Neuroscience Chapter on Neurons and Synapses

Functional Ways to Classify Neurons

• Sensory nerves send information about the internal and external environment to the central nervous system.
• Motor nerves control muscle and gland function.
• Motor response to sensory input depends on the integration of information.
• Interconnections between nerves are called reflex arcs.

The Synapse

• The synapse is the junction between one neuron and the next cell.
• The synapse is a specialized structure that converts electrical impulses into chemical signals for communication between cells.
• Synaptic communication can occur in:
  • Nerve-Nerve
  • Nerve-Organ/Organ-Nerve
  • Nerve-Muscle
  • Nerve-Gland
• Neuromuscular junctions are synapses between nerve and muscle cells.

Neurotransmitters

• Neurotransmitters are typically small, rapid-acting molecules.
• Some neurons can transmit up to 500-1000 impulses per second.

The Action Potential

• The strength of the action potential is an intrinsic property of the cell.
• The strength of the stimulus is encoded in the frequency of the action potentials it generates.
• Inhibitory synapses hyperpolarize the postsynaptic membrane.
• Gamma aminobutyric acid (GABA) can act as an inhibitory neurotransmitter.
  • GABA opens ligand-gated chloride (Cl−) channels.
  • Chloride ions increase the membrane potential.

Myelination of Neurons

• Most neurons are encased in a fatty sheath called the myelin sheath.
• Myelin functions as an electrical insulator.
• Myelin is formed by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system).
• The voltage-gated sodium channels of myelinated neurons are concentrated at the nodes of Ranvier.
• Action potentials "jump" from one node of Ranvier to the next, which speeds up the speed of propagation.

Categorization of Muscle Types

• Muscle cytoplasm is mostly actin and myosin filaments.
• Nuclei and organelles are located at the edge of the muscle cell.
• The sarcoplasmic reticulum is arranged in a system of tubes around groups of myofibrils.

Skeletal Muscle

• Skeletal muscle is attached to bones and allows for voluntary movement.

Peripheral Nervous System and Neuromuscular Junctions

• Motor neurons connect the CNS to muscles and glands.
• Somatic motor nerves innervate skeletal muscle.
• Autonomic nerves innervate cardiac or smooth muscle.

Motor Neurons Innervate Skeletal Muscle Fibers

• Motor neuron cell bodies are located in the ventral horn of the spinal cord.
• Motor neurons have thick, myelinated axons.
• Motor neurons lose their myelin sheath at the motor end plate.
• Each axon terminal forms a neuromuscular junction with a single muscle fiber.

The Neuromuscular Junction

• Motor neurons and skeletal muscle fibers are chemically linked at the neuromuscular junction (NMJ).
• Action potentials traveling down motor neurons cause the contraction of skeletal muscle fibers.
• The NMJ always has an excitatory effect on skeletal muscle.
• Single vesicle release creates miniature end plate potentials (mEPP).

End Plate Potential

• Muscle fibers have a resting membrane potential of −80 mV.
• Influx of sodium ions reduces the membrane potential, creating an end plate potential.
• The magnitude of the end plate potential depends on the amount and duration of acetylcholine (ACh) present.
• The depolarizing effect of the end plate potential opens voltage-gated sodium channels, eliciting an action potential in the muscle fiber.
• The action potential leads to muscle contraction.