3.5 Reflexes in the Nervous System Flashcards
What is a reflex ?
- An automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (a muscle or gland) without _______________ Example: the constriction of the pupil to light stimulus.
- An involuntary coordinated pattern of muscle contraction and relaxation elicited by _____________.
- -> Example: knee jerk when the patella tendon is hit with a tendon hammer.
- -> Example: jerking our hand away after touching a hot surface even before we consciously feel the heat.
- Magnitude and timing are determined respectively by the __________________
- Reflexes differ from voluntary movements in that ______________
reaching the level of consciousness;
peripheral stimuli;
intensity and onset of the stimulus;
once they are released, they can’t be stopped.
What are the components of a reflex arc (5)?
The sensory receptor (labelled 1), picks up the external stimulus and sends impulses which travel via the preganglionic neuron, through the _____________ and enters the spinal cord dorsally (labelled 2) to the integrating center which relays impulses to the anterior root. The anterior root exits ventrally (labelled 4) into a _______________- to the effector organ
- sensory receptor (responds to a stimulus by producing a generator of receptor potential)
- sensory neuron (axon conducts impulses from receptor to integrating center)
- integrating center (one or more regions within the CNS that relay impulses from sensory to motor neurons)
- motor neuron (axon conducts impulses from integrating centers to effector)
- effector (muscle or gland that responds to motor nerve impulses)
dorsal root ganglion;
mixed spinal nerve
In order for reflex to occur, afferent fibers need to bring impulses into the spinal cord.
- At the bottom trace, a reflex can be elicited by stretching a muscle. The muscle is taken from a particular length to a longer length, so it is stretched within the space of a second.
- What happens in general is that the force produced by that stretched muscle will increase, providing afferent signals to the spinal cord.
- This reflexively produces efferent signals via the ______________ to the skeletal muscle fibers and leads to muscle contraction.
- If the dorsal roots are cut such that there is no afferent input and the muscle is stretched, ___________________
alpha motor neuron;
production of force by the muscle ceases.
Monosynaptic vs disynaptic stretch reflex
There is a delay between the afferent volley and the recorded impulse, as this action potential reaches this synapse, activates this cell and it activates it in such a way that it brings that cell closer to its firing threshold, cross the firing threshold, and causes that neuron to fire.
Monosynaptic (stretch) reflex: On stimulating the afferents in the extensor nerve, you can notice that the delay between the afferent volley coming into the spinal cord and the efferent or excitatory postsynaptic potential being recorded on the left is just over half a millisecond. This is indicative of a monosynaptic connection.
- For example, in a knee jerk reaction, there is ________________-leading to extension of the knee.
Disynaptic effect: On stimulating the afferents in the flexor nerve, the time between the activation of this afferent fiber and the activation of the efferent fiber is significantly prolonged, and it is in the opposite direction. It is actually inhibited and has received an _______________ which has made its likelihood of firing less by making it even more negative.
- In a knee jerk reaction, there is relaxation of the knee flexors to allow the knee to extend.
contraction of the knee extensors ;
inhibitory postsynaptic potential
The Monosynaptic (stretch) reflex This is a diagram showing a simple stretch reflex, which involves hitting the patella ligament which is located just below the knee cap or patella. Hitting the ligament will stretch the quadriceps muscle. That stretch will be signaled by an afferent neuron, and via a \_\_\_\_\_\_\_\_\_\_\_\_\_\_, will cause the quadriceps muscle to be contracted, and the leg will exhibit the classic knee jerk response.
At the same time, the excitation of the receptor in the muscle will cause, via a ______________, an inhibitory response to the motor neurone supplying the antagonist. So this reflex can be described as having a monosynaptic excitatory contraction, and at the same time stretch of this muscle via a disynaptic connection causes the antagonist muscle on the other side of the limb to become relaxed.
monosynaptic connection;
disynaptic connection
The Hoffman (H-) Reflex
In reflex test with the H reflex, the afferent signal is generated artificially by electrically stimulating the nerve that __________________.
If we deliver an electrical stimulus to the nerve at a particular intensity we will activate both the sensory and motor axons. Now, since the motor axons are generating signals, which will go down towards the muscle, the response we see first after stimulating the nerve is the ___________________ in the electromyography below (EMG).
