2.3 Flashcards
Spinal reflexes
Involuntary, instantaneous, and usually protective
Steps for spinal reflexes
- Stimulus activates peripheral sensory receptor associated with pseudounipolar neuron.
- Sensory neuron projects to spinal cord where it synapses with interneuron or motor neuron.
- Motor neuron joins peripheral nerve that carries it to skeletal muscle
- The motoneuron stimulates the muscle and causes it to contract.
Categories of spinal reflexes
- Intrasegmental - same level
- Intersegmental - 2 of more levels
- Suprasegmental - to brainstem
Intrasegmental reflex
Sensory branches allow motor response from same level
Suprasegmental reflex
Influenced by neurons of the cerebral hemispheres or brainstem (thalamus or sensory cortex)
Example: postural reflexes
Components of spinal reflexes
Sensory receptors
Sensory nerves
Motor nerves
Skeletal muscle
Extrafusal muscle fibers are innervated by
alpha motor neurons
Intrafusal muscle fibers are innervated by
gamma motor neurons
Alpha motor neurons
The largest neurons innervate typical skeletal muscle fibers, referred to as extrafusal fibers
When stimulated, cause contraction of the gross muscle.
Gamma motor neurons
Gamma motoneurons are smaller neurons and innervate the contractile portions of the intrafusal fibers of muscle spindles.
The main function of the gamma motoneurons is to increase the sensitivity of the intrafusal fibers to changes in muscle length and indirectly cause the reflex stimulation of the alpha motoneurons that stimulate contraction of the gross muscle
Muscle spindle function
Change in tension
Gamma motor neurons provide proprioceptive feedback
- length (degree of stretch)
- velocity (speed of stretch)
Muscle spindles are sensors of deep tendon reflexes.
Unique to skeletal muscle that continually monitor changes in muscle length.
They have a key function during movement and in the maintenance of muscle tone
Muscle spindles are oriented parallel to the longitudinal axes of the extrafusal muscle fibers
Skeletal muscle receptors in muscle spindle
within the muscle
Skeletal muscle receptors in the golgi tendon organ
within muscle tendon to detect tension
Golgi tendon organ function
Detect tension
Muscle shortens –> compresses GTO receptors –> signal travels to CNS
When the muscle contracts, it produces movement by puling a bone. The pull of the contracting muscle stretches the tendon, triggering GTO response.
It has no contractile element of motor neuron.
They receive sensory innervation.
Arranged parallel to the skeletal muscle fibers and contain intrafusal fibers
Two types of intrafusal fibers
There are two morphologically and functionally distinct types of intrafusal fibers: Nuclear bag fibers and nuclear chain fibers.
- Skeletal muscle fibers (extrafusal fibers) contain multiple nuclei. Nuclear bag fibers and nuclear chain fibers are also multinucleated cells, and the organization of their nuclei is their most distinctive characteristic.
Specific spinal reflexes
Muscle stretch reflex (simplest)
Reciprocal inhibition
Autogenic inhibition
Flexor reflex
Crossed extension reflex
Muscle stretch reflex = myotatic
Deep tendon reflex
Monosynaptic
Enhanced by reciprocal inhibition
Reciprocal inhibition
Primary mover contracts, antagonist muscle relaxes
Involved inhibitory interneuron
One muscle (e.g., the extensor) is stimulated to undergo contraction, while another muscle (e.g., the flexor, it’s antagonist) is simultaneously inhibited from undergoing contraction and, therefore, relaxes.
Autogenic inhibition
GTO fine tunes movement and inhibits further contraction
Sends inhibitory signal when too much tension is being placed on one muscle.
GTOs are specialized receptors that monitor the tension applied to tendons generated mainly by muscle contraction.
Muscle contraction causes the sensory endings terminating on tendons (the GTOs) to discharge.
Flexor reflex
Withdrawal reflex
Response of an entire limb to noxious stimulus resulting in flexion/ withdrawal
The flexor reflex is elicited by stimulation of cutaneous receptors and includes a reflex response across several joints involving an entire limb.
Crossed extension reflex
Maintains balance/posture
The crossed extension reflex occurs to maintain balance and upright posture following contact with a noxious stimulus.
The “crossed extension reflex” occurs in the contralateral limb that does not encounter the noxious stimulus.
Radiculopathy
compression of the nerve at the level of the nerve root
A motor unit that innervates only three muscle fibers is likely to innervate muscle in which of the following?
Thumb
Motor units are smallest in the parts of the body that have the most precise motor control, such as thumb, fingers, and tongue.
Which of the following effects is caused by activation of gamma motor neurons during active contraction of extrafusal muscle fibers?
Maintained sensitivity of the Ia afferents during unexpected stretch
Coactivation of gamma motor neurons with alpha motor neurons shortens the intrafusal muscle fibers during contraction of extrafusal fibers so that sensitivity of the Ia stretch receptors in the intrafusal fibers is maintained when unexpected stretch occurs during the extrafusal muscle contraction.
Which of the following observations would suggest that an upper motor neuron lesion rather than a lower motor neuron lesion is present?
Spasticity
Upper motor neuron lesions produce spasticity, involving hyperactive stretch reflexes. Lower motor neurons don’t.
