Upper motor neurons originate in the brain and brainstem and project to the spinal cord or motor nuclei of cranial nerves. They facilitate voluntary movement by exerting control over lower motor neurons. Lower motor neurons originate in the spinal cord and project to skeletal muscles. There are two main types - alpha motor neurons directly innervate skeletal muscle fibers to enable contraction, while gamma motor neurons innervate the contractile portions of muscle spindles to regulate muscle tone and proprioception. Damage to upper or lower motor neurons can be identified by distinct clinical signs such as weakness, spasticity, hyperreflexia and abnormal reflexes.

1. MOTOR SYSTEM
PHYSIOLOGY: NEUROLOGY

2. UPPER MOTOR
NEURONS

3. UPPER MOTOR NEURONS
OVERVIEW
‣ Upper motor neurones make up one half of the body’s
somatic nervous system – the other half being made of
lower motor neurones
‣ Both can be damaged in various ways and both have
distinct clinical presentations
‣ Learning Goal
‣ To consider the definition, function and clinical
significance of the upper motor neurones

4. UPPER MOTOR NEURONS
INTRODUCTION TO UMNS
‣ The general definition of an UMN is a neuron whose cell
body originates in the cerebral cortex or brainstem and
terminates within the brainstem or spinal cord
‣ Neurons which give rise to the various descending motor
tracts are all UMNs
‣ In addition, neurons which input to the nuclei of
the extrapyramidal tracts (such as the rubrospinal tract) are
also UMNs

5. https://teachmephysiology.com/nervous-system/motor-system/upper-motor-neurone/

6. UPPER MOTOR NEURONS
INTRODUCTION TO UMNS
‣ The term UMN is typically used to describe descending motor
neurons within:
‣ corticospinal tracts - pre-central gyrus -> ventral horn of the
spinal cord
‣ corticobulbar tracts - pre-central gyrus -> motor nuclei of cranial
nerves
‣ General path: UMN -> interneurons -> LMN -> muscle
‣ UMNs exert their effects via LMNs, which may be one singular LMN
or several LMNs

7. UPPER MOTOR NEURONS
FUNCTION
‣ The function of the UMN is to facilitate voluntary
movement
‣ One cortical motor neuron is responsible for activating a
group of muscles in order to facilitate a certain movement
‣ The neurotransmitter typically involved in the transmission
from upper to lower motor neurons is glutamate

8. UPPER MOTOR NEURONS
FUNCTION
‣ Let’s consider corticospinal and corticobulbar tracts
‣ Both tracts originate within the pre-central gyrus, also known as M1
‣ The pre-central gyrus is somatotopically organized
‣ This means that particular areas of the body correspond to specific points of
the cortex = there is a relationship between where a neuron originates and its
function
‣ This gives rise to the motor homunculus
‣ areas of the body with a larger precision of motor control (eg. the hands) have
a larger cortical representation
‣ areas which do not have a great deal of motor precision have a smaller
representation in the cortex

9. https://en.wikipedia.org/wiki/Cortical_homunculus

10. UPPER MOTOR NEURONS
SIGNS OF PATHOLOGY
‣ Damage to an UMN causes
‣ weakness (-paresis)
‣ paralysis (-plegia)
‣ … of movement for the group of muscles it innervates
‣ This paresis or paralysis is typically widespread in the form of
either a mono/hemiparesis or mono/hemiplegia
‣ The function of UMNs can also be understood through observing
what other clinical signs occur when they are damaged

11. UPPER MOTOR NEURONS
SIGNS OF PATHOLOGY - HYPERTONIA
‣ UMNs are believed to have a modulatory role in governing muscle
tone through what is known as descending inhibition, although this is
not fully understood
‣ It is thought that UMNs regulate inhibitory interneurons and their
effect on alpha and gamma motor neurons
‣ Loss of UMN input, and thus descending inhibition, means firing of
alpha and gamma motor neurons is favoured, causing hypertonia
‣ Furthermore, damage to cortical inputs to other descending tract
nuclei, especially the medullary reticulospinal tract, known to inhibit
spinal reflexes, may also be implicated

