MD137 Skeletal Muscle Reflex Responses

Skeletal Muscle Reflex Responses

• Skeletal muscles are typically voluntary, controlled by higher brain regions.
• They can also contract unconsciously in response to stimuli.
• Reflex responses involve muscle spindle apparatus and Golgi tendon organs.

Principles of Physiology

• MD137 course discusses skeletal muscle reflex responses.
• Lecturer is Dr. K.McCullagh

Nervous System Control of Movement

• See Chapter 5 in Medical Physiology, Rhoades & Bell, 4th Edition, for further details.

Skeletal Muscle Reflexes

• Skeletal muscles are usually voluntary and controlled by higher brain regions.
• They can also contract unconsciously in response to stimuli.

Classification of Muscle Types;

• Muscles are categorized by control mode (voluntary/involuntary), anatomical type (skeletal, cardiac, visceral), and histological structure (striated, smooth).
• Voluntary muscles, like skeletal muscles, are categorized as either involuntary reflex or voluntary.

Motor Control System

• The motor control system includes: cerebral cortex, basal ganglia, thalamus, brainstem, cerebellum, spinal cord, and motor neurons, working together to control skeletal muscle movement.

Local Control of Motor Neurons

• Local control systems receive instructions from higher brain centers.
• They make adjustments based on sensory receptor information from muscles, tendons, and joints.

Muscle Sensory Organs

• Muscle spindle apparatus responds to muscle length, and the Golgi tendon organs respond to muscle tension on tendons.
• Muscles requiring precise control (extraocular, e.g.) have more spindles. Stretching a muscle causes spindles to stretch.
• Golgi tendon organs sense tension a muscle applies to a tendon.

Muscle Spindle Apparatus

• Contains intrafusal fibers.
• Two fiber types: nuclear bag fibers and nuclear chain fibers.
• Two types of sensory nerve cells (afferent nerves) surround the fibers: primary (type Ia) and secondary (type II).

Muscle Spindle (Diagram)

• Contains intrafusal muscle fibers.
• Sensory endings (primary and secondary) are connected to extrafusal fibers via efferent and afferent nerves.

Muscle Spindle Apparatus (during slow contractions)

• Passive stretch of muscle fibers from resting length.
• Alpha nerve stimulation causes la response to cease.
• Alpha and gamma nerve coactivation maintains la responsiveness.

Muscle Spindle Apparatus (continued)

• Gamma motor neurons adjust muscle spindle sensitivity to respond appropriately as extrafusal fibers contract and shorten.
• Maintaining spindle tautness prevents slackening.
• Slackening would reduce sensory information during rapid shortening.
• Alpha-gamma coactivation prevents this loss of information.

Muscle Spindle Apparatus (rapid stretch)

• During rapid stretches, type I a endings significantly increase firing rate. Type II show a modest increase.
• Upon release of stretch type I a ceases firing and type II slows down.
• Type la endings report both velocity and muscle length. Type II endings only report muscle length.

Monosynaptic Stretch Reflex

• The simplest reflex, involves a sensory neuron synapsing on a motor neuron in the spinal cord.
• Maintains optimal resting length of skeletal muscles.
• Stimulated by striking the patellar ligament (knee-jerk reflex).

Knee-Jerk Reflex (Diagram)

• Sensory neuron activated by spindle stretch, which stimulates motor neuron.
• Motor neuron stimulates extrafusal muscle fibers to contract.
• Striking patellar ligament stretches tendon and quadriceps femoris muscle.

Stretch Reflex (protects from damage)

• Load added to a muscle, stretches muscle and spindles.
• Muscle spindle afferents increase firing rate.
• Reflex contraction restores position and prevents damage from overstretching.

Monosynaptic Stretch Reflex (continued)

• Stretch on muscle stretches spindle fibres.
• Activates sensory neuron.
• Sensory neuron activates alpha motor neuron.
• Motor neuron stimulates extrafusal muscle fibres to contract.
• Stretch on spindle is reduced.
• Main purpose of muscle spindle and stretch reflex is injury prevention and muscle tone maintenance.

Golgi Tendon Organs

• Constantly monitor tension in tendons.
• Sensory neuron activates interneuron in spinal cord.
• Interneuron inhibits motor neuron, lowering tension in tendons.
• Disynaptic reflex (involving two synapses).

Golgi Tendon Organ (Diagram)

• Located at the junction of muscle and tendon.
• Connected to sensory neurons.
• Sensing tension in the tendon.

Golgi Tendon Organs (continued)

• Tension on tendon activates sensory neuron.
• Sensory neuron stimulates interneuron in spinal cord.
• Interneuron inhibits motor neuron.
• Tension in tendon is reduced.

Golgi Tendon Organs (ultimate protection role)

• GTOs protect muscle and surrounding connective tissue from injury due to sudden, unaccustomed movement or excessive load.

Golgi Tendon Organs (in athletic training)

• Minimizing GTO influence is called disinhibition. It's part of athletic training.
• Allows pushing performance to the limits of tissue capacity.
• Risks include muscle/tendon rupture and broken bones.

Flexor Withdrawal Reflex (Diagram)

• Painful skin stimulation triggers it.
• Flexor muscles on the same side (ipsilateral) contract, while they relax on the other side.
• The limb moves away from the harmful stimulus and is a withdrawal reflex.
• Contra lateral leg maintains stance by reciprocal inhibition (flexors on contralateral leg inhibited, while extensors stimulated).

Flexor Withdrawal Reflex (continued)

• Painful skin stimulation activates flexor muscles and inhibits contralateral leg's extensor muscles.
• This reflex moves the affected limb away from the harmful stimulus, to avoid injury.
• The same stimulus elicits the opposite response (reciprocal inhibition) on the contralateral leg (opposite side) so the body can maintain its balance while the affected leg withdraws.

Explore the fascinating world of skeletal muscle reflex responses in this MD137 course quiz. Understand the distinction between voluntary and involuntary muscle actions, and learn about the mechanisms like the muscle spindle apparatus and Golgi tendon organs. Dive into Chapter 5 of Medical Physiology for deeper insights.