lower motor neurons and brainstem-spinal level reflexes

What are lower motor neurons

carry final motor command

encode force and timing of contraction of individual muscles

lower motor neurons

Located in the spinal anterior/ventral horn motor neurons and motor neurons located in the cranial nerve somatic motor nuclei whose axons innervate skeletal muscles are called lower motor neurons

• Activate skeletal muscle - final motor pathway 

• Activity of LMN influenced by two sources

  • Descending control 

  • Peripheral sensory input

• Axons of LMN will contact skeletal muscle at a specialized synapse called the motor end plate or neuromuscular junction

types of lower motor neurons

Alpha motor neurons

gamma motor neurons

interneurons

alpha motor neurons

innervate extrafusal fibers: fibers that generate tension of skeletal muscle

gamma motor neurons

innervate intrafusal fibers: special type of skeletal muscle fiber found within/at polar ends of the muscle spindle;

Regulate tension of muscle spindle

interneurons

axons distribute locally in the spinal gray matter - regulate the activity of alpha and gamma motor neurons; they can excite or inhibit these motor neurons;

integrate descending input and sensory input to modulate/modify activity of LMN (alpha and gamma motor neurons)

neuromuscular junction

alpha and gamma motor neurons communicate with muscle fiber at the neuromuscular junction

One to one relationship between the firing of the motor neuron action potential and the activation/triggering of the muscle fiber - unique chemical synapse

• Alpha motor neuron and the motor end plate

• Synaptic cleft (located between the axon terminal of motor neuron and muscle fiber membrane)

• Active zones at motor end plate

  • contains large numbers of docking proteins located in close proximity to voltage-gated Ca2+ channels

  • large numbers of neurotransmitter (acetylcholine) filled vesicles

• Sub-junctional folds on surface of muscle fibers: contain clusters of nicotinic acetylcholine receptors (AChRs) which are ionotropic receptors selective for cations

  • Activation of the receptor causes influx of NA+ (predominantly) which depolarizes the muscle fiber membrane leading to cross bridge cycling

    • ACh is removed from synaptic space by enzymatic breakdown: acetylcholinesterase (when removed, cross bridging will stop)

Synaptic Cleft for Lower motor neurons

located between axon terminal of motor neuron and muscle fiber membrane

active zones at the motor end plate

• contains large numbers of docking proteins located in close proximity to voltage-gated Ca2+ channels

• large numbers of neurotransmitter (acetylcholine) filled vesicles

sub junctional folds on muscle fibers

• contain clusters of nicotinic acetylcholine receptors (AChRs) which are ionotropic receptors selective for cations

  • Activation of the receptor causes influx of NA+ (predominantly) which depolarizes the muscle fiber membrane leading to cross bridge cycling

    • ACh is removed from synaptic space by enzymatic breakdown: acetylcholinesterase (when removed, cross bridging will stop)

motor unit

 is the single alpha motor neuron and the muscle fibers it innervates

motor unit size

 size of muscle mass, speed of contraction needed and type of control all factor into motor unit size

• Small motor units - small muscles that generate lower levels of force

  • Large motor units - large muscles that generate higher levels of force

motor unit type

• Type I - slow twitch: generate lower levels of tension but for longer periods of time (fatigue resistant) - postural control muscles

  • Type II - generate higher levels of force but for shorter periods of time - IIa: fast twitch oxidative; IIb: fast twitch glycolytic

grading force of muscle contractions

• Size principle - smaller motor units are recruited first followed by larger units

  • Rate code - as the need for greater force and speed increases, synaptic input increases the firing rate of the neurons increase

t/f: Lower motor neurons are the final motor pathway, allowing a neural signal to be converted into a mechanical event.

• This statement is true: lower motor neurons are the final motor pathway. The action potential of a motor neuron will cause a muscle fiber to depolarize setting off cross bridge cycling.

Where are lower motor neurons that innervate limb muscles located?

