Control of Movement: Spinal reflexes Flashcards
What makes up a lower motor neuron (LMN)
What gives the info to LMN
Peripheral nerve, Neuromuscular junction and skeletal muscle
Peripheral nerve gives info to LMN
Afferent: conducted ____ CNS
Efferent: Conducted ____ from CNS
Afferent: conducted towards CNS
Efferent: Conducted away from CNS (towards PNS)
Overview of motor system organisation
- Motor Cortex
a) Brain stem - Spinal Cord- Alpha motor neuron
b) Brain stem - alpha motor neuron
c) Alpha motor neuron
Alpha Motor Neuron is the FINAL COMMON PATH
Axons leave the spinal cord via ventral roots & travel via their efferent spinal nerves to muscle.
Where do LMN orginate and where do they synapse?
Where do Upper motor neurons orginate and where do they synapse?
Originate in brain stem or spinal cord & synapse on muscle fibres (effector organs)
Originate in the cerebral cortex & travel down to the brain stem or ventral horn of spinal cord.
Motor system organisation elements
- Descending tracts
eg. the cortico-spinal (pyramidal) tract (fine movement); reticulo-spinal tract (gross movements). - Spinal interneurons (excitatory or inhibitory)
- Propriospinal neurons (coordinate upper and lower limbs)
- Afferent fibres
(type Ia) from muscle spindles
Why are alpha motorneurons called the final common path
All motor control signals ULTIMATELY synapse on α-motoneurons, which are the “final common pathway” for CNS control of behaviour
What feedback system is the final common pathway?
Diagram to explain as well
All motor control signals ULTIMATELY synapse on α-motoneurons, which are the “final common pathway” for CNS control of behaviour
there must be continuous sensory feedback provided to the CNS so that adjustments can be made in the muscle force & movement produced.
describe the anatomical relationship between motor neurons and muscles
motor neuron pool and the motor unit
- Motor neurons are clustered in columnar, spinal nuclei called motor neuron pools (or motor nuclei).
* All of the motor neurons in a motor neuron pool innervate a single muscle, and all motor neurons that innervate a particular muscle are contained in the same motor neuron pool.
* Thus, there is a one-to-one relationship between a muscle and a motor neuron pool. - A single motor neuron can innervate many muscle fibers. The combination of an individual motor neuron and all of the muscle fibers that it innervates is called a motor unit.
Draw a diagram showing the levels of control of movement from a motor unit and motor neuron pool
Lecture Slide
Main receptors controlling movement and what they sense
- Muscle spindles
(monitor muscle length (L) and speed of length change: dL/dt) - Golgi tendon organs
(monitor muscle tension) - Pain (nociceptors) receptors in the skin (respond to damaging or potentially damaging stimuli)
- Joint receptors
(monitor position, signal hyperextension or hyperflexion at the joint)
Whats a muscle spindle AND what are the types
Every skeletal muscle has encapsulated
muscle fibres scattered throughout called muscle spindles.
Muscle spindle fibres are intrafusal, whereas those making up the bulk of the muscle are extrafusal.
Draw a diagram showing the difference endpoints for gamma and alpha neurons
Lecture Slide
Relationship between Muscle spindles anf extrafusual fibres
Muscle spindles lie in parallel with extrafusal fibres and respond to stretch. Eg Group 1a afferent fibres from muscle spindles (a stretch receptor that monitors the rate of muscle stretch change)
Golgi tendon organ (GTO)
- Type of organ
-Contribute to?
- Location?
-Respond to?
-Mechanoreceptive organs.
Tendons and GTOs both contribute to neuromusculoskeletal dynamics.
GTO receptors are located at the myotendinous junction of skeletal muscles and are innervated by a single
Group Ib afferent.
Respond to muscle tension
Define reflex
- when does it occur
-reflex arc
A reflex is an involuntary, stereotypical response of the effector tissue following stimulation of receptors.
Occurs following successive activation of mutually connected neurons, with the last neuron innervating the effector tissue (usually a muscle).
These neurons and the effector tissue make up the reflex arc, which is the basic unit of a reflex.
Types of reflexes AND what determines the type
Based on how many neurons participate in one arc, the reflexes can be monosynaptic or polysynaptic.