12. UPPER MOTOR NEURONS
SIGNS OF PATHOLOGY - HYPERREFLEXIA - CLONUS
‣ Hyperreflexia
‣ Descending inhibition from the UMNs also regulates the
extent to which the myotatic stretch reflex is elicited
‣ Therefore, loss of the UMN causes brisk reflexes
‣ Clonus
‣ This is a series of contractions which occur in a muscle when
it is suddenly stretched and held in that position
‣ This is thought to arise due to an exaggerated stretch reflex

13. UPPER MOTOR NEURONS
SIGNS OF PATHOLOGY - SPASTICITY
‣ This describes the phenomenon where a muscle is tight and stiff on
passive movement in a velocity dependent manner
‣ In spasticity, the amount of resistance is directly proportional to the speed
of passive movement
‣ For example, on rapid passive movement of a limb there will be a point
where the muscle becomes suddenly resistant to further stretch
‣ Furthermore, a ‘clasp knife reflex’ may also be present
‣ This phenomenon describes how following a sudden increase in
resistance there is a swift decrease in tone of a muscle, allowing the
muscle to easily be stretched

14. UPPER MOTOR NEURONS
SIGNS OF PATHOLOGY - POSITIVE BABINSKI SIGN
‣ The Babinski test is a fundamental component of a neurological examination used to
assess UMNs
‣ The clinician will stroke a blunt object along the lateral border of the plantar surface of the
foot
‣ A normal response is flexion of the large toe and adduction of the other toes
‣ this a negative Babinski sign
‣ However, in patients who have an UMN syndrome an abnormal planar reflex is elicited
whereby the large toe extends and there is abduction of the other toes
‣ this is a positive Babinski sign
‣ In infants below the age of two, a positive Babinski sign is normal
‣ This is because the corticospinal tracts are not yet fully developed

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16. UPPER MOTOR NEURONS
SIGNS OF PATHOLOGY
‣ Long term disuse of a muscle due to paralysis may cause disuse
atrophy
‣ It is also important to note that although hypertonia and
hyperreflexia are long term consequences of UMN damage
‣ There is typically an initial hypotonia and hyporeflexia
immediately following damage
‣ This is seen in strokes of the cerebral cortex or within the
internal capsule and occurs contralateral to the side of the
lesion

17. UPPER MOTOR NEURONS
CLINICAL RELEVANCE - STROKE
‣ A stroke, or cerebrovascular event, is a clinical syndrome
involving a sudden loss of brain function caused by
disruption of the blood supply to the brain
‣ A stroke and a transient ischaemic attack (TIA) differ in that
the disturbances last 24 hours or less and resolve
completely in a TIA
‣ In a stroke the disturbances persist or result in death

18. UPPER MOTOR NEURONS
CLINICAL RELEVANCE - STROKE
‣ A stroke is the consequence of either
‣ infarction (80-85%)
‣ intracranial haemorrhage (10-15%)
‣ subarachnoid haemorrhage (5%)
‣ Common causes of stroke include large vessel atheroma or
thrombosis, cardiac emboli, atherothromboemboli from the carotids
and hypertensive microaneurysm rupture
‣ Other causes include carotid dissection, vasculitis and venous sinus
thrombosis

19. UPPER MOTOR NEURONS
CLINICAL RELEVANCE - STROKE
‣ Risk factors for strokes are similar to that of other cardiovascular disease such
as a myocardial infarction
‣ These include:
‣ Smoking
‣ Poor diet
‣ Alcohol
‣ History of cardiovascular risk factors e.g. hypertension and atrial fibrillation
‣ Hyperlipidaemia
‣ Diabetes mellitus

20. https://teachmephysiology.com/nervous-system/motor-system/upper-motor-neurone/