• Motor neurons that innervate the muscles of the trunk and limbs are located in the anterior horn of the spinal cord. Motor neurons that innervate muscles of the head and neck are located in the cranial nerve somatic motor nuclei (like the trigeminal and facial motor nuclei).

Which of the following neurons innervates the contractile ends of the muscle spindle?

• Gamma motor neurons will innervate the infrafusal fiber (contractile poles of the muscle spindle) and will be important in maintaining the tension in the muscle spindle. Alpha motor neurons innervate the extrafusal fibers which are the tension generating fibers of skeletal muscle. Interneurons (local circuit) neurons will integrate sensory information from the periphery and descending control signals from higher brain regions and they will use this information to regulate the activity of the lower motor neurons.

Which neurotransmitter is released at the neuromuscular junction?

• Acetylcholine is the neurotransmitter released at the neuromuscular junction. It will bind the nicotinic acetylcholine receptors allowing Na+ to enter the cell resulting in the depolarization of the muscle fiber.

topographical organization of LMN in SC

Orderly arrangement between LMN pools and the muscles they supply in the spinal cord: topographical organization

Longitudinal arrangement

• Lmn pools that innervate the UL are located in the cervical enlargement of the cord

• LMN pools that innervate the trunk are located in the thoracic regions of the cord

• LMN pools that innervate the lL are located in the lumbar enlargement of the cord

where are LMN pools that innervate UL located?

cervical enlargement of the cord

where are LMN pools that innervate the trunk located

thoracic regions of the cord

where are LMN pools that innervate the LL located?

lumbar enlargement of the cord

topographical organization of LMNs in the Spinal Gray Matter

• Flexor extensor rule

  • Flexor LMNs; located poseriorly 

  • Extensor LMNs: located anteriorly 

• Proximal-Distal Rule

  • LMNs innervating proximal/axial muscles are located medially 

  • LMNs innervating distal muscles are located laterally

where are Flexor LMNs located

posteriorly

where are extensor LMNs located

anteriorly

where are LMNs for proximal muscles located

medially

where are LMNs for distal muscles located?

laterally

topographical organization of local circuit neurons

Similar somatotopic organization is evident in the local circuit neurons and the connections they make in the SC

• Medially located local circuit neurons project to multiple spinal segments and cross the midline (bilateral projections) and they communicate with medially located LMNs - postural control 

• Laterally located local circuit neurons project to fewer spinal segments and most are ipsilateral (ipsilateral projections) and they communicate with laterally located LMNs - control of skilled limb movements

medially located local circuit neurons

Medially located local circuit neurons project to multiple spinal segments and cross the midline (bilateral projections) and they communicate with medially located LMNs - postural control 

laterally located local circuit neurons

Laterally located local circuit neurons project to fewer spinal segments and most are ipsilateral (ipsilateral projections) and they communicate with laterally located LMNs - control of skilled limb movements

role of sensory input

Sensory input conveyed to higher centers is used to plan and monitor movements indirectly impacting the activity of LMNs - think voluntary movements 

Sensory input at the level of the spinal cord or brainstem can directly influence the activity of LMNs - think reflexive movements and localized control (control not dependent on higher centers)

• muscle spindles, gogli tendon organs, nociceptors

role of muscle spindle sensory input

monitor muscle length, mediate monosynaptic stretch reflex

role of Golgi tendon organs sensory input

monitor muscle tension and mediate autogenic inhibitory reflexes

role of nociceptors sensory input

transmit information about pain; mediate withdrawal reflexes

reflexive movements

General Concepts

• All circuits have an afferent limb and an efferent limb

• Spinal and brainstem somatic motor reflex pathways target alpha motor neurons or interneurons

• They can produce complex coordinated activity at multiple joints

• They are involuntary, but descending input can alter the threshold for their activation or their gain (strength of response) 

  • modulation by higher centers; depending on context, reflexes may operate differently

