Classes of Neurotransmitters • Acetylcholine • Amino Acids • aspartate • glutamate • GABA (-aminobutyric acid: from glutamate) • glycine
Classes of Neurotransmitters • Amines: derived from amino acids • monoamines • catecholamines (tyrosine derivatives) • norepinephrine • epinephrine • dopamine • serotonin [5HT: 5-hydroxytryptamine] (from tryptophan) • [Monoamine oxidase (MAO) inactivates the mono-amines listed above. Reductions in norepinephrine and serotonin levels in the brain are associated with depression. Therefore, MAO inhibitors can be used as antidepressants.] • histamine (from histidine)
Classes of Neurotransmitters • Purines and Pyrimidines • adenosine • ATP • UTP • Gases • NO (nitric oxide) • Peptides Alberts et al., Molecular Biology of the Cell cf. Fig. 31-1 Ganong
Neuropeptides • There are many, probably hundreds, of neuropeptides. • Examples include • posterior pituitary hormones: oxytocin and vasopressin (ADH) • hormones which regulate secretions from the anterior pituitary: hypothalamic releasing hormones and release-inhibiting hormones • substance P (P for pain) • endorphins and enkephalins: endogenous opiates (i.e., natural pain killers)
Neurotransmitters Table 4-1, Ganong see also - Table 12.3
Neurotransmitters:Mechanisms of Action • binding to channel-linked receptors (ligand-gated channels) • generation of postsynaptic potentials • EPSP = excitatory postsynaptic potential • depolarization (e.g., due to Na+ in) • e.g., ACh @ nicotinic cholinergic receptor • IPSP = inhibitory postsynaptic potential • hyperpolarization (e.g. due to K+ out) or stabilization (e.g. due to Cl- in and out) • It’s harder to reach threshold. The effect is to put adjacent, action-potential-producing areas into a relative refractory period.
Neurotransmitters:Mechanisms of Action • binding to non channel-linked receptors (G-protein linked and catalytic receptors) • adaptive/survival advantage? • better regulation • The variety of neurotransmitters and of receptors allows much fine tuning of function. At each step along the pathway from sending cell to physiologic effect(s) of receiving cell there is the possibility for pharmacological intervention.
The Neuron • e.g., somatic motor neuron • dendrites • cell body (soma) • nucleus • Nissl bodies • ribosomes: protein synthesis • neurofilaments • intermediate filaments • axon hillock: trigger zone • axon Neurofilaments Fig. 12.4
The Neuron • e.g., somatic motor neuron • axon terminal with synaptic end bulbs • Nerve Fiber = axon and associated Schwann cells (neurolemmocytes) • Nerve = bundle of nerve fibers Fig. 12.4
Graded Potentials and Action Potentials Fig. 12.24 • Graded potentials are characteristic of the membranes of the cell body and dendrites. • Graded potentials are proportional to the stimulus. • Action potentials are characteristic of the membranes of axons. • Action potentials are all-or-none. Fig. 12.13
Fig. 4-1 Ganong Trigger Zone • There are thousands of synapses on the dendrites and cell body. • The individual EPSPs and IPSPs from each synapse passively spread to the trigger zone. • trigger zone = axon hillock of cell body and initial segment of axon. • At the hillock the electrical signals add together (are summated). This summation is both over time and space. Fig. 12.4
Summation of Graded Potentials Fig. 12.26 at the trigger zone Fig. 12.25
A larger EPSP results in a greater number of action potentials being produced. • If the summation of the post-synaptic potentials at the axon hillock exceeds threshold, it will cause action potentials to be generated at the initial segment of the axon. • The greater the depolarization, the greater the number of action potentials fired. B: summation at axon hillock C: action potentials generated at the initial segment of the axon Moffett, Moffett and Schauf, Human Physiology see also Fig. 12.28
Graded (Local) Potentials and Action Potentials • Graded potentials are an AM system; Action potentials are an FM system. • AM = amplitude modulation • for graded potentials: the greater the stimulus, the greater the amplitude (size) of the graded potential • FM = frequency modulation • for action potentials: the greater the stimulus, the greater the frequency of firing [For additional comparisons, see Table 12.2.]