control of movement:

Question 1

Describe the structures of a skeletal muscle:

Answer 1

• They have two types of muscle fibres:
  
  1. Extrafusal muscle fibre= provide the muscles motive forces and are served by axons of the alpha motor neuron, they are found outside the muscle spindle.
  2. Intrafusal muscle fibres= specialised sensory organs that are served by a sensory and motor axon, they are found within the muscle spindle.
     ○ The central region contains sensory endings that are sensitive to the stretch applied to the muscle fibre. The efferent axon (gamma motor neuron) causes the muscle to contract, though this is weak contraction. Its function is to modify the sensitivity of the fibres afferent encoding to stretch.

Question 2

What does a single muscle fibre consist of?

Answer 2

• A bundle of myofibrils (overlapping of strand of actin and myosin).
  
  • The myosin filaments interact with the actin filaments to produce muscular contractions- called the striated muscle as they produce dark stripes when overlapped.

Question 3

List the steps involved in neurotransmission at the neuromuscular junction that results in muscle fibre contraction:

Answer 3

1. When an axon fires, acetylcholine is released from the terminal buttons and produces a depolarisation of the postsynaptic membrane of a muscle cell.
   
   2. The depolarisation is an endplate potential which is larger than an excitatory postsynaptic potential in synapses between neurons and it always causes the muscle fibre to fire and propagate the potential along its length.
   3. It causes a contraction of the muscle fibre, which produced by myosin attaching itself to actin strands, to bend in one direction, detach itself, bend back and the reattach itself to the actin further along the strand.
   4. It is repeated along the actin and myosin as they slide past each other to shorten the muscle (contracting it).
   5. It is not an all-or-nothing phenomenon but is determined by the firing rate- if lots of units are firing, the contraction will be string, if only a few a firing then it will be weak.

Question 4

Contrast the types of information detected by different axons of muscle spindles and Golgi tendon organs:

Answer 4

Sensory endings detect the length of a muscle through relaxing the sensory endings when the muscle shortens and stretching them when the muscle lengthens.
Golgi tendon organs detect the total amount of stretch exerted by the muscle, through the tendons to provide feedback on how hard the muscle is pulling.
They both help protect muscles from extreme movement that could damage the muscle or bone by regulating its activity due to the sensory feedback provided.

Question 5

Explain the functions of monosynaptic stretch reflexes:

Answer 5

• Aids posture (when standing up we need to keep our centre of gravity above our feet- monosynaptic stretch reflexes control visual and vestibular cues.
  
  • Only one synapse is encourage along the route from receptor to effector- afferent impulses are conducted to terminal buttons in the grey matter of the spinal cord (starting at the muscle spindle), the terminal button synapse on the alpha motor neuron innervates the extrafusal muscle fibres of the same muscle.
  • To aid movement without the use of the brain for quicker responses.

Question 6

Contrast the structures and functions of polysynaptic reflexes with monosynaptic reflexes:

Answer 6

• Monosynaptic= only one synapse. All others are polysynaptic (use multiple synapses).
  
  • Monosynaptic reflexes= a muscle contracts in response to it being too quickly stretched, involves a sensory neuron and a motor neuron, with one synapse between them.
  • Polysynaptic reflexes= (Golgi tendon organs) detect muscle stretch, if it stretches too much there will be a risk of damage to the tendon and the bone. Information needs to be relayed about how hard the muscle is pulling to the brain and help decrease the strength of muscular contraction to prevent damage- a polysynaptic reflex is used here.

Question 7

Describe the pathways and functions of cortical regions involved in control of motor behaviour.

Answer 7

• Primary motor cortex: lies on the precentral gyrus, just rostral to the central sulcus. When particular parts are activated, it causes movement in particular parts of the body (somatotopic organisation). It’s organised in terms of particular movements of body parts- each movement may be accompanied by the contraction of several muscles. Therefore, complex neural circuitry is located between individual neurons in the primary motor cortex, and the motor functions in the spinal cord that cause motor units to contract. The movements are assisted and modified mostly by the cerebellum and the basal ganglia.
  
  • Supplementary motor area (SMA) and premotor cortex: both lie immediately adjacent to the primary motor cortex and are important in the control of movement. They both receive sensory information from the parietal and temporal lobes and send efferent axons to the primary motor cortex.
    ○ SMA: located on medial surface of the brain, just rostral to the primary motor cortex.
    ○ Premotor cortex: located primarily on the lateral surface, just rostral to the primary motor cortex.

Question 8

Describe the components and functions of the descending pathways:

Answer 8

• Lateral group: primarily involved in control of independent limb movement (hands and fingers). Right and left limbs make different movements or one limb moves while the other is still.
  ○ Corticospinal tract: consists of axons of cortical neurons that terminate in the grey matter of the spinal cord.- largest concertation of cell bodies responsible for these axons is located in the primary motor cortex. The corticospinal pathway controls arm, hand, finger movement- indispensable for moving the finger independently when reaching and manipulating.
  ○ Centrilobular tract: controls movements of the face, neck, tongue, and parts of the extracellular eye muscles.
  ○ Rubrospinal tract: originates in the red nucleus of the midbrain and controls independent movements of the forearms and hands.
  
  • Ventromedial group: control more automatic movements, gross movements of the nucleus of the trunk and coordinated trunk and limb movements involved in posture and locomotion. Contains the vestibulospinal tract, tectospinal tracts, vestibulospinal tract and ventral corticospinal- control motor neurons in the ventromedial part of the spinal cord grey matter. The neurons in all of the tracts receive input from the portions of the primary motor cortex that control movements of the trunk and proximal muscles. The neurons play a role in posture and control several automatic functions.

Question 9

Explain the functions of the supplementary motor area and the premotor cortex in planning and initiating movement:

Answer 9

• Supplementary motor area: involved in learning and performing behaviours that consist of sequences of movements.
  
  • Premotor cortex: involved in learning and executive responses that are signalled by the presence of the arbitrary stimuli.
  • Both involved in planning movements and they execute these plans through their connections with the primary motor cortex.