  • gain: withdrawal response may be greater than usual

  • threshold: lower or increased threshold depending on context

  • may inhibit response based on needs/context

muscle spindles

Monitor muscle length and mediate the monosynaptic stretch reflex

• Structure of muscle spindle

  • Intrafusal fibers: nuclear bag and nuclear chain fiber innervated by gamma motor neurons

  • Sensory endings: group Ia and group II afferents

    • Group Ia afferents: responsive to small phasic stretches; velocity dependent

    • Group II afferents: responsive to sustained stretch 

  • Afferents send signal and cause muscle to contract

  • when muscle stretches, spindle stretches, afferents fire, activates alpha motor neuron, causes muscle to contract

spinal level reflexes: monosynaptic stretch reflex

Monosynaptic stretch reflex: sensory response to muscle stretch (aka: muscle stretch ms reflex; deep tendon reflex)

• Circuit: afferent limb: Muscle spindle afferents; efferent limb: alpha motor neurons

  • Muscle spindle afferents: form monosynaptic excitatory connections with alpha motor neurons in the ventral horn of the SC; these motor neurons innervate the same muscle that was stretched (feedforward excitation) = contraction of the agonist

  • Reciprocal inhibition: via local circuit neurons inhibitory connections (feedforward inhibition) are made with the alpha motor neuron of the antagonist muscle = relaxation of the antagonist;

  • agonist will contract; antagonist will relax

    • e.g hamstring contract, quads relax

  • Function: plays a major role in the maintenance of posture and ensures coordination of agonist and antagonist muscle activity in response to changing conditions; it is also an important reflex to be tested to determine neurological damage

muscle spindles alpha gamma co-activation

Alpha gamma co-activation

• During voluntary movement both alpha and gamma motor neurons will be activated simultaneously

• Gamma motor neurons cause polar ends of muscle spindle to contract; keeps middle section tense enough to sense ongoing changes in muscle length  

• Activation of the alpha motor neuron results in muscle fiber contraction

• Activation of the gamma motor neuron maintains the sensitivity of the muscle spindle during ongoing movement

  • Allows the system to provide feedback regarding muscle length during movement

Gamma Loop

Gamma loop: activation of gamma motor neurons can indirectly cause activation of an alpha motor neuron thereby mediating muscle contraction

Gamma Bias

Gamma bias (gain): amount of force generated in response to a muscle fiber stretch 

• High gain: small amount of stretch produces large increase in # of motor units recruited and increased firing rates resulting in large increases in tension

  • Making small adjustments with movement on the train 

• Low gain: greater stretch needed to produce the same level of tenison

  • Stretching in yoga: takes greater stretch to produce contraciotn in muscle; you dont want to activate stretch reflex

• Gain is modulated by descending control (UMN) to meet demands of functional situations

t/f: During voluntary movements the motor system will activate both alpha and gamma motor neurons simultaneously.

• This statement is true: during voluntary movement both alpha & gamma motor neurons will be activated simultaneously. Activation of the alpha motor neuron results in muscle fiber contraction, while activation of the gamma motor neuron maintains the sensitivity of the muscle spindle during movement allowing the spindle to provide ongoing proprioceptive information.

What are the ways in which higher brain centers can modulate the activity of reflexes? Select all that apply.

• They can decrease the threshold for activation of the reflex

• They can decrease the gain of a reflex

• They can increase the threshold for activation of a reflex

• They can increase the gain of a reflex

  • All of the above are correct: higher brain centers can alter the threshold (decreasing or increasing it) for activating the reflex or change the gain (strength of the response once activated). This allows higher centers to produce adaptive behavioral responses based on context.

During the monosynaptic stretch reflex, sensory information will activate which of the following neurons to mediate reciprocal inhibition so that the antagonist muscle will relax?

• During the monosynaptic stretch reflex, sensory information from the muscle spindle will activate an alpha motor neuron directly causing contraction of the agonist (the same muscle that was stretch). Simultaneously, the sensory single will activate an inhibitory interneuron, which will inhibit the alpha motor neuron of the antagonist muscle (reciprocal inhibition).

