Neurons Synapses and Communication

The most remarkable abilities of nervous systems stem from interactions among neurons. It is these interactions that process and integrate information. Our nervous systems can orchestrate complex behaviors, deal with complex concepts, and learn and remember because large numbers of neurons interact with one another. The mechanisms of these interac tions depend on synapses between cells. Synapses, as we saw above, are structurally specialized junctions where one cell influences another cell directly through the transfer of an electrical or chemical message. The cell that sends the message is the presynaptic cell, and the cell that receives it is the postsynaptic cell. The most common type of synapse in the nervous system is the chemical synapse—one in which chemical messages released by a presynaptic cell induce changes in a postsynaptic cell.

The neuromuscular junction is a classic chemical synapse

Neuromuscular junctions are synapses between muscle cells and the neurons that innervate them. They are excellent models for how chemical synaptic transmission works. The neurons that innervate muscles cells are called motor neurons. Like most other neurons, a motor neuron has only one axon, but that axon can have many branches, each with an axon terminal that forms a neuromuscular junction with a muscle cell. At each axon terminal is an enlarged knob or buttonlike structure that contains many vesicles filled with neurotransmitters. The neurotransmitter used by all vertebrate motor neurons is acetylcholine. The portion of the axon terminal plasma mem brane that forms a synapse with a muscle cell is called the presynaptic membrane. Acetylcholine is released by exocytosis when the membrane of a vesicle fuses with the presynaptic membrane.

Where does the neurotransmitter come from? Some neu-rotransmitters, like acetylcholine, are synthesized in the axon terminal and packaged in vesicles. The enzymes required for acetylcholine biosynthesis, however, are produced in the cell body of the motor neuron and are transported down the axon to the terminals along microtubules. Other kinds of neurotransmitters, such as peptide neurotransmitters, are produced in the cell body and transported down the axon to the terminals.

Motor neuron

44.13 Synaptic Transmission Begins with the Arrival of a Nerve Impulse This diagram shows the sequence of events involved in synaptic transmission at the neuromuscular junction, a typical chemical synapse.The events shown here for acetylcholine are similar for other neurotransmitters.

Motor neuron

44.13 Synaptic Transmission Begins with the Arrival of a Nerve Impulse This diagram shows the sequence of events involved in synaptic transmission at the neuromuscular junction, a typical chemical synapse.The events shown here for acetylcholine are similar for other neurotransmitters.

| Depolarization of the terminal membrane causes voltage-gated Ca2+ channels to open.

Acetylcholine Neuromuscular Junction

^ Ca2+ enters the cell and triggers fusion of acetylcholine vesicles with the presynaptic membrane.

Action potential

^ Acetylcholine molecules diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.

| Activated receptors open chemically gated Na+ channels and depolarize the postsynaptic membrane. The spreading depolarization fires an action potential in the postsynaptic membrane.

Postsynaptic cell

Acetylcholine Synaptic (muscle cell) receptors cleft

I Acetylcholine in the synaptic cleft is broken down by the enzyme acetylcholinesterase and the components are taken back up by the presynaptic cell for resynthesis.

) Na+ channels open, depolarizing the axon terminal membrane.

| Depolarization of the terminal membrane causes voltage-gated Ca2+ channels to open.

^ Ca2+ enters the cell and triggers fusion of acetylcholine vesicles with the presynaptic membrane.

Action potential

^ Acetylcholine molecules diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.

| Activated receptors open chemically gated Na+ channels and depolarize the postsynaptic membrane. The spreading depolarization fires an action potential in the postsynaptic membrane.

Postsynaptic cell

Acetylcholine Synaptic (muscle cell) receptors cleft

I Acetylcholine in the synaptic cleft is broken down by the enzyme acetylcholinesterase and the components are taken back up by the presynaptic cell for resynthesis.

The postsynaptic membrane of the neuromuscular junction is a modified part of the muscle cell plasma membrane called a motor end plate. The space between the presynaptic membrane and the postsynaptic membrane is called the synaptic cleft. In chemical synapses, the synaptic cleft is, on average, about 20-40 nm wide. Neurotransmitter released into the cleft by the presynaptic cell diffuses across to the postsynaptic membrane (Figure 44.13).

The motor end plate contains acetylcholine receptor proteins. These receptors are chemically gated channels that allow both Na+ and K+ to pass through. Since the resting membrane of the postsynaptic cell is already permeable to K+, the major change that occurs when these channels open is the movement of Na+ into the cell. When a receptor binds acetylcholine, the pore of its channel opens, and Na+ moves across the membrane, depolarizing the motor end plate (Figure 44.14).

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Responses

  • abdullah
    How does acetylcholine bind to receptors?
    5 years ago
  • Marisa
    What is the components of a typical synapse.?
    5 years ago
  • mario
    Is the motor end plate in the presynaptic postsynaptic or in synaptic cleft?
    5 years ago
  • ROBERT
    How does the Ca2 channel open and close?
    5 years ago
  • ANA
    What are the parts of Neuromuscular Junction?
    5 years ago
  • amalda
    Does a neuron have a plasma membrane?
    5 years ago
  • antje
    What is the space between plasma membrane of neurons?
    5 years ago
  • Bildat
    How does neuron stimulates the motor end plate at the neuromuscluar junction?
    4 years ago
  • Ilse
    What is the sequence of events involved in the process of impulse transmission across a synapse?
    4 years ago
  • madoc
    What is the structure of the neuromuscular junction?
    4 years ago
  • walter
    Where on the synapse is a chemicallygated channel involved?
    4 years ago
  • YOLANDA
    Which of these is not directly involved in the synapse at the neuromuscular junction?
    4 years ago
  • rose
    What forms synapses or junctions?
    4 years ago
  • mikey
    What neuron is involved in neuromuscular junction?
    4 years ago
  • Aatifa
    Which of theses in not directly involved in the synapse at the neuromuscular junction?
    4 years ago
  • ulrich
    What is a synaptic plasma membrane protein?
    3 years ago
  • semere
    What is the importance of the plasma membrane for motor neurons?
    1 year ago
  • angelica
    How is message transported through a synapse or neuromuscular junction?
    11 months ago
  • Olindo
    Does a motor neuron have a plasma membrane?
    10 months ago

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