Spinal reflexes include:
- The stretch (or myotatic) reflex
- The reverse (or ‘inverse’) myotatic reflex
Ø Sensors of muscle force in the tendon (Golgi tendon organs) - The flexion (withdrawal) reflex, and the crossed-extension reflex
Why test reflexes in exam?
Testing reflexes during clinical examination allows the integrity of the motor system to be determined, providing information on the condition of upper and lower motor neurons.
Draw the Monosynaptic reflex arc
Lecture Slide
What are the 5 elements to stretch reflexes
- Receptors:
(in muscle spindles) detect: ∆L and dL/dt
2.Afferent (sensory) fibers:Type Ia
3.Synaptic central relays: Ia afferents to α MNs (also MNs to muscle)
4.Efferent (motor) fibers:Axons of α MNs
- Effectors: skeletal muscles (extrafusal)
Stretch reflex & reciprocal inhibition:
Describe and example
Stretch reflex & reciprocal inhibition includes additional synaptic relay: Ia afferents to Ia interneurons which inhibits MNs innervating antagonist muscle(s)
Group 1a vs Group II in response to stretch
Lecture Slide
- Muscle spindles in parallel with extrafusal fibres respond to changes in length.
- Intrafusal fibres have spring-like properties.
- Spindle afferent mechanoreceptors respond to length changes, sensed by mechanoreceptors.
Commonly tested stretch reflexes:
Assess what segment?
Jaw Jerk
Biceps Jerk
Triceps jerk, Finger-flexion reflex
Knee Jerk
Ankle Jerk
Jaw Jerk: 5th Cranial Nerve
Biceps Jerk: C5, C6
Triceps jerk, Finger-flexion reflex: C7, C8
Knee Jerk: L3, L4
Ankle Jerk: S1
The extrafusal muscle fibres are innervated by the (____) ____.
The intrafusal muscle fibres are innervated by the (____) _____.
The EXTRAFUSAL muscle fibres are innervated by the (EFFERENT ) ALPHA motor neurons.
The INTRAFUSAL muscle fibres are innervated by the (EFFERENT) GAMMA motor neurons.
Afferent fibres are what types
Afferent fibres are type Ia and type II fibres.
Gamma Motorneurons:
- Unique feature
-innervate what fibres
- The muscle spindle is the only sensory receptor to have its own motor supply.
- γ-motoneurons innervate the striated portions of
the intrafusal fibres (aka “fusimotor neurons”). - There are two types of afferents that innervate muscle spindles, distinguished by structure and function into primary (type Ia) and secondary (type II) endings.
Gamma vs alpha
-size of cell bodies
- inputs
- Gamma motor neurons have small cell bodies and lack direct input from proprioceptive sensory afferents.
- Alpha motor neurons have large cell bodies, and most receive direct group Ia–derived proprioceptive sensory input.
What is the role of gamma motoneurons
gamma MNs keep muscle spindles at appropriate length so that they remain active during muscle shortening evoked by α motoneurons.
The reverse myotatic reflex:
Receptors
Afferents
Synaptic relays
Efferents
Effectors
Receptors: Golgi tendon organs (in series with muscle)
Afferents: Ib fibers
Synaptic relays:
Ib afferents to Ib interneurons which inhibits α MNs
Efferents: axons of α MNs
Effectors: muscles
* Under extreme conditions this reflex protects muscle from overload.
* Normal function: to maintain muscle tension in optimal range
The flexion (flexor, withdrawal) reflex:
Receptors
Afferents
Synaptic relays
Efferents
Effectors
Receptors: nociceptors
Afferents: type III and IV fibers
Synaptic relays: to excitatory interneurons which excite flexor α MNs (afferents project to multiple spinal segments !)
Efferents: axons of αMNs
Effectors: flexor muscles
The flexion withdrawal & crossed extension reflex:
- Purpose
- explain in terms of flexors and extendors
Significant for maintaining balance when nociceptive receptors in a foot are activated.
During a withdrawal reflex, the flexors in the withdrawing limb contract and the extensors relax, while in the other limb, the opposite occurs as part of the crossed extensor reflex.
