Yuste C11 - Reflexes Flashcards
Difference between the lower and upper nervous system
Voluntary (cortically initiated) vs involuntary (spinal reflexes) movement
Purpose of the nervous system
To make the animal move. And to generate and control movements in a way that is intelligent.
How does our brain generate most movements?
Top down - motor cortex is involved in planning, initiating, and directing voluntary movements. The basal ganglia and cerebellum help with this. Motor commands are then sent through he brainstem into the spinal cord.
Extrapolating the logic of lower and upper NS
Motor cortical areas are involved in decision making and awareness and must generate some idea of the self.
What does the upper motor system involve?
The cortex, basal ganglia, and cerebellum
What does the lower motor system involve?
The spinal cord and the brain stem. Focuses on reflex and locomotion.
What is a reflex?
Involuntary response to a sensory stimulus.
Role of the spinal cord in locomotion
S.C has sensory inputs and motor neurons; these sensory-motor loops generate motor reflexes locally in S.C.
S.C is partly reflexive and partly top-down-controlled.
How do spinal reflexes implement control theory?
Use feedback.
Locomotion is mostly build by what?
by circuits => Central Pattern Generators
What do skeletal muscles do?
Move the bones and different parts of our body by contracting or relaxing.
How are muscles activated
By the firing of motor neurons in the ventral horn of the S.C.
How are muscles divided
They are divided into motor units, which are all the muscle fibres that a particular motor neuron innervates.
Muscle fibres
In a motor unit, not next to each other because axons from a motor neuron branch in the muscle. This results in force exerted by the contraction of all the muscle fibres being spread out through the entire muscle, generating a smoother movement.
Duration and the strength of the contraction of a muscle
Depends on the motor neuron you activate
Three types of muscle fibres
- Slow
- Fast fatigue-resistant
- Fast fatigable
Slow fibres
Slow to respond, generate relatively little force, do not get tired.
Fast fatigue-resistant fibres
Fast to act, generate more force, resist fatigue.
Fast-fatigable fibres
Fast to act, generate a lot more force, but tire out quickly.
Why do we need all these types of muscle fibres?
For different types of behaviour, need to recruit different proportions of muscle fibres.
Which fibres we use when we exercise
We first engage the slower fibres, then the fast fatigue-resistant, leaving the muscle fibres that fatigue quickly until the end.
Ordered recruitment
Idea that motor neurons and their motor units are recruited in order, only using the fast and fatiguable fibres for the most strenuous behaviours. This idea is known as the size principle because it correlates with the size of the motor neuron and the motor unit.
When the human body evolved to adapt to hunter-gatherer lifestyles
Paleolithic period. Humans have ability to ride on our slow or fatigue-resistant motor units for trekking - outlast prey.
Sheerington’s contribution to reflexes/spinal cord
Argued that the motor neurons in the spinal cord were the “final common path” - all behaviour gets funnelled through the activity of these neurons.
Alpha motor neurons.
Alpha motor neurons
Motor neurons in the S.C. Largest and located in the ventral horn.
How are ventral motor neurons related to columns?
The ventral horn motor neurons associated with a. particular muscle are in columns that run up and down the spinal cord. The columns are elongated clusters of neurons that innervate the same muscle.
Topography of muscle innervation in the spinal cord
Neurons innervating the musculature that is axial (closer to the centre of the body) are located on the medial part of the ventral horn on the S.C. Muscles that control the limbs (more lateral) are innervated by neurons that are located on the lateral ventral horn.
Local connectivity within the spinal cord
Projections on the ipsilateral side, but motor neurons are decussate to innervate the contralateral side. Local connectivity builds motor reflexes (among other unknowns).
Proprioceptive system
Monitors the position and tension of all of our muscles and tendons and provides real time info on the state of our skeleton and body.
Axons that carry proprioceptive info
The thickest and most myelinated of axons = the fastest.
Muscle spindles
Sit in the middle of muscle fibres. Have intrafusal muscle fibres; group A axons wrap themselves around them with stretch receptors in the membrane of their nerve endings. Monitored by proprioceptive sensory axons.
How are APs in the group A axons sent to the S.C
When the muscle contracts, the intrafusal fibres also contract, generating APs in the group A axons.
Some implications of axons monitoring stretch, length etc.
