Brain and Behavior - University of Texas at Arlington - Chapter Two

Synaptic Transmission

The process at a synapse by which a neurotransmitter is released from one neuron and diffuses to another neuron.

Reflex

Automatic muscular response to stimuli

Reflex Arc

A circuit from sensory neuron to muscle response

Speed of A Reflex

Speed of a reflex is a delayed transmission evidenced by longer time taken for impulse to finish its chain of effects (15m/s as opposed to 40m/s). The delayed reaction from initial pinch to muscle flexing in response is important because it implies the existence of synapses, that impulses must travel in the spaces between axons and that synaptic transmission is slower than axon conduction.

Temporal Summation

Term used to describe several weak stimuli at different times summing up to a larger response than one weak stimulus. This is only possible with synaptic transmission since regenerating action potentials, which is whats responsible for axon conduction, follows the “all-or-none” rule.

Presynaptic Neuron

Neuron that delivers transmission to another neuron

Postsynaptic Neuron

Neuron that receives transmission from another neuron

How does temporal summation generate an action potential?

Multiple stimuli, one after another, generate several EPSPs which accumulate in a post-synpatic nerve.

Excitatory Post-Synaptic Potential (EPSP)

Depolarization of a cell membrane that can be graded before reaching the threshold for excitation. Decays as it moves certain distances along the membrane. Used in temporal summation.

Spatial Summation

Several weak stimuli applied to slightly different locations, which stimulate several synapses that converge onto one post-synaptic cell, causing the cumulative EPSPs to go past the threshold of excitation.

Inhibitory Post-Synaptic Potential (IPSP)

A temporary polarization of a post-synaptic cell, produced by a flow of chloride ions into the cell or when potassium ions leave the cell.

How do IPSP and EPSP work together to allow animals to walk?

When a flexor muscle is excited, this excitation will simultaneously decrease the activity of the paired extensor muscle. For example, when a dog lifts one leg, the flexor muscles in that leg contract and the extensor muscles in that leg will relax, but in the other three legs, the opposite will happen. When the first leg is raised, the sensation of the leg “pinching” makes interneurons block the activity of both flexor muscles in the other legs and extensor muscles in the leg that’s being raised. This is because in order to keep balance, the dog must extend its other legs and flex the leg its raising. Relies on EPSP to excite flexor muscles in raised leg and IPSP in other legs to prevent excitation of flexor muscles.

How does the number of synapses affect response time?

The more synapses that a response travels through, the longer the response takes. The probability of an action potential on a given neuron (like one connected to two synapses) depends on the ratio of EPSPs to IPSPs at a given moment. For example, lets say an inhibitory signal and an excitatory signal are headed to the same neuron, but the inhibitory signal has to pass through an intermediary neuron first. The excitatory signal would reach the target neuron first since it only has to pass through one synapse, but the inhibitory signal would take longer. This would result in the neuron being excited for a short while before being inhibited. This setup is useful for timing certain reactions. (If it helps, think of it like redstone in Minecraft)

Spacial Summation: A or B

Axon from either cell A or cell B stimulates the post-synaptic neuron enough to reach its threshold.

Spacial Summation: A and B

Axons from both cell A and cell B stimulate the post-synaptic neuron together but neither one by itself reaches the threshold. The combination of both at the same time reaches the threshold.

Spacial Summation: A and B if not C

The axons from A and B can combine to reach the threshold for the post-synaptic cell, but the axon from C can inhibit it enough to prevent a response.

Spontaneous Firing

Sometimes post-synaptic neurons can fire off action potentials without synaptic input. EPSPs increase the frequency of action potentials above the spontaneous (making it more likely spontaneous firing will occur, also known as facilitation) whereas IPSPs decrease it (blockade).

Chemical Synapses

Communication across the synapse done by chemical means aka neurotransmitters

Where are neurotransmitters synthesized?

Smaller neurotransmitters, like acetylcholine, are synthesized in the axon terminal. Larger neurotransmitters, like neuropeptides, are synthesized in the cell body (soma).

Amino Acids

Neurotransmitters that are acids containing an amine group (NH2). Includes glutamate, GABA, glycine, and aspartate.

Acetylcholine

A modified amino acid that includes a tri-methylamine group N(CH3)3, instead of NH2

Monoamines

Nonacidic neurotransmitters containing an amine group (NH2) formed by a metabolic change of an amino acid. Has two subgroups: indoleamines and catecholamines.

