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.