Synapses and Neurotransmitters Notes | Zoology Optional Notes for UPSC PDF Download

Chemical Transmitters and Synapses

Chemical Transmitters called “Neurotransmitters” carry a signal across Synapses (& at Neuromuscular Junctions) A point of contact between two neurons is called a synapse At the synapse there is a break in electrical transmission (the action potential cannot cross). Instead chemicals called “neurotransmitters” are released that carry the signal to the next nerve. There is a delay at synapses, because chemical transmission between neurons is slower than electrical transmission (action potential) within a neuron. A neuromuscular junction (NMJ) is a contact between a neuron and a muscle: it is like a synapse in that the action potential stops and the signal is carried by a chemical neurotransmitter released by the neuron.

Neurotransmitters Production and Release

Neurotransmitters Are Made and Stored in the Pre-synaptic Terminal The end of the neuron enlarges into an axon terminal Neurotransmitters are produced in the cell body of a neuron and then transported to the ends of the axon terminals in small membrane-enclosed sacs called “synaptic vesicles”. At the axon terminus, neurotransmitters are stored in these tiny synaptic vesicles so that they can be released when an action potential reaches the axon terminus At a synapse, axon termini of the pre-synaptic neuron contact the dendrites of the post-synaptic neuron. Because the neurotransmitter is only located in the axon termini on one side of a synapse, the impulse can go in only one direction.

Calcium and Neurotransmitter Release

Calcium is Required for Neurotransmitter Release Neurotransmitter release requires Ca2 ions Normally, the concentration of Ca2 in the pre-synaptic neuron l is kept very low (by the action of a Ca pump). The arrival of an action potential at the axon terminus opens voltage-gated Ca channels, and Ca2 ions rush inside the pre-synaptic neuron The inrushing Ca2 ions cause some of the neurotransmitter-containing vesicles to fuse to the pre-synaptic membrane, releasing neurotransmitter into the synaptic cleft

Neurotransmitter Binding and Receptors

Neurotransmitter Diffuses Across the Synaptic Cleft and Binds to a Receptor The synaptic cleft is narrow and the neurotransmitter travels across it by simple diffusion On the far side of the synaptic cleft, the neurotransmitter binds to a specific receptor protein in the postsynaptic cell membrane Not all neurotransmitters fit in all receptors o Sort of like a lock and key, or a puzzle o If it “fits” the receptor it will bind and the receptor will become” activated” Most neurons respond to 3-5 different types of neurotransmitters Some diseases are due to defects in the neurotransmitter receptors, e.g. in myasthenia gravis an autoimmune reaction destroys acetycholine receptors in the neuromuscular junction and causes muscular weakness or paralysis Many drugs and toxins block neurotransmitter receptors, including curare, strychnine, atropine, and antihistamines

Effect of Neurotransmitter Binding on Post-synaptic Cell

When Neurotransmitter Binds to a Receptor on the Post-synaptic cell, it Produces an EPSP or an IPSP When the transmitter binds to the receptor, ion channels in the post-synaptic cell membrane are opened (ligand-gated channels) Once the channels are opened, ions rush into the post-synaptic cell If the ions depolarize the post-synaptic cell (e.g., by Na ions rushing in) they produce an excitatory postsynaptic potential (EPSP) o Most neurotransmitters produce EPSPs (e.g., acetylcholine, epinephrine, norepinephrine, and glutamate) If the ions hyperpolarize the postsynaptic membrane (e.g., by Cl ions rushing in) they produce an inhibitory postsynaptic potential (IPSP) o The major neurotransmitters producing IPSPs are glycine and GABA (gamma amino-butyric acid) Each neuron may have as many as 10,000 synapses with other neurons, and there are both excitatory and inhibitory neurons coming into most synapses, thus the behavior of the post-synaptic neuron is determined by temporal and spatial summation of the EPSPs and IPSPs. In summation, if there are enough EPSPs the postsynaptic membrane will be depolarized to the threshold level and an action potential will be produced, then the signal will travel along the second nerve or muscle cell. In summation, if there are enough IPSPs to keep the membrane potential of the post-synaptic cell negative (hyperpolarized) and cancel out the EPSPs, then the post-synaptic neuron will NOT generate an action potential, and the signal will stop.