The activation of gamma motor neurons by higher brain centers to increase the tension of the muscle spindle so that a small stretch of the muscle will produce a greater than typical increase in muscle tension is an example of which of the following?

• This is known as gamma bias. This allows the system to set the spindle for more adaptive responses.

spinal level reflexes: autogenic inhibition

Autogenic Inhibition: sensory response to muscle tension (aka: GTO reflex arc)

• Circuit: afferent limb: GTO afferents; efferent limb; alpha motor neurons

  • Golgi tendon organ afferents: excite an inhibitory interneuron that then inhibits the alpha motor neurons in the ventral horn of the SC that innervate the same muscle that was producing the tension (feedforward inhibition) = reduced contraction of the agonist

  • Reciprocal excitation: via local circuit neurons excitatory connections (feedforward excitation) are made with the alpha motor neuron of the antagonist muscle = increased contraction of the antagonist

    Function: plays a major role in protective responses to inappropriate levels of muscle tension and spreads the work of force generation equally across motor units to ensure efficient generation of force; works with MS reflex to ensure coordination of agonist and antagonist muscle activity in response to changing conditions

spinal level reflexes: flexor withdrawal and flexion crossed extension

Flexor withdrawal and flexion-crossed extension: withdrawal of a limb in response to pain (flexor withdrawal) coupled with the opposite response in the contralateral limb (crossed-extension)

• Circuit: afferent limb: pain afferents; efferent limb; alpha motor neurons

• Afferents form excitatory and inhibitory connections with interneurons

• Excitatory interneurons synapse with alpha motor neurons in the ventral horn of the sc that innervate the muscles required for the flexor withdrawal 

• Inhibitory interneurons synapse with alpha motor neurons in the ventral horn of the sc that innervate the antagonist muscles

• Excitatory interneurons synapse with alpha motor neurons in the ventral horn of the sc that innervate the muscles required for extension of the opposite limb

• Inhibitory interneurons synapse with alpha motor neurons in the ventral horn of the sc that innervate the antagonist muscle of the opposite limb

Function: plays a major role in responding to dangerous stimulation and coordinating activity on both sides of the body for balance and postural adjustments

central pattern generators

Are neuronal circuits that produce patterns of self-sustained neural activity that control rhythmic movements associated with activities such as walking, running, swimming, chewing, and swallowing

• CPGs have been located in the spinal cord and brainstem 

• They are centrally generated - not dependent on sensory stimulation

• They can be modified by sensory input

The golgi tendon organ is the afferent limb of which of the following reflexes?

• The golgi tendon organ is the sensory receptor that mediates the autogenic inhibition reflex. Muscle spindles mediate the monosynaptic stretch reflex and nociceptive information mediates the flexor withdrawl reflex.

The flexor withdrawl & flexion-crossed extension reflex involve the activation of both excitatory and inhibitory interneurons that mediate complex patterns of movement on both sides of the body.

• This statement is true: this is a very complex reflex. Exciatory input from nociceptors will activate both excitatory interneurons to activate appropriate alpha motor neurons for contraction of agonists, and inhibitory interneurons to mediate reciprocal inhibition of the appropriate antagonists on both sides of the body.

brainstem level reflexes: jaw jerk

Jaw jerk reflex (CN V): like monosynaptic stretch reflex 

carried on trigeminal nerve

Circuit

• Afferent limb: proprioceptive fibers of the mandibular division of the trigeminal nerve (mecensephalic nucleus)

• Efferent limb: motor fibers from the trigeminal motor nucleus traveling in the mandibular division of the trigeminal nerve

brainstem level reflexes: corneal reflex

Corneal reflex (CN V & CN VII) - direct and consensual ; protect the eye; blink when something touches the eye

• Consensual response: blink happens in both eyes 

Circuit

• Afferent limb: nociceptive fibers of the ophthalmic division of the trigeminal nerve project to the spinal trigeminal nucleus (pain)

