a2 biology: muscles

Muscles - three types

1. cardiac muscle cell
2. skeletal muscle cell
3. smooth muscle cell

Structure

• they are made up of elongated cels called fibres
• contraction is possible because the fibres contain filaments
• these filaments are made up of the proteins actin and myosin.

Involuntary muscle (smooth)

• this muscle is innervated (distrubution of nerves to an organ or body part) with neurones
• these neurones are from the autonomic nervous system
• as a result they are not under voluntary (concious) control

Types of smooth muscles

• Walls of intestines
• arranged in circular and longitudinal bundles
• Peristalsis - moves food along the intestines
• Iris of the eye
• circular and radial bundles
• controls the intensity of the light entering
• contraction of radial muscles dilates the pupil
• contraction of circular muscle constricts the pupil
• Walls of arteries and around arterioles' wall and cervix of uterus
• circular bundles
• important in temperature regulation, regulation of BP and redirecting blood flow
• contraction narrows vessel diameter
• relaxation causes dilation

Smooth muscle cells

• They are not striated like voluntary and cardiac muscle
• they are described as being 'spindle shaped'
• they contain small bundles of actin and myosin and a single nucleus
• when relaxed they are 500 µm and 5µm wide
• contraction is slow but the muscle tires very slowly

CARDIAC MUSCLE

• these are three types of cardiac muscle
• atrial
• ventricular
• specialised excitatory and conductive muscle fibres
• it contracts continuously, powerfully and without fatigue
• contraction of atrial and ventricular muscle is similar to skeletal muscle but for longer
• the excitatory and conductive fibred contract feebly
• they conduct electrical impulses and control the rhythmic heartbeat
• some cardiac muscle is myogenic
• capable of stimulating contraction without a nerve impulse
• cardiac cycle
• initiation of this rhythm comes from a patch of specialised excitatory and conductive muscle fibres in a small part of the right atrium
• this is called sino-atrial node (S.A.N)
• waves of electrical activity spread out rapidly over oth atria.
• the atrioventricular septum between the atria and the ventricles does not conduct the cardiac impulse from SAN
• another node made of specialised excitatory and conductive muscle fibres, the atrioventricular node (AVN) and t picks up atrial impulse
• this is transmitted along a bundle of modified cardiac fibre in the inerventricular septum
• this bundle of fibres is called the bindle of His
• when the impulse reaches the apex of the heart it spreads rapidly up the ventricular walls in a network of conductive fibred called the purkyne fibre

Role of autonomic nervous system

• neurones of the autonomic nervous system can carry impulses to the heart and regulate the rate of contraction
• sympathetic stimulates an increase
• parasympathetic stimulates a decrease
• Intercalated discs
• these are areas where adjacent cardiac muscle cells meet
• they are cell membranes with gap junctions with free diffusion ions
• this allow action potentials to pass easily and quickly

Voluntary (skeletal/striated) muscle

• Result in the movement of the skeleton at joins
• muscle cells form fibred about 100µm in diameter
• these contains several nuclei
• Each fibre is surrounded by a cell surface membrane called sarcolemma
• muscle cell cytoplasm is called sarcoplasm
• contains following organelles
• many mitochondria
• extensive sarcoplasmic reticulum
• a numer of myofibrils
• Myofibrils
• these are contractile elements
• each consists of a chain of smaller contractile units called sarcomeres
• this is the smallest contractile unit of a muscle
• within myofibrils there are two types of myofilaments
• thin actin
• thick myosin

The sarcomere

• the span from Z-line to the next is the sarcomere
• relaxed its around 2.5µm and shorter when contracted
• Z line gets closer durig contraction

Actin

• thin filaments are two strands
• mainly actin couled around each other
• each strand is made up of actin sub units
• tropomysin coils around actin to reinforce it
• a troponin complex is attached to the tropomyosin
• each troponin consist of 3 polypeptides (binding sites) for
• actin
• tropomyosin
• calcium ions

Myosin

• thick filaments arranged as bundles of protein
• each molecule is made up of a tail and two heads
• each filament consists of many myosin molecules with heads sticking out from opp ends