21. UPPER MOTOR NEURONS
CLINICAL RELEVANCE - STROKE
‣ Strokes are a common cause of UMN signs since ischaemia can affect the function of UMNs
‣ The clinical signs caused by strokes typically relate to the affected vessel and its respective
blood distribution
‣ In particular, UMN signs are seen in total or partial anterior circulation strokes as these affect the
motor cortex
‣ In addition, subcortical lacunar stokes of the internal capsule will result in UMN signs
‣ 80% of patients who have a stroke have either hemiparesis or hemiplegia and 40% will go on to
have long term impairment of function in the upper limb
‣ Sensory deficits, such as numbness, dysphagia, dysarthria, homonymous hemianopia, and other
symptoms may also be present depending on what cerebral vessels are effected
‣ It is not uncommon for patients to regain some function in a previously plegic limb as
the rubrospinal tract is able to compensate for the loss of corticospinal fibres to a certain extent

22. UPPER MOTOR NEURONS
REVIEW QUESTIONS
‣ What is the general definition of an UMN?

23. UPPER MOTOR NEURONS
REVIEW QUESTIONS
‣ UMNs are descending motor neurons within
the corticospinal and corticobulbar tracts, which arise
from the pre-central gyrus and terminate in the ventral
horn of the spinal cord and motor nuclei of cranial nerves
respectively

24. UPPER MOTOR NEURONS
REVIEW QUESTIONS
‣ What does it mean that the pre-central gyrus is
somatotopically organized?

25. UPPER MOTOR NEURONS
REVIEW QUESTIONS
‣ This means that particular areas of the body correspond to
specific points of the cortex = there is a relationship
between where a neuron originates and its function

26. UPPER MOTOR NEURONS
REVIEW QUESTIONS
‣ How is the Babinski test performed and what outcome
would indicate UMN damage (>2yrs age)?

27. UPPER MOTOR NEURONS
REVIEW QUESTIONS
‣ Negative Babinski sign
‣ Normal response is flexion of the large toe and adduction of
the other toes
‣ Positive Babinski sign
‣ Abnormal response when large toe extends and there
is abduction of the other toes
‣ UMN damage!
‣ Remember is normal <2yr old

28. LOWER MOTOR
NEURONS

29. LOWER MOTOR NEURONS
OVERVIEW
‣ A lower motor neuron (LMN) is a multipolar neuron which
connects the upper motor neurone (UMN) to the skeletal
muscle it innervates
‣ An UMN may synapse directly or indirectly, via interneurons,
onto a LMN
‣ Learning Goal
‣ To consider the location of LMNs and the different types, as
well as the classical signs and symptoms that are found
when they are damaged

30. LOWER MOTOR NEURONS
INTRO TO LMNS
‣ The cell body of a LMN lies within the ventral horn of the spinal cord or
the brainstem motor nuclei of the cranial nerves which have motor
modalities
‣ Therefore, the cell body of a LMN lies within the central nervous system
(CNS)
‣ The axon of a LMN exits the CNS and forms the somatic motor part of
the peripheral nervous system (PNS)
‣ Finally, the LMN terminates on the muscle fibres which it innervates
‣ The combination of the LMN and these fibres is known as a motor unit

31. LOWER MOTOR NEURONS
INTRO TO LMNS
‣ It is important to note that although one LMN will innervate several muscle
fibres, a single muscle fibre is innervated by only one LMN
‣ The bridging gap between the LMN’s axon terminal and the muscle fibre it
supplies is known as the neuromuscular junction (NMJ)
‣ It is here that the motor neuron releases the neurotransmitter
acetylcholine, which causes firing of an action potential in the receiving muscle
fibre
‣ The term LMN is often used interchangeability with α-motor neuron – but when
clinicians refer to a LMN syndrome they are referring to damage to α-motor
neurons
‣ There are also another type of LMNs, known as γ-motor neurons

32. LOWER MOTOR NEURONS
TYPES OF LMNS
α-Motor Neurons
‣ α-motor neurons are the type of LMNs which, when damaged, produce
the characteristic clinical signs of a LMN syndrome
‣ Within the spinal cord, the cell bodies of these neurons originate
in laminae VIII and IX of the ventral horn and are somatotopically
organized
‣ neurons which innervate distal musculature are located lateral to
those which innervate axial muscles
‣ neurons which innervate extensors are ventral to those which
innervate flexors

