Chapter 2: Synapse Flashcards
Synapse:
specialized gap between neurons.
Sherrington
studied reflexes (automatic muscular responses to stimuli) and introduced the term synapse
Reflex arc:
the circuit from sensory neuron to muscle response
- Temporal summation:
Repeated stimuli within a brief time having a cumulative effect.
Presynaptic neuron:
The neuron that delivers the synaptic transmission.
Postsynaptic neuron:
The neuron that receives the message.
Graded potential:
Either depolarization (excitatory) or hyperpolarization (inhibitory) of the postsynaptic neuron.
excitatory postsynaptic potential (EPSP)
A graded depolarisation which occurs when Na+ ions enter the postsynaptic neuron.
- EPSPs are not action potentials: The EPSP’s magnitude decreases as it moves along the membrane.
- A quick sequence of EPSPs combine to exceed the threshold and produce an action potential
Spatial summation:
Several synaptic inputs originating from separate locations exerting a cumulative effect on a postsynaptic neuron.
a. For example, pinching one point does not produce a reflex but pinching two points at once does.
b. This happens because the two points activated separate sensory neurons, whose axons converged onto a neuron in the spinal cord.
c. Spatial summation critical to brain functioning
Inhibitory postsynaptic potential (IPSP):
A temporary hyperpolarization of a postsynaptic cell (this occurs when K+ leaves the cell or Cl- enters the cell after it is stimulated).
Spontaneous firing rate:
The ability to produce action potentials without synaptic input (EPSPs and IPSPs increase or decrease the likelihood of firing action potentials).
The Discovery of Chemical Transmission at Synapses
- T.R. Elliott (1905):
- Otto Loewi (1960):
- T.R. Elliott (1905): Suggested that the sympathetic nerves stimulate muscle by releasing adrenaline or a similar chemical. However, the evidence was not convincing and most scientists continued to belief that synapses communicated electrically.
- Otto Loewi (1960): Fluid from a stimulated frog heart was transferred to another heart. The fluid caused the new heart to react as if stimulated.
- Lead to the conclusion that synaptic transmission depends on chemical rather than electrical stimulation.
- This discovery revolutionized our understanding and led to research developing psychiatric drugs.
neurotransmitters:
a. Chemicals that are released by one neuron at the synapse and affect another neuron
Categories of neurotransmitters include:
Amino acids Monoamines Acetylcholine Neuropeptides (chains of amino acids) Purines Gases (such as nitric oxide, which dilate nearby blood vessels, increasing blood flow to nearby brain areas)
Catecholamines (dopamine, epinephrine, and norepinephrine):
Three closely related compounds containing a catechol and an amine group.
Synthesis of Transmitters
Neurons synthesize neurotransmitters (from amino acids) from precursors derived originally from food.
Vesicles:
Tiny nearly spherical packets that store high concentrations of neurotransmitters near the presynaptic terminal
- Nitric oxide is an exception to this rule, as neurons do not store nitric oxide for future use.
MAO (monoamine oxidase):
break down transmitters into inactive chemicals because it is possible for a neuron to accumulate excess levels of a neurotransmitter
- The first antidepressant drugs that psychiatrists discovered were MAO inhibitors, by blocking MAO they increased the brain’s supply of serotonin, dopamine and norepinephrine
exocytosis:
When an action potential reaches the axon terminal, the depolarization causes voltage-dependent calcium gates to open. As calcium flows into the terminal, the neuron releases a neurotransmitter into the synaptic cleft within 1-2 milliseconds
A neurotransmitter can have two types of effects when it attaches to the active site of the receptor:
ionotropic (faster but shorter effect) or metabotropic (slower but longer effects) effects.
Ionotropic effects:
- Sodium and potassium channels along an axon are transmitter-gated or ligand-gates channels. When the neurotransmitter attaches, it opens a channel.
- Well suited to conveying visual and auditory information along with anything else that needs to be updated ASAP
Metabotropic Effects:
when the neurotransmitter attaches to the receptor, it alters the configuration of the rest of the receptor protein, enabling a portion of the protein inside the neuron to react with other molecules. Activation of the receptor by the neurotransmitter leads to activation of G-proteins, which are attached to the receptor.
- These synapses are better for more enduring effects such as taste, smell and pain
G-proteins:
A protein coupled to the energy-storing molecule guanosine triphosphate (GTP)
Second messenger:
Chemicals that carry a message to different areas within a postsynaptic cell; the activation of a G-protein inside a cell increases the amount of the second messenger.
