Communication between Neurons: The Synapse
Communication within neuron = electrical, communication between neurons = chemical
Axodendritic: axons that form synapses with dendritic spines
Axosomatic: axons forming synapses with the soma of the neurons
Dendrodendritic synapses: dendrites form synapses with other dednrites
Axoaxonic synpases: axons for synapses with other axons
Terminal button of axon contains small packages of neurotransmitters (vesicles) located next to active
zones )protein accumulation on membrane that allow vesicle to deposite its contents into synapse)
- Action potential causes Ca2+ channels to open and calcium rushes into neuron.
- exocytosis: increase of Ca2+ causes neurotransmitter to be released into synapse membrane.
vesicle fuses with active zone (axonmal membrane) results in opening in vesicle allowing
neurotransmitter to flow into synapse
post synaptic = neurotransmitter binds to protein embdedded in the postsynaptic membrane (receptor)
- receptors are neurotransmitter specific (key and lock)
2 types of receptors in postsynaptic membrane:
- transmitter-gated ion channels/ionotropic: control ion channel. When neutrotransmitter binds
here it either opens or closes and changes ionic concentrations. Often in situations with fast
o function depends on ion. If receptor controls channel permeable to Na+ then it will
depolarize dendrite (excitatory postsynaptic potential EPSP).
o If receptor controls channel permeable to chloride, it would hyper polarize dendrite
(inhibitory postsynaptic potential IPSP)
- G-protein-coupled receptors/ metabotropic: slower, more diverse responses. Occur more
o neutrotransmitter binds to receptor, subunit of G-protein beraks away and binds to ion
channel or triggers synthesis of other chemicals.
o Can result in IPSPs or EPSPs or ersult in changes in gene expression
Presynaptic = autoreceptors. Metabotropic and are in presynaptic cell membrane and bind
neurotransmitter lreased. Regulates and monitors amount of neurotransmitter in synapse
Neurotransmitter must break away and go back to synapse. 2 mechanisms for this
- Reuptake – more common. Presynaptic neuron reabsorbs neurotransmitter and repackaiging in
- Enxymatic degradation - neirtransmitter breaks down into inactive for by enzyme in synapse
Small molecule: acetylcholine, monoamines, soluable gases, amino acids. Faster responses
Large molecule: eg. Neuropeptides. released diffusely, activating metabotropic recptors and produce
meatbolic or genetic alterations within neuron. Slower, longer-lasting ersponses.
Acetylcholine (Ach) . cholinergic neurons release this. Used by all motor neurons in CNS.
- synthesized by enzymatic conversion from choline.
- Ach is deactivated into choline and acetic acid by acetylcholinesterase (AchE) - Fast degredation and choline reabsorbed presynaptically
- Drugs that inhibit AchE prevent breakdown of ACh = insecticides and nerve gases. Decreases
heart rate, blood pressure, respiration and death
- 2 types of Ach
o Muscarinic: metabotropic receptor. Found in brain and cardiac and smooth buscle
o Nicotinic: ionotropic, excitatory and can be blocked by poson. Found in striated muscles.
Few location in brain
Monoamines derived from single amino acid. 2 groups
- Tryptophan (indoleamines)
o Serotonin – 5-HT. Involved in eating, sleep, emotional behaviour. Rare in nervous
system. Antidepressants affect rate of reabsorbtion.
- Tyrosine (catecholamines)
o Norepinephrine NE
o Epinephrine E
o Dopamine DA
o Converted by using using different enzymes from tyrosine to dopa. Dopa converted
to DA ME E
o Regulate movement, mood, motivation, attention. Numerous in nervous system
o DA are metabotropic
o Involved with movement and reward
o Delpetion of DA = parkinson’s disease. Drug L-dopa improves disease
o Stimulating release of DA = addictive
o Adrenergic neurons (use NE or E) located throughout brain, many in locus coeruleus.
They’re metabotropic and mediates hormonal effects of catecholamines
Eg. Drugs that treat asthma affect adrenergic neurons and act by causing
stimulation of adrenergic receptors which relaxes bronchial muscles and
contraction of smooth buscles in bronchi
o E also hormone that is released by adrenal glands
Soluble gases includes NO and CO. Rapidly synthesized within nervous system and rapid degeneration
NO is small it can easily cross neural membrane. Doesn’t need receptor. It is a retrograde messenger.
Viagara affects NO. NO plays role in learning. Less functions known for CO
Amino acids there are 4
- Glutamate – most prevalent excitatory amino acid neurotransmitter
- Gamma-aminobutyric acid (GABA) – most prealent inhibitory amino acid neurotransmitter
- All ionotropic thus very fast
- Involved with memory and learning
Neuropeptides – large molecule class.
- Endorphins- role in pain mediation
o Codeine and heroin act on endorphin receptors that relieves pain
- Substance P - role in sensory transmission such as touch, temperature, pain
o Capsaicin produces “hot” feeling in mouth. Sitmulates cells that release substance P
o Cholycystokinin and insulin involved in regulation of hunger and ingestion - Neuropeptides are made, stored and transported differently from small molecules.
- They adjust neurosensitivity of neurons. Modulates effects of other neurotransmitters
The nervous system
- Is complicated
- Is inconsistent
Positional terms allow you to describe location precisely and many parts are named after where they
Always relates to spinal cord
Neuraxis – line through spinal cord up through front of brain. It’s straight for quadrupeds but bipeds, it
pends 90 degrees
Dorsal – back / top of your head
Ventral – front / part of brain running along the bottom surface and the part of the spinal cord closest to
Anterior/rostral – objects toward head/close to nose
Posterior /caudal– toward one’s behind. Back of head is most posterior as is the lower portion of spinal
superior – topmost
Inferior – bottompost
Medial – toward middle
Lateral – away from middle
Ipsilateral – on same side
Contralateral – opposite side
Horizontal – slices through neuraxis parallel to ground
Sagittal – brain slice taken parallel to side of brain (left and right)
Midsagittal – line right down center
Coronal – brain slice taken parallel to face
Divisions of the nervous system
Central nervous system /CNS– encased in bone. Eg. Skull and spinal column
Peripheral nervous system /PNS– outside bone
- Includes autonomic nervous system (ANS) responsible for regualting internal states.
o Conveys info to CNS (afferent).
o Nerves from the CNs projects motor information (efferent)
Parasympathetic – nonemergency behaviours
Sympathetic – prepares body for vigorous activity
Both maintain balance
- Somatic nervous system (SNS)
o Interacts with eternal environment. Largely afferent. Receives sensory signals and
projects efferents that convey motor signals from CNS.
Spinal cord It’s gray on the inside and white on the outside.
Gray matter – mostly cell bodies and blood vessels
White matter – composed of myelinated