33. LOWER MOTOR NEURONS
TYPES OF LMNS
α-Motor Neurons
‣ The function of α-motor neurons is to cause contraction of the muscle
fibres they innervate
‣ This is called ‘the final common pathway’, as α-motor neurons are
essential for muscle contraction
‣ This can either been under voluntary control, through the action of
UMNs, or through eliciting the myotatic stretch reflex, as α-motor
neurons form the efferent portion of the reflex arc
‣ Therefore, there can be no coordinated muscle contraction if the α-
motor neurons are not functioning

34. LOWER MOTOR NEURONS
TYPES OF LMNS - GAMMA MOTOR NEURONS
‣ γ-motor neurons have a key function in the regulation
of muscle tone and maintaining nonconscious
proprioception
‣ Although γ-motor neurons fall under the umbrella term
LMN, a LMN syndrome results from damage to α-motor
neurons only
‣ γ-motor neurons also arise from laminae VIII and IX in the
ventral horn of the spinal cord

35. LOWER MOTOR NEURONS
TYPES OF LMNS - GAMMA MOTOR NEURONS
‣ These innervate fibres that form the contractile parts of the muscle spindles in
skeletal muscle
‣ α-motor neurons receive input from both muscle spindle Ia sensory afferents
and UMNs
‣ γ-motor neurons are solely under control from the UMNs
‣ These fibres are important in signalling the length and velocity of a muscle
‣ The function of γ-motor neurons is to keep the fibre taut by causing
contraction of its polar ends
‣ Maintaining tension in these fibres is necessary for preserving sensitivity to
muscle stretch by muscle spindles

36. https://teachmephysiology.com/nervous-system/motor-system/lower-motor-neurones/

37. LOWER MOTOR NEURONS
TYPES OF LMNS - GAMMA MOTOR NEURONS
‣ To illustrate their importance, consider the myotatic stretch reflex
‣ Upon stretching the muscle, (for example through a patellar tendon tap) the
muscle spindle will stretch and sensory afferent fibres will fire
‣ This in turn will cause the α-motor neurons to fire and lead to muscle contraction
through the reflex arc
‣ The contraction of muscle causes the intrafusal fibres to become slack, reducing Ia
afferent firing
‣ A slack fibre attenuates the firing of muscle spindle afferents and they are no
longer sensitive to stretch
‣ Therefore, if the muscle were to be stretched again, there would no firing and no
α-motor neuron activation

38. LOWER MOTOR NEURONS
TYPES OF LMNS - GAMMA MOTOR NEURONS
‣ The γ-motor neuron is essential in resetting the sensitivity of the muscle spindle by
contracting both ends of the fibre
‣ This makes the fibre taut and the muscle spindle sensitive to stretch once again
‣ In voluntary movement, both the γ-motor neurons and α-motor neurons are
activated simultaneously by UMNs
‣ This maintains the muscles spindles sensitivity to stretch upon movement
‣ Sensitivity to the stretching of muscle spindles allows for information on muscle
length and velocity to be relayed to the cerebellum via the various ascending
spinocerebellar tracts
‣ It is for this reason that γ-motor neurons play an important role in nonconscious
proprioception

39. LOWER MOTOR NEURONS
TYPES OF LMNS - GAMMA MOTOR NEURONS
‣ Finally, the firing of γ-motor neurons is directly
proportional to the tone of a muscle
‣ This is observed in pathological states
‣ Tone is increased through increasing γ-motor neuron firing
as muscle spindles become hypersensitive to stretch
‣ This hypersensitivity in turn causes greater activation and
recruitment of α-motor neurons via the reflex arc, creating
a stiff muscle on passive movement

