MD137 Muscle Lecture 4 PDF

Physiology School of Medicine 2024 Skeletal Muscle: Reflex Responses MD137: Principles of Lecturer: Dr K.McCullagh Physiology Muscle Spindle Apparatus and the Golgi Tendon Organs. Involvement in reflex responses Nervous system control of movement S...

Physiology School of Medicine 2024 Skeletal Muscle: Reflex Responses MD137: Principles of Lecturer: Dr K.McCullagh Physiology Muscle Spindle Apparatus and the Golgi Tendon Organs. Involvement in reflex responses Nervous system control of movement See Chapter 5 in Medical Physiology Rhoades & Bell, 4th Edition.  Skeletal Muscle Reflexes Skeletal muscles are usually referred to as voluntary and are controlled by higher brain regions. They can also contract unconsciously in response to certain stimuli. CLASSIFICATION OF THE TYPES OF MUSCLES Involuntary Reflexes Motor Control System  Local Control of Motor Neurons Local control systems receive instructions from higher brain centers and make adjustments based on information received from sensory receptors in the muscles, tendons, and joints of the body part to be moved. 6  Muscle Sensory Organs Muscle spindle apparatus: respond to muscle length – Muscles that require finer control have more spindles e.g. extraocular muscles. – Stretching a muscle causes spindles to stretch. Golgi tendon organs: respond to tension a muscle puts on a tendon  Muscle Spindle Apparatus Contains thin muscle cells called intrafusal fibers Two types: – Nuclear bag fiber – Nuclear chain fibers Two types of sensory nerve cells (afferent nerves) around the fibers: – Primary (type Ia) on Nuclear bag & Nuclear chain fibres – Secondary (type II) on Nuclear chain fibres  Muscle Spindle α Motor Extrafusal Neuron muscle fibers γ Motor neurons Afferent/Sensory nerves Ia: Chain and Bag fibres II: Chain fibres Muscle Spindle Apparatus (during a slow contraction) No Alpha or Gamma nerve Type Ia Alpha nerve, No Gamma Alpha and Gamma nerve (coactivation)  Muscle Spindle Apparatus contd…. Gamma motor neuron function is to adjust sensitivity of the muscle spindles, so that they respond appropriately as extrafusal fibres contract and shorten. So that the spindle remains taut and does not slacken as shown in (B) on the previous slide, if there was no gamma nerve activity. Slackening of muscle spindles would reduce the available sensory information about muscle length during rapid shortening contractions Alpha-gamma coactivation prevents this loss of information  Muscle Spindle Apparatus contd…. Response sensory endings (Type Ia & II) during a rapid muscle stretch Type Ia Type II During a rapid stretch type Ia endings shows an increase in firing rate, while type II show a modest increase. Release of the stretch, Ia cease firing, while type II slows Type Ia endings reports both velocity and length of muscle stretch Type II endings report the length  Monosynaptic Stretch Reflex Simplest reflex Only involves a sensory neuron synapsing on a motor neuron in the spinal cord – One synapse = monosynaptic Maintains optimal resting length of skeletal muscles Can be stimulated by striking the patellar ligament in the “knee-jerk reflex” Knee-Jerk Reflex 5. Stretch on spindle is reduced. The stretch reflex protects from damage Human Physiology 8th Edition Silverthorn  Monosynaptic Stretch Reflex 1. Stretch on a muscle stretches spindle fibers. 2. This activates sensory neuron. 3. Sensory neuron activates alpha motor neuron. 4. Motor neuron stimulates extrafusal muscle fiber to contract. 5. Stretch on spindle is reduced. Main Point: The purpose of muscle spindles and the stretch reflex is to protect your body from injury caused by overstretching and to maintain muscle tone.  Golgi Tendon Organs Constantly monitor tension in tendons – Sensory neuron stimulates interneuron in spinal cord. – Interneuron inhibits motor neuron. – Tension in tendon is reduced. Disynaptic reflex involving two synapses Golgi Tendon Organ Golgi Tendon Organs Human Physiology, Vander’s 8th Edition Ultimately, the GTOs protect the muscle and surrounding connective tissue from injury due to a sudden, unaccustomed movement or an excessive load. Golgi Tendon Organs (GTOs) The process of minimizing the influence of GTOs is referred to as disinhibition. Practicing disinhibition is part of athletic training: To push performance to the limits of tissue capacity. Risk of rupturing muscles or tendons or broken bones Flexor Withdrawal Reflex Human Physiology, Vander’s  Flexor Withdrawal Reflex Painful stimulation of the skin, as occurs from stepping on a tack, activates the flexor muscles and inhibits the extensor muscles of the ipsilateral (on the same side of the body) leg. The resulting action moves the affected limb away from the harmful stimulus, and is thus known as a withdrawal reflex. The same stimulus causes just the opposite response in the contralateral leg (on the opposite side of the body from the stimulus). In the contralateral leg motor neurons to the extensors are activated while the flexor muscle motor neurons are inhibited (called Reciprocal inhibition). This crossed-extensor reflex enables the contralateral leg to support the body’s weight as the injured foot is lifted by flexion.