Neurotransmitter Breakdown and Recycling

The Neurotransmitter is Broken Down and/or Recycled Once the signal has been delivered the neurotransmitter must be removed from the synaptic cleft so that new signals may be received In some cases the neurotransmitter is broken down by an enzyme in the synapse In other cases the neurotransmitter is recycled: it is transported back into the pre-synaptic neuron by endocytosis and then stored in vesicles of the axon terminus for reuse. This process is referred to as “reuptake” In still other cases, these 2 methods are combined Some drugs inhibit the enzymes that break down transmitters (e.g., nerve gases, physostigmine) Other drugs act by inhibiting “reuptake” (e.g., Prozac, cocaine)

Types of Neurotransmitters

There are Dozens of Neurotransmitters in the Nervous System In this class we will deal with only a few class of transmitters:

• Acetylcholine: Responsible for muscle stimulation, learning, emotion, and memory.
• Amino acids: GABA, glutamate, and glycine.
• Biogenic Amines: Epinephrine, norepinephrine, dopamine, and serotonin.
• Neuropeptides: Substance P and endorphins.
• Gases: NO (nitric oxide) and CO (carbon monoxide).

There are dozens of other neurotransmitters in the central nervous system (CNS) and new ones are being discovered every year. A high percentage of pharmaceutical drugs affect the function of neurotransmitters at the synapse or neuromuscular junction.

Understanding Synapses

Another way to think about SYNAPSES…. The Synapse, or synaptic cleft, is the gap that separates adjacent neurons or a neuron and a muscle. Transmission of an impulse across a synapse, from pre-synaptic cell to post-synaptic cell, is chemical. In chemical synapses, action potentials are transferred across the synapse by the diffusion of neurotransmitters, as follows:

• Calcium (Ca2) gates open: When an action potential reaches the end of an axon, the depolarization of the membrane causes voltage-gated channels to open that allow Ca2 to enter.
• Synaptic vesicles release a neurotransmitter: The influx of Ca2 into the terminal end of the axon causes synaptic vesicles to merge with the pre-synaptic membrane, releasing a neurotransmitter into the synaptic cleft.
• The neurotransmitter binds with post-synaptic receptors: The neurotransmitter diffuses across the synaptic cleft and binds with specialized protein receptors on the post-synaptic membrane. Different proteins are receptors for different neurotransmitters.
• The post-synaptic membrane is excited or inhibited: Depending upon the kind of neurotransmitter and the kind of membrane receptor, there are two possible outcomes for the post-synaptic membrane: If positive ion gates open (allowing more Na and Ca2 to enter than K to exit), the membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated. If K or chlorine ion (Cl–) gates open (allowing K to exit or Cl– to enter), the membrane becomes more polarized (hyperpolarized), which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane.
• The neurotransmitter is degraded and recycled: After the neurotransmitter binds to the post-synaptic membrane receptors, it is either transported back to and reabsorbed by the secreting neuron (“reuptake”) or broken down by enzymes in the synaptic cleft. For example, the common neurotransmitter acetylcholine (ACh) is broken down by acetylcholinesterase (AChE).

Specific Examples of Neurotransmitters

• Acetylcholine: Responsible for muscle stimulation, learning, emotion, and memory.
• Norepinephrine: Associated with high alertness, heart rate, and blood pressure.
• Dopamine: Involved in reward mechanisms, thought, feeling, motivation, behavior, and movement.
• GABA (Gamma Aminobutyric Acid): Acts as a brake to excitatory neurotransmitters, reducing anxiety.
• Glutamate: Most common neurotransmitter in the central nervous system, important for memory and learning.
• Serotonin: Associated with emotion, mood, depression, anger control, and perception.
• Endorphin: Elevates mood, reduces pain, and structurally similar to opioids.

Consider: Heroin slows heart-rate, respiration, and metabolism in general - exactly what you would need to hibernate.