40. LOWER MOTOR NEURONS
LMN SIGNS
‣ Both α and γ motor neurons have important roles in regulating
voluntary movement, reflexes and tone
‣ When either are damaged, the clinical signs present reflect
impairments in these areas
‣ A LMN syndrome is the term used by clinicians to describe the
collection of signs and symptoms present when a patient damages
α-motor neurons
‣ This damage can occur anywhere between the origins of the LMN in
the ventral horn or brainstem nuclei and its termination on a muscle

41. LOWER MOTOR NEURONS
LMN SIGNS
‣ The signs and symptoms of a LMN syndrome include:
‣ Hyporeflexia/areflexia
‣ Since the efferent portion of the reflex arc is damaged, eliciting the myotatic stretch reflex
will produce decreased or absent reflexes depending on the extent of that damage
‣ Hypotonia/atonia
‣ Tone is a product of the contraction of the extrafusal fibres in response to the stretch of a
muscle
‣ Therefore, loss of α-motor neurons leads to reduced or absent muscle tone
‣ Flaccid muscle weakness or paralysis
‣ Depending on the extent of the lesion, α-motor neuron damage means muscles will
receive a weakened signal to contract or no signal at all when attempting to elicit
voluntary movement

42. LOWER MOTOR NEURONS
LMN SIGNS
‣ The signs and symptoms of a LMN syndrome include:
‣ Fasciculations
‣ When α-motor neurons are damaged, they can fire spontaneous action
potentials, causing contractions in the fibres of the motor unit
‣ This can be seen as small involuntary muscle twitches, often compared to
having ‘a bag of worms under the skin’
‣ Muscle atrophy
‣ The loss of neurotrophic factors from the α-motor neuron nerve terminal,
which typically support the muscle, causes atrophy
‣ This is different to the disuse atrophy which may been observed in an UMN
syndrome

43. LOWER MOTOR NEURONS
LMN SIGNS
‣ It is important to differentiate between weakness of a
muscle as a result of damage to the nerve, disease of the
NMJ or disease of the muscle itself
‣ In primary muscle disease, there is no sensory loss, which
would be present if the nerve was damaged
‣ In addition, weakness is symmetrical and reflexes are often
lost later than in nerve damage

44. https://teachmephysiology.com/nervous-system/motor-system/lower-motor-neurones/

45. https://teachmephysiology.com/nervous-system/motor-system/lower-motor-neurones/

46. LOWER MOTOR NEURONS
REVIEW QUESTIONS
‣ Which of the following is not a symptom of LMN damage?
‣ Hypertonia / Hyperreflexia
‣ Fasciculations
‣ Muscle Atrophy
‣ Flaccid paralysis

47. LOWER MOTOR NEURONS
REVIEW QUESTIONS
‣ Which of the following is not a symptom of LMN damage?
‣ Hypertonia / Hyperreflexia
‣ Fasciculations
‣ Muscle Atrophy
‣ Flaccid paralysis

49. LOWER MOTOR NEURONS
REVIEW QUESTIONS
‣ Where does the cell body of the LMN exist?
‣ Peripheral NS
‣ ANS
‣ CNS
‣ ENS

50. LOWER MOTOR NEURONS
REVIEW QUESTIONS
‣ Where does the cell body of the LMN exist?
‣ Peripheral NS
‣ ANS
‣ CNS
‣ ENS

52. LOWER MOTOR NEURONS
REVIEW QUESTIONS
‣ Which of the following is NOT true regarding Spinal Muscular
Atrophy (SMA)?
‣ autosomal dominant inherited disorder
‣ most common genetically determined cause of neonatal
death
‣ presentation of SMA consists of the signs and symptoms
seen in a LMN syndrome
‣ there is often an accompanying bulbar palsy

53. LOWER MOTOR NEURONS
REVIEW QUESTIONS
‣ Which of the following is NOT true regarding Spinal Muscular
Atrophy (SMA)?
‣ autosomal dominant inherited disorder
‣ most common genetically determined cause of neonatal
death
‣ presentation of SMA consists of the signs and symptoms
seen in a LMN syndrome
‣ there is often an accompanying bulbar palsy