- A metabotropic synapse, by way of its second messenger, influences activity in much or all of the cell and over longer time
Neuropeptides/ neuromodulators:
b. Whereas the neuron synthesizes most other neurotransmitters in the presynaptic terminal, it synthesizes neuropeptides in the cell body and then slowly transports them to other parts of the cell.
c. Whereas other neurotransmitters are released at the axon terminal, the neuropeptides are released mainly by dendrites, and also by the cell body and the sides of the axon.
d. They are better for hunger, thirst and other long-term changes in behaviour and experience
Hallucinogenic drugs:
Drugs that distort perception.
i. Many hallucinogenic drugs like lysergic acid diethylamide (LSD) resemble serotonin and bind to serotonin type 2A (5-HT2A) receptors.
ii. These drugs provide stimulation at inappropriate times or for longer than usual durations.
A drug that chemically resembles a neurotransmitter:
can bind to its receptor.
Nicotine:
Compound found in tobacco.
i. Stimulates the nicotinic receptor (a type of acetylcholine receptor) in both the central nervous system and neuromuscular junction of skeletal muscles.
ii. Can also increase dopamine release by attaching to neurons that release dopamine in the nucleus accumbens.
Opiate drugs:
Derived from, or chemically similar to those derived from, the opium poppy.
i. Familiar opiates include morphine, heroin, and methadone.
ii. Opiate drugs exert their effects by binding to the same receptors as endorphins.
Inactivation and Reuptake of Neurotransmitters
a. Neurotransmitters need to be inactivated because they may continue to excite or inhibit the receptor. Neurotransmitters are inactivated in different ways.
b. Acetylcholine is broken down by the enzyme acetylcholinesterase (AChE); the two components are reabsorbed and recycled.
c. Serotonin and the catecholamines (i.e., dopamine and norepinephrine) are taken up by the presynaptic neuron. This process is called reuptake; it occurs through specialized proteins called transporters.
d. Some serotonin and catecholamine molecules are converted into inactive chemicals by enzymes such as COMT (converts catecholamines) and MAO (converts both catecholamines and serotonin).
e. Stimulant drugs, including amphetamine and cocaine, inhibit the transporters for dopamine, thus decreasing reuptake and prolonging dopamine’s effects.
Methylphenidate (Ritalin):
Stimulant drug often prescribed for attention- deficit/hyperactivity disorder.
i. Works like cocaine by blocking reuptake of dopamine at presynaptic terminals.
ii. The effects of methylphenidate are much longer lasting and less intense as compared to cocaine, yet also come with a risk of addiction.
Autoreceptors:
Presynaptic receptors sensitive to the same neurotransmitter they release. Detect the amount of transmitter released and inhibit further synthesis and release (that is, they provide negative feedback).
Electrical Synapses
- Gap junction:
A few special-purpose synapses operate electrically because it is faster than chemical transmission (somewhat confirming Sherrington’s hypothesis).
- Gap junction: In an electrical synapse, the membrane of one neuron comes into direct contact with the membrane of another.
a hormone:
chemical that is secreted primarily by glands but also by other cells and is conveyed by blood to other organs whose activity it influences
- Unlike neurotransmitters, which are released directly to another neuron, hormones convey messages to any organ that can receive them.
Endocrine glands:
produce hormones
Protein hormones and peptide hormones:
composed of chains of amino acids
- Protein and peptide hormones bind to membrane receptors and activate a second messenger within the cell—exactly the same process as at a metabotropic synapse.
Hormones secreted by the brain control the secretion of other hormones.
a. The pituitary gland is attached to the hypothalamus and consists of two distinct glands, the anterior pituitary and the posterior pituitary, which each produce different hormones.
b. The posterior pituitary is composed of neural tissue like the hypothalamus. Two hormones, oxytocin and vasopressin (also known as antidiuretic hormone), are released from the posterior pituitary. However, both of these hormones are synthesized in the hypothalamus.
c. The anterior pituitary, composed of glandular tissue, synthesizes six hormones.
d. The hypothalamus controls the release of these six hormones by secreting releasing hormones that stimulate or inhibit the release of other hormones.
- The hypothalamus maintains fairly constant circulating levels of certain hormones through a negative feedback system