Indoleamines

A subgroup of monoamines. Includes serotonin.

Catecholamines

A subgroup of monoamines that contain a catechol and an amine group. Includes dopamine, epinephrine, and norepinephrine.

Neuropeptides

Neurotransmitters formed from chains of amino acids. Includes endorphins, substance P, and neuropeptide Y.

Purines

Neurotransmitters that are a category of chemicals including adenosine and several of its derivatives (ATP).

Gases

One of the categories of neurotransmitters, includes nitric oxide and possibly others.

Choline

Precursor to acetycholine. Found in eggs, milk, and nuts.

Tryptophan

Precursor to serotonin. Food with high concentration of it is soy and food with low concentration is American corn. Competes with other large amino acids that share the same transport system. Diffusion into brain can be increased by consuming carbohydrates since they increase the release of insulin which takes competing amino acids out of the bloodstream.

Phenylalanine and Tyrosine

Precursors of catecholamines. Present in proteins.

How long does it take to transport large-sized neurotransmitters from the cell body to the axon terminal?

Can take hours or days in the largest axons.

Where and how are neurotransmitters stored?

In vesicles (tiny spherical packets located in the presynaptic terminal). Most neurotransmitters can be stored in vesicles, excluding nitric oxide. There are also neurotransmitters located outside the vesicles but these are subject to “breakdown” into inactive chemicals by metabolizing enzymes.

Exocytosis

When an action potential reaches the axon terminal, the depolarization causes voltage-dependent calcium channels to open. As calcium flows into the terminal, it triggers the movement of vesicles toward the membrane and fusion with the membrane. Then a neurotransmitters is released into the synaptic cleft within 1-2 ms.

How long does diffusion across the synaptic cleft take?

No more than 0.01 ms across a cleft that is 20-30 nanometers wide.

Complex Messages

Most individual neurons release at least two or more different types of neurotransmitters from different branches of their axons. This combination release make’s the neuron’s message more complex.

Ionotropic Effects

Refers to when a neurotransmitter bonds to one type of the receptors known as “ionotropic receptors” and immediately opens ion channels. Glutamate is excitatory (opens sodium channels), Acetylcholine is inhibitory (opens chloride channels) or excitatory depending on the case, and GABA and Glycine are inhibitory.

Ligand-Gated Receptors

Receptors that are controlled by specific neurotransmitters instead of cell membrane potential.

Ionotropic Receptors

Effects occur quickly and are very short lasting. Most classical, small-sized neurotransmitters produce these effects such as glutamate, GABA, and acetylcholine. When a neurotransmitter bonds to the receptor, the inner and outer portions of the receptor fold out, allowing the ion channel “surrounded” by the receptor to open.

Metabotropic Effects

Occur when a neurotransmitter binds to a receptor and initiates a sequence of slower and longer-lasting metabolic reactions compared to ionotropic effects. Include such behaviors as taste, smell, and pain.

Neuromodulators

The name for neurotransmitters that activate metabotropic receptors (also called neuropeptides). Includes dopamine, norepinephrine, serotonin, and neuropeptides (and sometimes Glutamate and GABA too). Can be released by dendrites, soma, or sides of the axon. Can spread out and effect many receptors at once.

Metabotropic Receptor

When a neuromodulator bonds to the receptor, it changes the configuration of the protein embedded in the cell membrane. This leads to the activation of G-Proteins which are attached to the receptor (a protein coupled with GTP which is an energy storing molecule). The G-Protein then leads to an increased concentration of a secondary messenger (usually cyclic AMP) inside the cell. This second messenger carries information to other areas of the cell, either opening other ion channels or activate/inactivate part of a chromosome.

Enzyme-Linked Receptors

Trigger enzymatic reactions in the cell

Channel-Linked Receptors

Ionotropic Receptors

G-Protein-Coupled Receptors

Metabotropic receptors

Intracellular Receptors

Metabotropic receptors (control gene expression)

G-Protein-Coupled Channels

A type of ion channel which is linked to a G-protein and activated by the secondary messenger triggered by a G-Protein-Coupled Receptor

What happens to neurotransmitters after they’ve bonded to a receptor?