54. MUSCLE STRETCH
REFLEX

55. MUSCLE STRETCH REFLEX
OVERVIEW
‣ A reflex is defined as an involuntary, unlearned, repeatable,
automatic reaction to a specific stimulus which does not require
input from the brain
‣ The muscle stretch reflex is the most basic reflex pathway in the
body and as such, understanding this allows understanding of
more complex reflexes
‣ Learning Goal
‣ To discuss the components of a reflex arc, the
monosynaptic reflex and relevant clinical issues

56. MUSCLE STRETCH REFLEX
REFLEX ARC COMPONENTS
‣ A reflex arc is a neural pathway that controls a reflex
‣ Most sensory neurones have a synapse within the spinal cord, allowing for reflexes to
take place without the involvement of the central nervous system (CNS)
‣ speeding up the process
‣ The pathway can be described as a ‘reflex arc’ which is made up of 5 components:
‣ A receptor – muscle spindle
‣ An afferent fibre – muscle spindle afferent
‣ An integration centre – lamina IX of spinal cord
‣ An efferent fibre – α-motoneurones
‣ An effector – muscle

57. https://teachmephysiology.com/nervous-system/motor-system/muscle-stretch-reflex/

58. MUSCLE STRETCH REFLEX
THE MONOSYNAPTIC STRETCH REFLEX
‣ A monosynaptic reflex, such as the knee jerk reflex, is a simple reflex
involving only one synapse between the sensory and motor neurone
‣ The pathway starts when the muscle spindle is stretched (caused by the
tap stimulus in the knee jerk reflex)
‣ The muscle spindles are responsible for detecting the length of the
muscles fibres
‣ When a stretch is detected it causes action potentials to be fired by Ia
afferent fibres
‣ These then synapse within the spinal cord with α-motoneurones which
innervate extrafusal fibres

59. MUSCLE STRETCH REFLEX
THE MONOSYNAPTIC STRETCH REFLEX
‣ The antagonistic muscle is inhibited and the agonist muscle
contracts i.e. in the knee jerk reflex the quadriceps contract
and the hamstrings relax
‣ The sensitivity of the reflex is regulated by gamma
motoneurones – these lead to tightening or relaxing of
muscle fibres within the muscle spindle
‣ It is thought that this takes place to allow preservation of
the stretch reflex when muscles are contracted, although
not much is known about it

60. https://teachmephysiology.com/nervous-system/motor-system/muscle-stretch-reflex/

61. https://teachmephysiology.com/nervous-system/motor-system/muscle-stretch-reflex/

62. https://teachmephysiology.com/nervous-system/motor-system/muscle-stretch-reflex/

63. MUSCLE STRETCH REFLEX
REVIEW QUESTIONS
‣ What are the 5 components of the reflex arc in order?

64. MUSCLE STRETCH REFLEX
REVIEW QUESTIONS
‣ ‘reflex arc’ which is made up of 5 components:
‣ A receptor – muscle spindle
‣ An afferent fibre – muscle spindle afferent
‣ An integration centre – lamina IX of spinal cord
‣ An efferent fibre – α-motoneurones
‣ An effector muscle

65. MUSCLE STRETCH REFLEX
REVIEW QUESTIONS
‣ Which of the following reflexes correlates with the C5/C6
spinal level?
‣ Triceps reflex
‣ Patellar reflex
‣ Biceps reflex
‣ Achilles reflex

66. MUSCLE STRETCH REFLEX
REVIEW QUESTIONS
‣ Which of the following reflexes correlates with the C5/C6
spinal level?
‣ Triceps reflex
‣ Patellar reflex
‣ Biceps reflex
‣ Achilles reflex

67. References
These slide reflect a summary of the contents of
TeachMePhysiology.com and are to be used for educational
purposes only in compliance with the terms of use policy.
Specific portions referenced in this summary are as follows:
‣ https://teachmephysiology.com/nervous-system/motor-system/upper-motor-
neurone/
‣ https://teachmephysiology.com/nervous-system/motor-system/lower-motor-
neurones/
‣ https://teachmephysiology.com/nervous-system/motor-system/muscle-stretch-
reflex/
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