Spinal reflexes Flashcards
define the term motor unit + describe how the force of muscle contraction is controlled by impulse frequency and by recruitment of motor units.
A motor unit refers to the basic functional unit of muscle contraction, comprising a single motor neuron and all the muscle fibers it innervates.
* A motor unit consists of a:
* neuronal cell body in lamina IX of the ventral horn of the spinal cord
* its motor axon, and
* the set of muscle fibres it innervates
Contraction of one motor unit is the smallest unit of contraction a muscle can produce
When a motor neuron receives a signal from the nervous system, it fires an action potential that travels down its axon and reaches the muscle fibers it controls, causing them to contract. The strength of a muscle contraction can be influenced by two primary mechanisms:
- Impulse Frequency (Rate Coding)=
What it is: Changing how fast signals (impulses) are sent to muscles.
How it works: A single signal causes a quick muscle contraction and relaxation. If signals come faster, muscle contractions combine for a stronger and continuous contraction. By varying signal speed, muscles can contract with just the right force needed. - Recruitment of Motor Units=
What it is: Using more muscle fibers for stronger contractions.
How it works: Muscles are made up of different types of fibers controlled by motor units. For light tasks, only a few motor units with slow-twitch fibers are used. For heavy or fast tasks, more and larger motor units with fast-twitch fibers join in. This way, muscle force can be adjusted from very light to very strong, depending on the task.
what is a muscle spindle
- The muscle spindle is the receptor that mediates all the tendon reflexes.
- It is often called a proprioceptor as it responds to movements of the body rather than external stimuli
- It consists of a complex encapsulated stretch receptor inside a connective tissue sheath.
There are normally about 4 to 20 muscle spindles in a muscle
Stretch of the muscle also stretches the muscle spindle and activates the muscle spindle sensory afferents.
The main sensory nerve fibre from the muscle spindle is called a ____________
Stretch (THIS IS WHAT ACTIVATES A MUSCLE SPINDLE) of the muscle also stretches the muscle spindle and activates the muscle spindle sensory afferents.
The main sensory nerve fibre from the muscle spindle is called a 1A AFFERENT (Ia afferent)
what are intrafusal muscle fibres + why is there role important?
Intrafusal muscle fibers are specialized fibers found inside muscle spindles, which are sensory receptors in muscles that help detect changes in muscle length and movement. Unlike regular (extrafusal) muscle fibers that contract to produce movement, intrafusal fibers don’t significantly contribute to moving the body. Instead, their main role is sensory, helping the body understand where it is in space and how it is moving (PROPRIOCEPTION, INTRAFUSAL FIBRES ARE RESPONSIBLE FOR PROPRIOCEPTION)
extra info:
When muscles stretch, intrafusal fibers stretch too, which triggers sensors attached to them. These sensors send messages to the brain and spinal cord about how the muscle is stretching or contracting. This information is crucial for maintaining balance, posture, and for the smooth coordination of movements.
- In intrafusal fibres all the contractile apparatus is at the ends.
- This leaves the centre of the fibre without actin or myosin but full of cell bodies and mitochondria.
- This central region has no connective tissue and so is very easily stretched.
The 1a (Ia) sensory afferents coil around the centre region. When it is stretched, they are deformed and fire action potentials.
most muscle spindles are constantly firing to tell brain our position in space (part of our autonomic nervous system), when a muscle is stretched, muscle length detectors sense this and increase their firing rate. Knowing this, explain why when we tap a tendon there is a response?
When a tendon is tapped, the muscle is only stretched a very small amount; this, however, is sufficient to briefly increase the firing of the spindles.
what kind of motor neuron supplies intrafusal muscle fibres ( we know that alpha motor neurons innervate the main body of muscle)
the gamma motor neurons supply intrafusal muscle fibres
- The gamma motor neurone stimulates contraction of the intrafusal fibres and ‘pre-stretches’ their centres.
This increases the response of Ia afferents to muscle stretch. - Gamma motor neurones are NOT reflexively activated by the muscle spindle afferents.
They are driven from descending motor pathways, such as the corticospinal tract.
describe alpha- gamma coactivation
- During any voluntary movement of a muscle, the brain (via the corticospinal tract) activates both the alpha and gamma motor neurones.
- This co-activation serves to keep the muscle spindle feedback at the right level during movement.