Some neurotransmitters are taken back into the pre-synaptic cleft intact by transporters to be re-used. Other neurotransmitters are inactivated by a specific metabolizing enzyme which breaks them down into metabolites. Acetylcholine is broken down by Acetylcholinesterase, catecholamines are broken down by COMT (catechol-O-methol-transferase), and serotonin is broken down by MAO (monoamine oxidase).

Autoreceptors

Receptors located in the pre-synaptic membrane that detect excess transmitter and inhibit further synthesis and release.

Antagonist

Term used to describe an effect of drug use that blocks the effects of a neurotransmitter. Usually high affinity but low efficacy.

Agonist

Term used to describe an effect of drug use that boosts the effects of a neurotransmitter. Usually low affinity but high efficacy.

What are stages of synaptic processing that can be interrupted by drugs?

1. Altering Synthesis → Can increase or decrease release of neurotransmitters

2. Disrupting Vesicles → Decreases release

3. Increasing or decreasing release by other neurochemicals

4. Decreasing Reuptake → Increases or prolongs effects of neurotransmitters

5. Blocking Breakdown Into Inactive Chemicals → Increases or prolongs effects of neurotransmitters

6. Stimulating or blocking postsynaptic receptors

Amphetamine, Cocaine, and Methylphenidate (Ritalin)

Blocks reuptake of dopamine and other neurotransmitters. Gradually, in the case of ritalin.

MDMA (Ecstasy)

Releases dopamine, serotonin, and norepinephrine

Nicotine

Stimulates nicotinic-type acetylcholine receptor, which increases dopamine release

Opiates (e.g., Heroine, Morphine)

Stimulates endorphin receptors (alleviates pain, decreases stress, improves mood)

Cannabinoids (Marijuana)

Excites negative-feedback receptors on presynaptic cells

Hallucinogens (e.g., LSD)

Stimulates serotonin type 2A receptors (5-HT)

Electrical Synapses

Faster than chemical transmissions. Depolarization occurs in both the presynaptic and postsynaptic cell, resulting in the two neurons acting as if they were one.

Hormones

A chemical secreted by a gland or other cells that is transported via the bloodstream to other organs where they alter activity. Synthesized by many kinds of endocrine glands both inside and outside the brain.

What kinds of hormones are secreted by endocrine glands outside the brain?

Mainly steroids which bind to intracellular receptors in order to alter gene expression

What kinds of hormones are secreted by endocrine glands inside the brain?

Mainly peptides which bind to membrane receptors, play a key role in immune response

Releasing Hormones

Control the release of other hormones. Peptides secreted by the brain in the hypothalamus to control the release of hormones in the pituitary gland. Also referred to as Hypothalamic-Releasing Hormones (HRHs) or Hypothalamic-Releasing Factors (HRFs).

Stimulating Hormones

Secreted by the pituitary gland which secretes several types of stimulating hormones.

Anterior Pituitary

Secretes hormones that stimulate the secretion of other hormones outside the brain. Composed of glandular tissue with secretory cells which synthesize and secrete six different hormones. Thyroid-Stimulating Hormone (TSH), Luteinizing Hormone (LH), Follicle-Stimulating Hormone (FSH), Adrenocorticotropic Hormone (ACTH), Prolactin, and Growth Hormone/Somatotropin (GH)

Posterior Pituitary

Composed of neural tissue, mostly axons, and can be considered an extension of the hypothalamus. Oxytocin and Vasopression are released from the axonal terminals of the hypothalamic and reach the posterior pituitary

Peripheral Endocrine Hormones

Secreted by glands located outside the brain.

Thyroid-Stimulating Hormone (TSH)

Stimulates Thyroid gland.

Luteinizing Hormone (LH)

Increases production of progesterone (female), testosterone (male); stimulates ovulation

Follicle-Stimulating Hormone (FSH)

Increases production of estrogen and maturation of ovum (female) and sperm production (male)

Adrenocorticotropic Hormone (ACTH)

Increases secretion of steroid hormones by adrenal gland

Prolactin

Increases milk production

Growth Hormone/Somatotropin

Increases body growth, including the growth spurt during puberty

Oxytocin

Controls uterine contractions, milk release, certain aspects of parental behavior, and sexual pleasure

Vasopressin

Constricts blood vessels and raises blood pressure, decreases urine volume

Pineal Gland

Secretes melatonin. Located in the back of the corpus callosum.

Melatonin

Increases sleepiness, influences sleep-wake cycle, also has role in onset of puberty