Your NS keeps careful track of the position, length, and forces of all of your muscles
Sherrington’s major contribution to neuroscience
Explaining how reflexes work.
Monosynaptic reflexes
Involve a single synaptic connection in the spinal cord, a connection from a sensory neuron to a motor neuron. AKA “stretch,” “deep tendon,” or “myotonic”.
When the tendon is hit (monosynaptic reflex) what happens?
The patellar tendon pulls and stretches the quadriceps. which activates and fire the proprioceptive axons in its muscle spindles. These axons enter the dorsal horn of the spinal cord and go down to the ventral horn, where they contact the motor neurons that activate the quadriceps, causing the muscle to contract and your leg to pull up. The proprioceptive axons also contact inhibitory neurones act inhibit the antagonist leg muscle, the biceps.
Why do we have stretch reflexes? Like the patellar tendon reflex
These reflexes are some of the many built-in mechanisms to ensure that muscle function is smooth and coordinated. The knee-jerk reflex probably avoids body imbalances created by sudden changes in posture or in terrain.
What is the neural circuitry underlying the knee-jerk reflex?
Feedback loop. Control theory = a set point (desired length of the muscle) and a feedback signal (proprioceptive input). New input generates a change in the system that compensates for it and maintains the system state. Cf. homeostasis.
Flexion-extension reflex
if you step on a pin, you immediately pull back (flex) your leg and straighten/extend your other leg so that you don’t fall.
Pathway of flexion-extension reflex
Afferent sensory neurons carrying the pain info from the foot enter the SC via the dorsal root to synapse onto interneurons, which project to motor neurons that activate the quads. You then flex and lift your leg. Simultaneously, these interneurons make synapses on the contralateral side of the SC, contacting the motor neurones that activate the biceps on the other leg. So you extend and straighten that other leg.
=> Excitatory connections.
Inhibitory connections of the flexion-extension reflex
Ipsilatoral and contralateral SC segment have inhibitory connections that inhibit the antagonist muscles (biceps of the leg you’re lifting and quads of the leg you’re straightening).
What is the problem with Sherrington’s reflex model
It doesn’t account for behaviour that occurs spontaneously, without sensory inputs.
Locomotion
A result of organising muscle activity in terms of space and time.
The flexor and extensor are activated antagonistically in different limbs at different times, and this pattern changes depending on whether the animal is walking, trotting, pacing or galloping.
Brown’s experiments
Cut all the sensory inputs into the SC by sectioning the dorsal root ganglion nerves. Cut the connection from the brain to the SC, leaving the SC completely isolated, with motor neurons coming out of the SC and no sensory neurons coming in.
Brown findings
His cats were still able to walk even though they had no sensory inputs. Proposed that the SC itself can generate a pattern of firing that is spontaneous and cyclical and occurs regardless of the sensory input. Generates locomotion.
Connection between motor neurones and interneurons
Motor neurons that correspond to a given muscle on one side fo the body connect with interneurons hat inhibit the same muscle on the contralateral side.
What happens when inhibition ends?
There is rebound excitation that happens automatically.
What is rebound excitation
Has to do with a property of sodium channels (release from their inactivation) which brings the neuron back to AP threshold. This can be aided if there are background excitatory inputs depolarising the neuron. These can be unspecific.
What happens when inhibition ends?
The contralateral flexor motoneuron fires, which inhibits the ipsilateral flexor motorneuron, which then fires later, etc. Crossed reciprocal inhibition within the SC and the small delay caused by the inhibition until the rebound excitation kicks in => movement to alternate between sides.
What is central pattern generator
A neural circuit that generates spontaneous rhythmic activity.
Hans Berger experiemnt
Using electroencephalography on his young son, discovered that when his eyes were closed, there was strong oscillatory activity in his visual cortex => the alpha rhythm. Oscillation = CPG.
Why are CPGs so widespread?
Generate spațio-temporal patterns of activity.
Llinas
Argues that CPGs can be cajoled for more abstract computations, where locomotor states now correspond to symbolic states in a model of the world such as memories, perceptions, ideas, emotions etc.
What is the relationship between CPG and attractors
The states of the CPG are homologous to the attractors of feedback neural networks.
What could the encephalisation of CPGs be responsible for
The evolution of the mind. Cf. Llinas.
