last lectura

Overview of Chapter 11 Learning Objectives

  • Remember: one two three FRE.
    • Important for understanding the central topics discussed in the lecture.
  • Focus on definitions related to action potentials and synapses.

Nervous Tissue

  • Composition and location of nervous tissue:
    • Found in the brain and spinal cord; comprises nerves that innervate various tissues in the body.
  • Function:
    • Sensory afferent pathways detect sensations via specialized receptors.
    • Information is sent to the brain for interpretation, leading to motor outputs via efferent pathways (efferent neurons).
  • Types of muscle tissue affected:
    • Smooth, cardiac, skeletal, and glands depending on visceral or somatic responses.
  • Neuroglia:
    • Types in the central nervous system vs. peripheral nervous system are critical.

Action Potentials

Muscle Action Potentials

  • Characteristics:
    • Simple functions via the motor end plate (neuromuscular junction).
    • Always excitatory in nature; respond to neuron signals to generate action potentials leading to contractions.

Neuron Action Potentials

  • Differences from muscle action potentials:
    • Complexity in phases compared to muscle cells with a basic action potential.
    • Neurons experience voltage changes that shape their action potentials through several phases.
  • Resting Membrane Potential:
    • Muscle: approximately −90-90 millivolts.
    • Neuron: approximately −70-70 millivolts.
  • Depolarization and Repolarization:
    • Process begins when sodium (Na+) channels open in response to neurotransmitter binding, leading to action potential generation.
    • Sodium influx causes depolarization followed by potassium (K+) efflux to cause repolarization.
  • Hyperpolarization:
    • Occurs when the membrane potential dips below rest following action potential generation.

Local Graded Potentials vs. Action Potentials

  • Local graded potentials:
    • Weak, decremental, and reversible; dissipate quickly after induction.
  • Action potentials:
    • Propagate over longer distances via voltage-gated channels, maintain strength throughout the signal.
  • Sodium-Potassium Exchange Pump:
    • 3 sodium ions out for every 2 potassium ions in, responsible for the negative resting potential.

Phases of Action Potentials

  1. Initial Phase:
    • Action potential generation requires reaching threshold at approximately −55-55 millivolts.
    • Sodium channels open, causing rapid depolarization.
  2. Peak Phase:
    • At about +30+30 millivolts, sodium channels close.
  3. Repolarization Phase:
    • Potassium channels open, leading to efflux of K+, restoring negative potential.
    • Overshooting occurs, leading to hyperpolarization at approximately −90-90 millivolts.
  4. Return to Resting Potential:
    • Sodium-potassium pump returns membrane to resting state.

Refractory Periods

  • Absolute Refractory Period:
    • Time during which a neuron cannot initiate another action potential irrespective of stimulus strength.
  • Relative Refractory Period:
    • Follows absolute refractory period; a stimulus stronger than usual can initiate a new action potential.

Propagation of Action Potentials

  • Propagation occurs in one direction along the axon.
  • Types of propagation:
    • Continuous Propagation:
    • Occurs in unmyelinated axons, slower due to sequential opening of voltage-gated channels.
    • Saltatory Propagation:
    • Occurs in myelinated axons; action potentials jump from node to node (nodes of Ranvier).
  • Factors affecting speed:
    • Thickness of myelin and diameter of axon influence signal speed.

Synapses

  • Definition:
    • Point of communication between two neurons; consists of presynaptic and postsynaptic neurons.
  • Presynaptic Neuron:
    • Releases neurotransmitters into the synaptic cleft.
  • Postsynaptic Neuron:
    • Contains receptors for neurotransmitters.
  • Types of Synapses:
    • Electrical Synapses:
    • Directly connected by gap junctions, allowing rapid signal transmission.
    • Chemical Synapses:
    • Use neurotransmitters; slower and more complex. Common in neuronal communication.

Process in Chemical Synapses

  1. Action potential reaches axon terminal of presynaptic neuron.
  2. Voltage-gated calcium channels open, leading to neurotransmitter release via exocytosis.
  3. Neurotransmitter binds to postsynaptic receptors, leading to ion channel opening (either excitatory or inhibitory).
  4. EPSPs (Excitatory Postsynaptic Potentials):
    • Cause depolarization in postsynaptic neuron; facilitate action potential propagation.
  5. IPSPs (Inhibitory Postsynaptic Potentials):
    • Cause hyperpolarization; inhibit action potential generation.

Signal Integration in Neurons

  • Neural Integration:
    • When presynaptic signals converge at a postsynaptic neuron; excitatory and inhibitory signals are summed to determine if threshold is reached.
  • **Types of Summation:
    • Temporal Summation:**
    • Multiple signals from the same synapse in rapid succession.
    • Spatial Summation:
    • Signals from multiple synapses occurring simultaneously.

Neurotransmitters

  • Acetylcholine:
    • Key excitatory neurotransmitter for muscle fibers and neurons; binds to cholinergic receptors.
  • Other Neurotransmitters:
    • Dopamine, serotonin, GABA (inhibitory), glutamate (excitatory), norepinephrine, and histamine are critical for various functions.
  • Receptors:
    • Ionotropic (immediate effects on ion channels) vs. Metabotropic (longer-term effects through metabolic pathways).

Practical Applications

  • Understanding action potentials and neurotransmitters is essential for comprehending neuromuscular functions, signal propagation, and certain neurophysiological conditions like multiple sclerosis.
  • Summation processes are important in understanding reflex actions and higher-level neural processing in the brain.