Reflex Physiology Flashcards
Describe the muscle spindle. [components and mechanism of activation of impulses]
[Muscle spindles are stretch receptors located within skeletal muscles. Their primary function is to detect changes in muscle length and the speed of these changes. Muscle spindles provide crucial information to the central nervous system for proprioception, which is essential for motor control, maintaining posture and ensuring stable gait.]
Each muscle spindle consists of several components:
(a) intrafusal muscle fibres
(b) connective tissue sheath
(c) sensory neurons
(d) gamma motor neurons [supply the contractile ends of the spindle fibres, adjusting their sensitivity and firing rate]
The intrafusal muscle fibers are parallel to the regular extrafusal muscle fibers. Intrafusal fibers are much thinner than extrafusal fibers.
Intrafusal fibers come in two types:
(a) nuclear bag fibers: multiply innervated and named based on their nuclei arrangement.
(b) nuclear chain fibers: also multiply innervated.
Sensory neurons (afferents) coil around the non-contractile central portions of the intrafusal fibers. There are two types of afferents:
(i) Primary (Group Ia) afferents: Transmit rapid changes in muscle length.
(II) Secondary (Group II) afferents: Transmit slower changes.
Activation and impulses:
When muscles lengthen (stretch), the spindles are stretched. This stretch activates the muscle spindle, leading to impulses sent to the spinal cord. The CNS processes this information for proprioceptive feedback.
Describe the stretch reflex [also state its function].
The stretch reflex, also known as the mytotatic reflex, is a rapid and automatic muscle contraction that occurs in response to stretching a muscle.
Function:
The primary function of the stretch reflex is to maintain muscle length at a constant level.
Mechanism:
⚚ When a muscle is stretched, specialized sensory receptors known as muscle spindles dectect this change in length. The afferent neurons [Group Ia or II afferents] from the muscle spindle transmit signals to the spinal cord.
⚚ The stretch reflex is a monosynaptic response that occurs within the spinal cord. The afferent neurons synapse with an alpha motor neuron of the same muscle, generating an immediate motor response in the form of a contraction. Simultaneously, the same afferent neuron will relay signals to an inhibitory interneuron, which communicates with an alpha motor neuron of an antagonistic muscle. The antagonistic muscle hence relaxes and won’t provide any resistance to the contracting muscle.
Explain alpha-gamma coactivation in the muscle spindle.
➤ During voluntary muscle contraction, alpha motor neurons stimulate the extrafusal muscle fibers to generate contract. Simultaneously, gamma motor neurons activate the intrafusal fibres within the muscle spindle. This is to prevent the sensory nerve fibers from going slack during muscle contraction.
➤ By maintaining tension in the intrafusal fibres, the muscle spindle remains responsive to changes in muscle length.
Describe the mechanism of the withdrawal reflex [also include the crossed extensor reflex].
➤ The withdrawal reflex, also known as the nociceptive flexion reflex or flexor withdrawal reflex, is a rapid and automatic protective refle triggered by noxious or painful stimuli.
➤ When a painful stimulus is perceived by free nerve endings in the skin, the sensory impulses travel to the spinal cord via sensory neurons. ➤ Within the spinal cord, the sensory neuron synapses with interneurons that connect to motor neurons.
➤ Some motor neurons send impulses to the flexor muscles in the affected limb, causing them to contract.
➤ Simultaneously, other motor neurons send inhibitory impulses to the extensor muscles, preventing them from interfering with flexion.
➤ In addition to flexion, the withdrawal reflex triggers the crossed extensor reflex (usually in the lower extremities). Contralateral motor neurons stabilize the uninjured side of the body (e.g. preparing the other leg to support the body weight). The crossed extensor reflex helps maintain balance during withdrawal.
