotransmitter (ACh):

    • Acetylcholine is the primary neurotransmitter at the NMJ.

    • Released from the motor neuron to stimulate muscle contraction.

  • Synaptic Cleft:

    • The space between the motor neuron and muscle fiber.

    • Contains acetylcholinesterase (AChE) to degrade ACh.

  • Motor End Plate:

    • Specialized region of the muscle fiber membrane.

    • Contains ACh receptors that bind to acetylcholine.

  • Anticholinesterases (AChE):

    • Inhibit the acetylcholinesterase enzyme.

    • Increase the duration of ACh action in the synaptic cleft.

Action Potential Propagation

  • Overview: Action potential propagation is the process by which an electrical signal travels along the axon of a neuron. The propagation method and speed depend on whether the axon is myelinated or unmyelinated. Myelination significantly increases the speed and efficiency of signal transmission.

  • Continuous Conduction (unmyelinated):

  • Occurs in unmyelinated axons.

  • Involves sequential regeneration of the action potential along the entire axon.

  • Slower due to the need to depolarize each adjacent segment of the membrane.

  • Saltatory Conduction (myelinated):

  • Occurs in myelinated axons.

  • Action potential "jumps" from one node of Ranvier to the next.

  • Faster because depolarization occurs only at the nodes.

  • Nodes of Ranvier:

  • Gaps in the myelin sheath where the axon membrane is exposed.

  • High concentration of voltage-gated Na+ channels.

  • Action potentials are regenerated at these nodes, allowing the signal to propagate quickly.

  • Conduction Speed:

  • Myelination increases conduction speed significantly.

  • Larger diameter axons also conduct faster due to reduced internal resistance.

  • Demyelinating Diseases:

  • Diseases (e.g., multiple sclerosis) that damage the myelin sheath.

  • Impair saltatory conduction, leading to slower or blocked nerve signal transmission.

  • Results in various neurological symptoms.