Muscle

Question 1

identify the three types of muscles

(where are they found / what do they do)

Answer 1

1) Skeletal muscle
- found in muscles attached to skeleton
- generates overall movement of body

2) Cardiac muscle
- make up most of heart tissue
- propels blood through chambers of heart and into major blood vessels

3) Smooth muscle
- found in walls of visceral organs (blood vessels, intestines, urinary tract)
- propels content through organs

Question 2

identify function of muscles

Answer 2

1. Produce movement (movement of bones / blood /contents through organs)
2. Produce heat (muscle contraction produces heat)
3. Maintain posture and body position
4. Stabilise Joints- maintains tone (tension)

Question 3

identify the characteristics of muscles

Answer 3

Excitability- muscles can receive and respond to a stimulus (nerve impulse in muscle causes contraction)

Contractility- muscle can shorten when adequately stimulated

Extensibility muscle can stretch (Extend) when no contracting

Elasticity- muscle can recoil and regain resting length after being stretched

Question 4

Describe anatomy of skeletal muscle, skeletal muscle fibres, myofibrils and sarcomeres

Answer 4

• Skeletal muscle
  
  • ​Muscle is covered in 3 layers of CT
    
    • Epimysium surrounds whole muscle
    • Perimysium surrounds bundles of muscle fibre
    • Endomysium surrounds individual fibres
  • muscle attached to bone via CT (can be directly / indirectly attached)
  • skeletal muscle attached to 2+ bones and cross joints to create movement
• Skeletal muscle fibres
  
  • ​long cells, multiple nuclei
  • each fibre is made up of many myofibrils (100’s-1000’s)
• Myofibrils
  
  • long rod like structures that run parallel along length of muscle fibre
  • Contain 2 myofilaments
    
    • thick filament that has many myosin molecules that have protruding heads which bind to actin
    • thin myofilament (that includes 2 strands of actin, which are twisted in helix shape, and provide binding sites for myosin heads
  • myofilaments are arranged into sarcomeres
• Sarcomeres-
  
  • speicfic arrangement of myofilement, creating smallest unit of skeletal mucle
  • • sarcomeres arranged End to end down length of myofibril
      
      • typical arrangement
        
        • filaments run parallel to each other
        • thick filament in the central portion
        • thin filaments run parallel and attached to ends of sarcomere

Question 5

describe a sarcomere and how it contracts

Answer 5

Motor cortex makes descision to respond,

Somatic upper motor neurons transmitted to spinal cord

Somatic lower motor neurons take impulse from spinal cord to muscle

At Neuromuscular junction, somatic lower motor neurons release Acetylcholine (Neurotransmitters which bind to receptors of skeletal muscle, causing AP in muscle

AP in muscles causes Ca to be released from the Smooth Endoplasmic reticulum of muscle

Ca binds to troponin and causes it to slide off

Question 6

identify the factors that contribute to the strength of a muscle contraction

Answer 6

Muscle contraction occurs when sensory input from somatic lower sensory neurons release Acetylcholine at neuromuscular junctions, causing stimulation of skeletal muscle)

strength of muscle contraction depends on

1. size of motor unit (motor unit = 1 somatic lower sensory unit + all the muscle fibres it innervates)
2. recruitment - # of motor units activated in a muscle
3. frequency rate the action potentials are occuring in the motor unit (more frequent = stronger muscle contraction)

Question 7

describe how graded muscle responses work

Answer 7

allow variation in muscle tension through summation based on

1) amount of recrutiment (# of) muscle units
2) frequency of action potentials to increase

if AP is low frequency, the movement may just be ‘twitches’ that don’t result in overall movement

As frequency increases, there is less time between impulses and so contraction ‘adds’ to the previous “sumating” to increase the force within the muscle, causing smooth continuous contraction

Question 8

briefly describe the two types of skeletal muscle contraction

Answer 8

1. Isotonic contractions (eg flex arm)
   
   • increase tension (cross bridges formed) shorten muscle (thin filaments pulled towards centre, shortening sarcomeres)
2. isometric contractions (e.g. maintain posture)
   
   • ​​increased tension (Cross bridges formed), but muscle doesn’t shorten
   • important for posture / body position / stabilising joints)

Question 9

Identify where smooth muscle is found in the body

Question 10

describe the anatomy of smooth muscle and smooth muscle fibres/ cells

Answer 10

• Smooth muscle cells are short, sligthly eongated, 1 nucleus
• Arranged in sheets of tightly packed cells, surrounded by Connective tissue that contains neural and vascular supply
• 2 types of sheets arranged at right angles

1. • longitudinal sheets- run parallel along long axis of organ.
   • contraction causes organ to shorten
2. • circular sheets- run around circumference of organ
   • contraction causes lumen (cavity) of organ to constrct
     3.

