Tenth lecture
c- Telany:
If the muscle fibres exposed to a contimuous stimulus, resulted
either complete or incomplete tetany.
Effect of temperature on the muscle:
Warming of a muscle leads to a stronger contraction than
normal, with shortening of all phases of the simple muscle twitch.
Due to acceleration of the metabolic reactions needed to provide
energy for muscle contraction. Warming decreases the viscosity of
the muscle facititales the process of contraction. During muscular
exercise the muscle temperature rises which makes contraction
stronger and more rapid. Cooling of the muscle produce the
opposite effects.
Effect of Fatigue
Rapid and repeated stimulation leads to muscle fatigue
manifested by decrease in strength of contraction and
prolongation of all phases of the simple muscle twitch specially
the relaxation phase which becomes in complete (contracture).
Contracture is a state of sustained muscle contraction which
occurs where the muscle become extremely fatigued. It is due
to depletion of ATP which is important for muscle relaxation.
In isolated muscles, fatigue occurs rapidly because of the
following:
Fatigue of the excitation contraction coupling mechanism due to
decreased ATP.
Decreased active transport of Ca++ ions into the sarcoplasmic
reticulum.
Decreased energy stores inside the muscle (ATP, Cr-P and
glycogen).
Accumulation of metabolites e.g. CO2 and lactic acid.
Decreased O2 supply.
Decreased pH of the muscle cells.
Electrolyte disturbance.
Smooth Muscle (Unstriated)
Smooth muscle cells are spindle-shaped, with centrally
placed nucleus, In uterus. Vascular smooth muscle. They are
contractile tissue present in the hollow viscera, blood vessels, the
bronchi, exocrine glandular ducts and certain structure in the eye
and skin.
It possesses no visible cross-striation. Smooth muscles
are arranged in bundles and layers. Vertebrate smooth muscles
are divided into two main types: multi-unit and single unit.
a- Multi-Unit smooth muscles:
it include the radial muscle of the iris of the eye, the ciliary
muscles, the pronotor muscle of the skin and the muscle of some
blood vessels.
These muscles contracts normally in response to
excitation of their extrinsic motor nerve; each cell is separately
innervated, repetitive stimuli are needed for maximal contraction.
b- Single-Unit smooth muscles (Visceral):
It include muscles of the gastro-intestinal tract, the ureter
and uterus. They are characterized by continuous rhythmic activity
(contraction & relaxation which is not dependent on extrinsic
.)innervation
In visceral smooth muscles, there are junctions between
the cells which are comparable with the intercalated disc in
cardiac muscles. These junctions facilitate conduction from one
cell to another & allow activity to spread from area of excitation to
surrounding regions. The vas defrens are intermediate between
multi and single unit types.
Physiological properties: the speed of contraction in smooth
muscles are very slow and the duration is often prolonged than
with little expenditure of energy as sphincter muscle of the bladder
that relax for urination. Persistent contraction of smooth muscle
.may be myogenic in origin, or neurogenic