Dental lecture: brain stem, ascending and descending pathways
1. Neurophysiology lecture topics
1. Role of brainstem and reticular formation
2. Ascending and descending tracts
muscle power
3. Maintenance of posture, equilibrium,
coordination
muscle tone
4. Functions of limbic system and basal ganglia
3. Role of brainstem
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Intermediate centre in controlling motor functions
Ascending and descending pathways cross brain stem
Contains vital centres
Contains reticular formation
Plays a vital role in attention, arousal and states of
consciousness
Brainstem injuries easily cause loss of consciousness
Most of the cranial nerves are connected to brainstem
Contain pain pathways
Involved in suprasegmental control of reflexes and muscle tone
Extrapyramodal tracts strats from the brain stme
4. Reticular formation
• Located in the core of the
brainstem
• Network of neurons
• Main centre of ascending
and descending tracts
• Functions:
consciousness, motor
control, pain modulation,
cardiovascular control,
sleep centres
6. Sensory area
in the brain
Ascending
Sensory pathway
Central Connections
Sensory nerve
Touch stimulus
Receptor
Sensory
modality
7. Two main ascending pathways
• Dorsal column - medial lemniscus
pathway
fast pathway
• Spinothalamic pathway
slow pathway
These two pathways come together at the level of thalamus
18. Corticospinal tract (Pyramidal tract)
• Starts from large cortical cells (pyramidal
cells) in the primary motor cortex
• These cells are called Betz cells
• From these cells starts the motor axon
• Divided into
– Lateral corticospinal tract
• Major part of the CST, cross to the opposite side
at the level of medulla
– Medial corticospinal tract (or anterior CST)
• Minor part, uncrossed tract, at the level of spinal
cord cross to the opposite side
19. Course of the corticospinal tract
• Descends through
– internal capsule
– at the medulla
• cross over to the other side
– descends down as the corticospinal tract
– ends in each anterior horn cell
– synapse at the anterior horn cell
21. Motor system
• Consists of
– Upper motor neuron
• Corticospinal tract (pyramidal tract)
• Extrapyramidal tracts
– Lower motor neuron
• Alpha motor neuron
• Gamma motor neuron
22. Lower motor neuron
• consists of mainly
• alpha motor neuron
– and also gamma motor neuron
gamma motor neuron
alpha motor neuron
24. alpha motor neuron
• this is also called the final common pathway
• Contraction of the muscle occurs through this
whether
– voluntary contraction through corticospinal tract
or
– involuntary contraction through gamma motor
neuron - stretch reflex - Ia afferent
25. Upper motor neuron
• Consists of
– Corticospinal tract (pyramidal tract)
– Extrapyramidal tracts
26. Extrapyramidal tracts
• starts at the brain stem
• descends down either ipsilaterally or
contralaterally
• ends at the anterior horn cell
• modifies the motor functions
27. Reticulospinal tract
• Starts from the reticular formation
• Maintain normal postural tone
• Controls mainly gamma motorneurons (lesser extent alpha
motor neurons)
• Inhibit antigravity muscles (extensor)
• End on interneurons
• Inhibited by cerebral influence
• Mainly ipsilateral
28. Reticular formation
• Loosely arranged cell bodies
in the central core of the brain
stem
midbrain
pons
• Pontine reticular area
medulla
• Medullary reticular area
spinal cord
29. Vestibulospinal tract
• Starts from the vestibular nuclei (present in the medullar region)
• Excitatory to alpha motor neurons of antigravity muscles
(extensor)
• End on interneurons
• Regulates posture and balance
• Mainly ipsilateral
• There are inputs from vestibular organs and cerebellum to
vestibular nuclei
30. • Rubrospinal and tectospinal tracts are not
functionally important in human nervous system
32. Suprasegmental control of reflexes and
muscle tone
• Alpha motor neuron is the final pathway
• Gamma motor neuron control
• Alpha-gamma coactivation
• Supraspinal control
– Pyramidal tract: activation of alpha
– Extrapyramidal: mixed effects on alpha and gamma
motor neurons
• Net effect: suppression of gamma motor neuron
33. Extrapyramidal
tracts
•Voluntary movement
•Muscle tone
Gamma
motor
neuron
Corticospinal
tract
Alpha motor
neuron
Muscle spindle
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There is a complex effect of corticospinal and extrapyramidal tracts on the alpha and
gamma motor neurons (in addition to the effect by muscle spindle)
There are both excitatory and inhibitory effects
