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Single unit smooth muscle cells are arranged in bundles such as the arrangement in a viscera eg intestine uterus and ureter Fig 2 11 These smooth muscle cells commun icate through hundreds of gap junctions separating the cell membranes by only 2 3 nm and from pacemaker tissue of variable location
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in gap junctions A chemical synapse consists of a neuronal presynaptic terminal a synaptic cleft and a subsynaptic or postsynaptic membrane with associated receptor proteins Fig 2 2 The chemical synapse is highly developed in the CNS It conducts the signal one way only and has a characteristic synaptic delay The presynaptic axon terminal typically
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developed till now are described here Nitro glycerine nitroprusside and similar drugs relax smooth muscles by transfer of NO from endothelial cells NO increases intracellular cGMP Fig 11 1 which is the basis for the beneficial effect of the drugs on cardiac cramps These second messengers activate protein kinases that phosphorylate effector proteins such as Ca2 +
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in diseases such as hypertension stroke asthma and many gastrointestinal diseases Smooth muscle cells can be divided into multi unit smooth muscle and single unit smooth muscle Fig 2 10 Contraction of multi unit smooth muscle cells vascular A single contraction is elicited by an electrical stimulus and later acetylcholine elicits tetanus Contraction of multi
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Neuropeptides Box 7 2 have slow excitatory or inhibitory transmitter actions Peptides cannot be synthesized locally in the axon terminals because they do not have ribosomes Fig 2 4 Peptide neurotransmitters Peptides are water soluble and act as hormones by binding to specific cell surface receptors Cell surface receptors are a family of guanosine
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of cross sectional area Hill developed an equation for the shortening velocity of isotonic muscle contractions Eq 2 2 The equation is illustrated in Hills force velocity diagram Fig 2 8 The maximum force is developed at the initial length Fig 2 8 right 18 g of load At 18 g there is no shortening the length is unchanged Stimulation of the unloaded
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If Ca2 + is present a new crossbridge cycle is initiated and may occur 100 times each s With a cycle movement of 10 nm this is 1000 nm per s for each half of the sarcomere Fig 2 7 Force length diagram Force is required to stretch a relaxed muscle because muscle tissue is elastic and the force increases with increasing muscle length Fig 2 7 The
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bundles of filaments that are visible on electron micrographs The A band contains the thick filaments of myosin and the I band contains thin filaments of actin and tropomyosin Fig 2 6 The thin filaments are anchored to a transverse structure termed the Z disc Fig 2 6 Each contractile unit contains the halves of two I bands with the A band in between This
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aminobutyric acid in the brain and glycine in the spinal cord Binding of GABA to the GABA receptor opens the pore for Cl influx whereby the subsynaptic cell membrane hyperpolarises Fig 2 3 The increase in Cl conductance stabilises the membrane potential and decreases the efficacy of excitatory transmission The GABA receptor pore is permeable to K+ besides Cl
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