• 2nd order neurons of the spinal trigeminal nucleus project to the facial motor nuclei

• Efferent limb: motor fibers from the facial motor nuclei traveling in the facial nerve project to orbicularis oculi muscles

brainstem level reflexes: pharyngeal/gag reflex

Pharyngeal reflex: aka gag reflex (CN IX & X) direct and consensual 

Circuit

• Afferent limb; nociceptive fibers of the glossopharyngeal and vagus nerves project to the spinal trigeminal nucleus

• 2nd order neurons of the spinal trigeminal nucleus project to the nucleus ambiguous bilaterally

  • Efferent limb: motor fibers from the nucleus ambiguous traveling in the glossopharyngeal and vagus nerve project to posterior oral and pharyngeal muscles

primitive reflexes

Are reflex actions that are exhibited by normal infants in response to particular stimuli

These reflexes are no longer present due to descending inhibitory control that occurs over the course of normal development

examples of reflexes

Examples: not complete list 

• TLR: review from vestibular lecture; present at birth; integrates at 3 ½ years

• STNR: review from vestibular lecture; appears ad around 6-9 months and integrates around 9-11

• ATNR: review from vestibular lecture; presenta t birth and integrates and around 6 months

• Moro reflex (aka startle reflex): when baby’s head moves quickly backward, responds by widely abducting and extending upper extremities then curls them into midline; often crying is associated with the motor response; can occur in response to loud noice; present at birth and integrates around 2-4 months

• Rooting reflex: when the skin near a baby’s mouth is stroked, they will turn the head in until the stimulus is found with the mouth; present at birth adn integrates at 3-4 months

• Sucking reflex: elicited when something is placed int he baby’s mouth; strongest when the palate is stimulated; present at birth and integrates and 3-4 months

• Palmar grasp reflex: stimulation across the palmar surface of the hand results in the fingers flexing into a tight grasp; present at birth and integrates around 4-6 months

  • Positive support/stepping reflex: when supported under the arms in upright, the infant will partial extension of the LEs and when leaning forward slightly, the infant will take reciprocating steps; present at birth and integrates around 2-4 months

Which nerve carries the afferent and efferent limbs of the jaw jerk reflex?

• The trigeminal nerve carries both the afferent and efferent limbs of the jaw jerk reflex.

When the left cornea is touched this nociceptive information will typically result in the activation of which of the following?

• The corneal reflex has a direct (ipsilateraly) and consensual (contralateral) response. When nociceptive information traveling on the ophthalmic division of the trigeminal nerve reaches the spinal trigeminal nucleus, second order neurons of this nucleus will activate motor neurons in the facial motor nuclei bilaterally.

  • The pharyngeal reflex (gag reflex) will also have a direct and consensual response resulting in activation of motor neurons in the nucleus ambiguus bilaterally.

muscle tone

steady level of tension in muscles - felt as a resistance of the muscle to passive stretchh

hypotonus

decreased tone of skeletal musclesh

hypertonus

increased tone of skeletal musclere

flexes: common deep tendon reflexes to assess

• common deep tendon reflexes to assess:

  • Upper limb: biceps (C5, C6); triceps (C6, C7, C8); brachioradialis (C5, C6); finger flexors (c7, c8)

    • Lower limb; patella (L2, L3, L4); achilles (s1, s2)

areflexia

•  lack of reflex activity 

hyporeflexia

• reflex activity is present but weaker than normal 

hyperreflexia

• reflex is activity is over active/stronger than normal 

Lower motor neuron signs:

damage to the final motor pathway will cause some combination of the following signs in the affected muscles

• Weakness and eventual muscle atrophy 

• Fibrilations or fasciculation: spontaneous twitches due to involuntary contractions of one motor unit (fibrillation) or groups of motor units (fasciculation) 

• Hypotonia

• Hyporeflexia or areflexia 

LMN damage: all ipsilateral; individual muscles will be affected NOT groups