Objectives

• I will know about involuntary movements

• I will know about more complex movements.

Movement

• Most muscles attach to points on the skeleton and cross one or more joints

  • skeletal muscles: muscles that are attached to bones in the body to produce voluntary movements

• Activation of the muscle can open/close the joints its spans

  • Flexor: a muscle that closes the joints it spans

  • Extensor: a muscle that opens the joints it spans

• Agonists: muscles that move joints in the intended direction

• Antagonist: muscles that oppose the intended direction of movement

• Each muscle is made of thousands of muscle fibers

• Each fiber is controlled by 1 alpha motor neuron in the brain or spinal cord

  • But 1 alpha motor neuron can control a few to 100+ muscle fibers

• Motor unit: the alpha motor neuron and the muscle fibers it controls

  • Motor units are a critical link between the brain and muscles

  • If motor neurons die, the person can not move

• Some muscles attach to soft tissue

  • Operate in a similar way to skeletal muscles

Involuntary Movement

• Reflexes: relatively fixed, automatic muscle responses to particular stimuli

  • All reflexes involve the activation of small sensory receptors in skin, joints, and in muscles

  • An important one is the reflexive knee movement produced by extensor muscles

    • A stimulus is sensed by muscle spindles

      • muscle spindles: receptors in muscles

    • Muscle spindles send this information to the spinal cord & brain about the length and speed of the shortening or lengthening of the muscle

      • This information is used to control voluntary/involuntary movements

    • This sudden stretch of the muscle sends a barrage of impulses to the spinal cord via muscle spindle sensory fibers

    • Fibers activate motor neurons in the stretched muscle causing the stretch reflex

      • stretch reflex: contraction of the muscle caused by activation of motor neurons

    • The same stimulus causes inhibition of motor neurons in the antagonist muscles

      • this is done by inhibitory neurons in the spinal cord

• “Flexion withdrawal (bottom) occurs when your bare foot encounters a sharp object. Your leg is immediately lifted (flexion) from the source of potential injury, but the opposite leg responds with increased extension so that you can maintain your balance. The latter event is called the crossed extension reflex.”

• The brain can also control the nature of feedback received as movements occur

• Reflexes occur rapidly & without attention because they are built into the system of spinal cord neurons

More Complex Movements

• Most complex movements require control of basic spinal mechanisms

• Motor cortex: important area in the frontal lobe responsible for voluntary movement

  • Exerts control over spinal cord in part through direct control of alpha neurons

• Movement control also involves the basal ganglia, thalamus, cerebellum, and neuron groups in the midbrain and brainstem (regions that send axons to the spinal cord)

• The basal ganglia and thalamus have connections with motor and sensory areas of the cerebral cortex

  • Dysfunction of basal ganglia is bad

    • Dopamine is supplied to basal ganglia by neurons in substantia nigra (midbrain cell group)

    • Parkinson’s kills off the substantia nigra

      • This causes the loss of dopamine for the basal ganglia

        • This loss of dopamine causes tremors, rigidity, and inability to move

          • All of these are hallmark symptoms of Parkinson’s

• Cerebellum: a brain structure at the back of the brain that is very important for coordination and adjusting skilled movement

  • Disturbances in cerebellar function lead to poor coordination of muscle control, disorders in balance and reaching, and difficulties in speech

  • The cerebellum gets direct information from sensory receptors in the head and limbs, as well as areas of the cerebral cortex

    • The cerebellum integrates information to ensure smooth coordination

  • The cerebellum also helps adjust motor output to deal with changing conditions

    • Tunes motor output to be appropriate to specific requirements of the task

    • As we learn to walk, speak, or play instruments, this detailed control information is stored in the cerebellum

      • This information can be called upon by commands from the cerebral cortex