Question 12

describe characteristcs of smooth muscle and how it contracts

Answer 12

Characteristics

• No myofibrils / sarcomeres (hence unstriated appearance)
• Myofilaments criss cross each other and spiral down length of cell- contraction causes a twisting, so cell gets shorter and fatter
• Thin filament (Actin) is covered by calmodulin (not troponin) ‘
• intermediate filaments’ arranged in a lattice with ‘dense bodies’ that resist tension and help transmit contraction to surrounding connective tissue
• • How smooth muscle contracts
• Autonomic neurons release NT’s across Diffuse junction, through use of varicosities
• NT’s generate AP in Smooth muscle cells, causing release of Ca which triggers calmodulin to be released from actin receptors
• Myosin now binds to Actin’s receptors- due to arrangement of myofilaments in a criss cross spiral arrangement, as well as the presence of intermediate filaments that are arranged with ‘dense bodies in the smooth muscle, the contraction causes a twisting momvement, pulling cell inwards and making it shorter and fatter.
• the AP is quickly transmitted to adjacent cells via gap junctions
• so whole layer of cells responds to stimulation together in slow synchronised contraction
• pace maker cells may set pace for muscle contraction and ensure muscle contracts in unison
• regulated by
• • ​both Autonomic nervous system (excitatory / inhibitory NT’s)
    
    • And other factors (hormones, pH, level of respiratory gases)
    • can be self excitatory (depolarise spontaneously without external stimuli
• the muscle may stretch, in response to increased tension
• eg uterus stretching as baby grows

Question 13

describe the anatomy of cardiac muscle

Answer 13

-Cells are short, brached and interconnected

• 1-2 nuclei
• membranes of adjacent cells ‘interlock’ (intercalated discs) with desmosomes (anchoring adjacent cells together) and gap junctions (allowing AP’s to be transmitted quickly)
• Cardiac fibres have Myofibrils and Sarcomeres
• Contain lots of mitochondria
• enables continuous supply of energy / resistance to fatigue

Question 14

briefly describe the characteristics of cardiac muscle and how it contracts

Answer 14

• Fibres are interconnected, enabling fibres to contract as single cooridinated unit
• Some muscle cells are self excitatory - depolarise without external stimuli (“auto-rhythmic”)Auto rhythmic cells maintain regular heartbeat, with ANS regulating rhythm (sympathetic + parasympathetic influence rhythm)

Question 15

Name and briefly describe three unquie and distinctive characteristics of muscle.

Answer 15

1) Excitability- muscle can detect / respond to stimulus - e.g. electricial impulse can stimulate contraction
2) Contractility

muscle can shorten with adequate stimulus

3) Extensibility - muscle can stretch when not being stimulated, beyond ‘resting length’
4) Elasticity - muscle can recoil back to resting length, after being stretched

Question 16

2. State three functions of skeletal muscle.

Answer 16

1) Heat production
2) Stabilise joints (isometric tension)
3) Produce movement
4) Maintain posture and body position

Question 17

A neurotransmitter binds to______________________ in the skeletal muscle fibres. This generates an action potential within the muscle and causes the _____________________ of the muscle fibre to release calcium ions. The calcium ions binds to ________________ and this causes tropomyosin to slide along the actin, exposing the ___________________ binding sites on actin. The myosin heads bind to actin when their binding sites are exposed, forming a ­­­­­­­­­­­­­­­­­­­________­­­­­­­­­___________________. The myosin heads flex, pulling the thin filaments towards the centre of the ________________________and this shortens the sarcomeres. Because each sarcomere shortens, the entire _____________________ shortens and as all myofibrils within a muscle fibre will be shortening at the same time, causing the entire muscle fibre to shorten.

Answer 17

A neurotransmitter binds to________receptors________ in the skeletal muscle fibres.

This generates an action potential within the muscle and causes the ________Smooth Endoplasmic reticulum_____________ of the muscle fibre to release calcium ions.

The calcium ions binds to troponin,________________ and this causes tropomyosin to slide along the actin, exposing the ________myosin___________ binding sites on actin.

The myosin heads bind to actin when their binding sites are exposed, forming a ­­­­­­­­­­­­­­­­­­­ cross bridge________­­­­­­­­­___________________.

The myosin heads flex, pulling the thin filaments towards the centre of the sarcomere________________________and this shortens the sarcomeres.

Because each sarcomere shortens, the entire myofibril_____________________ shortens and as all myofibrils within a muscle fibre will be shortening at the same time, causing the entire muscle fibre to shorten.