When the muscle reaches the desired length, the gamma motor neurone output adjusts to a level appropriate for this new length
= this is how the muscle spindle fibres maintain their responsiveness when the muscle is quite contracted
what proprioceptor in the muscles is activated by length/stretch and which proprioceptor is activated by tension
length/stretch= intrafusal fibre
tension= golgi tendon organ
glycine is a major inhibitory neurotransmitter in the spinal cord. Golgi organ has disynaptic interneurone connection to its own motor neurons. What is meant by a glycinergic inhibitory neurone like this golgi interneuron.
the glycinergic inhibitory interneuron in golgi tendon is going to turn the alpha motor neuron off to switch the muscle off (to avoid excess tension and damage to muscle) (when ppl drop weights because its too much its cuz of this)
- We increase or decrease the force of contraction in a muscle by recruiting more or fewer motor units
- Most muscles contain a mixture of small and large motor units.
- For fine control of force, we activate more or fewer small motor units.
- For full power we recruit large motor units.
- In motor nerve axons, we have an all-or-nothing frequency of activity.
- contrasts to sensory nerve fibres, which use frequency codes for stimulation intensity
Each motor axon will fire at its _____ ______ _________
- contrasts to sensory nerve fibres, which use frequency codes for stimulation intensity
- We increase or decrease the force of contraction in a muscle by recruiting more or fewer motor units
- Most muscles contain a mixture of small and large motor units.
- For fine control of force, we activate more or fewer small motor units.
- For full power we recruit large motor units.
- In motor nerve axons, we have an all-or-nothing frequency of activity.
- contrasts to sensory nerve fibres, which use frequency codes for stimulation intensity
Each motor axon will fire at its TETANUS FUSION FREQUENCY
- contrasts to sensory nerve fibres, which use frequency codes for stimulation intensity
what is a tetanus aka tetanic contraction
tetanus is the contraction of one motor unit by a single action potential which generates a twitch.
To produce a smooth contraction, the motor unit has to be activated by a train of action potentials at a frequency high enough to produce a smooth fused contraction
What is a reflex?
- What is a reflex? It is triggered by a sensory input, processed through neurones (and interneurones), and results in an involuntary motor action
- Classic examples: tendon jerk reflexes elicited by tapping the patella or Achilles tendon.
- These are also known as myotatic reflexes
- Reflexes are tiny ‘motor programs’ stored in the synaptic connections between sensory inputs, outputs, and interneurons.
Found in one or two spinal cord segments (one segment = distance between two spinal nerves)
why is the patella reflex known as a homonymous reflex
- The knee-jerk (patella) reflex is a myotactic reflex.
Note that the stretch of the quadriceps muscle activates the same (quadriceps) muscle to contract.
homonymous= because the same muscle (quadriceps) thats stretched, is also contracted
n.b.stretch reflex or myotatic reflex refers to the contraction of a muscle in response to its passive stretching
lots of small action potentials that alone wouldnt cause depolarisation, add together to trigger a response (e.g. 15mV + 20mV + 30mV) explain the science behind this
when a single action potential in a 1a sensory nerve fibre does not produce an action potential in the motor neuron. Instead it triggers a small depolarisation= excitatory post-synaptic potential EPSP
temporal summation (time dependent) is when the trigger in the 1a sensory nerve fibre is big enough (lots of EPSPs after each other so time is important they need to be coming quickly at the axon) to generate a action potential (aka response) in the motor neurone.
spatial summation (several neurons required). is when all these little EPSP’s are happening at same time so add together and trigger a response in the motor neuron. (input from multiple neurons simultaneuosly cause AP)
n.b. u need about +55mV to trigger an AP
what is reciprocal inhibition
So reciprocal inhibiton makes sure the opposing muscle (antagonist) triceps doesn’t contract at same time as agonist (biceps), so if biceps is contracting an inhibitory signal will stop triceps from contracting= physiological we can’t control this; any muscle contracted its opposing muscle will relax i.e. when extensor contracts, flexor relaxes
so when we step on smth painful and nocireceptors sense this our alpha motor neuron will respond by contracting the muscle to remove the foot from the pain, but another neuron will inhibit the opposing muscle (recirpocal inhibition)
N.b. although the extensor reflex does not involve an interneuron, the reciprocal inhibition neuron does involve an interneuron (aka relay neuron)