At the same time, the stimulus also activates the sensory neurons, which we know will send their signals back to the spinal cord, synapse on the motor neurone and then send another signal down the same motor neurone to the muscle, and we will get a similar trace in the EMG. But it happens, a lot later and is known as the ____________________.
usually carries the afferent signal from the muscle into the central nervous system;
direct motor response or the M wave;
H reflex so this is the one that is caused by activation of the sensory nerves.
When the stimulus intensity along the X axis, against the amplitude of the EMG response, the H reflex is initiated first, then the M wave
- At low stimulus intensities, the first fibers to be activated are the ______________.
- As the stimulus intensity increases, the _______________ are activated and the M waves start to appear.
H wave becomes smaller smaller at higher stimulus intensities as action potentials are generated in the motor fibers, which travel the opposite way. Action potentials are generated which travel in the ____________ direction will collide with action potentials traveling in the usual orthodromic direction and cancel each other out.
The M wave, which starts to occur at a slightly higher intensity will increase up to a plateau once __________________.
sensory fibers;
motor fibers;
antidromic or essentially the reverse;
all the motor neurons have been recruited
Flexion withdrawal reflex: comes about from stimulation of pain receptors in a foot which will cause, via various synaptic connections to the relevant muscles, the affected leg to ______________ in order for our foot to be removed from the painful stimulus,
flex at the knee and possibly for the hip to flex as well
Crossed extensor reflex: When you are withdrawing your leg from a painful stimulus, you do not fall. The adaptor reflex allows the other leg standing or staying on the floor to contract to therefore take the body weight in its entirety. Hence, in addition to flexion withdrawal reflex (reflex where the leg is withdrawn from the painful stimulus) we have the crossed extensor reflex. This helps the leg on the other side of the body or the contralateral leg to go into ________________ to hold the person upright.
extension
What is the Jendrassik manoeuvre?
If a person clenches their teeth or tries to pull their hands apart from grasped position during a knee jerk testing, the knee jerk response would be enhanced
Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex. In a simple experiment in an intact animal, if the cerebral cortex is separated in a technique known as decerebration, a muscle stretch would cause and much, much larger reflex to occur, followed by a maintenance of contraction of the muscle.
- The cerebral cortex in addition to providing voluntary control over muscle also provided a certain amount of inhibitory drive to reflex function.
- ___________ dominates in normal conditions (N).
- Decerebration reveals the excitatory control from _____________ (D).
- Rigidity and spasticity from damage to the cerebral cortex as seen in conditions such as stroke are accounted for by this loss of descending inhibition.
- Rigidity and spasticity can result from brain damage giving an ____________ stretch reflex
Inhibitory control;
supraspinal areas;
over-active or tonic
Gamma motor neurones
The receptors, which signal the changes in muscle length are not static organs.
- Not only do they move and stretch during muscle activity but they can also be regulated by the central nervous system, such that they maintain their ability to signal changes in length at different lengths.
- This is brought about by another collection of motor neurons - Gamma motor neurons
- The alpha motor neurons, which provide the voluntary or reflex contraction of these skeletal muscle fibers have these specialized receptors known as the muscle spindles embedded within a fibrous capsule. These are not separate from the extrafusal muscle fibers, so these are known as the ____________, They do not contribute to the overall contractility of a muscle, but they are there to sense ________________.
- These sensory neurons, which are wound around these intrafusal muscle fibers sense the changes in length and signal back to the spinal cord, which produce reflex contraction for these alpha motor neurons.
- However these gamma motor neurons can cause contraction of certain elements of these sensory organs such that the sensitivity of the reflex contraction can be adjusted. This is known as the __________________.
In order to allow further changes in length to be registered by the central nervous system, the gamma motor neurons can cause contraction, signalling this sensory organ, such that it can then start to fire again at the new length. –> The gamma motor neurone system is there to allow _____________________.
intrafusal muscle fibers;
the changes in length of the muscle;
Alpha Gamma Loop;
sensitivity of the muscle spindle to be altered
what does the corticospinal pathway do?
fine control of limb movements, body adjustments
What does red nucleus by the rubrospinal tract do?
automatic movements of arm in response to posture/balance changes
What does the vestibular nuclei do?
altering posture to maintain balance