Sum effect
– excitatory on alpha motor neuron
– Inhibitory on gamma motor neuron
34. Clinical Importance of the motor system
examination
• Tests of motor function:
– Muscle power
• Ability to contract a group of muscles in order to make an
active movement
– Muscle tone
• Resistance against passive movement
35. Basis of tests
• Muscle power
– Test the integrity of motor cortex, corticospinal tract
and lower motor neuron
• Muscle tone
– Test the integrity of stretch reflex, gamma motor
neuron and the descending control of the stretch
reflex
36. Muscle tone
• Resistance against passive movement
– Gamma motor neuron activate the spindles
– Stretching the muscle will activate the stretch reflex
– Muscle will contract involuntarily
– Gamma activity is under higher centre inhibition
37. Clinical situations
• Muscle power
– Normal
– Reduced (muscle weakness)
• muscle paralysis
• muscle paresis
• Muscle tone
– Normal
– Reduced
• Hypotonia (Flaccidity)
– Increased
• Hypertonia (Spasticity)
38. Main abnormalities
• Muscle Weakness / paralysis
– Reduced muscle power
• Flaccidity
– Reduced muscle tone
• Spasticity
– Increased muscle tone
43. Upper motor neuron lesion
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muscle weakness
spastic paralysis
increased muscle tone (hypertonia)
reflexes: exaggerated
Babinski sign: positive
superficial abdominal reflexes: absent
muscle wasting is very rare
clonus can be seen:
– rhythmical series of contractions in response to sudden
stretch
• clasp knife effect can be seen
– passive stretch causing initial increased resistance which is
released later
46. Babinski sign
• when outer border of the sole of the foot is
scratched
• upward movement of big toe
• fanning out of other toes
• feature of upper motor neuron lesion
• extensor plantar reflex
• seen in infants during 1st year of life (because
of immature corticospinal tract)
49. Site of lesions
quadriplegia (tetraplegia)
all 4 limbs are affected
cervical cord or brain stem lesion
hemiplegia
one half of the body including
UL and LL
lesion in the Internal capsule
paraplegia
both lower limbs
thoracic cord lesion
monoplegia
only 1 limb is affected either UL or LL,
lower motor neuron lesion
50. Conditions which cause increased
muscle tone
• Spasticity
– Stroke
• Rigidity
– Parkinsonism
• Lead pipe rigidity
• Cogwheel rigidity
• Brainstem lesions
– Decerebrate rigidity
– Decorticate rigidity
51. Reticular formation
• A set of network of interconnected
neurons located in the central
core of the brainstem
• It is made up of ascend-ing and
descend-ing fibers
• It plays a big role in fil-ter-ing
incom-ing stim-uli to
dis-crim-i-nate irrel-e-vant
back-ground stim-uli
• There are a large number of
neurons with great degree of
convergence and divergence
52. Functions
• Maintain consciousness, sleep and arousal
• Reticulospinal pathways are part of the
extrapyramidal tracts
• Several nuclei (PAG, NRM) are part of the
descending pain modulatory (inhibitory)
pathway
53. Basal ganglia
• These are a set of deep nuclei
located in and around the basal
part of the brain that are involved
in motor control, action selection,
and some forms of learning
• Purposeful movement
54. Basal ganglia
• Caudate nucleus
• Putamen
• Globus pallidus
–(internal and external)
• Subthalamic nuclei
• Substantia nigra
International Basal Ganglia Society
58. • Interconnecting circuitry through these nuclei
• These circuits start from the cortex and ends in
the cortex
• These circuits are very complex
• Their effect is excitatory or inhibitory on motor
functions
• They also have a role in cognitive functions.
60. Functions
• eg.
– writing letters of alphabet,
– cutting papers with scissors,
– hammering nails,
– passing a football,
– Vocalisation
– Cognitive control of movement
61. • Some of these circuits are excitatory and
some inhibitory
• This depends on the neurotransmitter
involved.
• Inhibitory: dopamine and GABA
• Excitatory: Ach
• Others: glutamate (from cortical projections)
enkephalin etc
62. Following pathways are known:
• Dopamine pathway from substantia nigra to
caudate nucleus and putamen
• GABA pathway from caudate and putamen to
globus pallidus and substantia nigra
• Ach pathway in the caudate and putamen
64. Functions of Basal Ganglia
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Motor control
Learning
Sensorimotor integration
Reward
Cognition
65. Basal Ganglia disorders
• Basal ganglia disorders are also called
extrapyramidal disorders
• Classical disorder is ―Parkinsonism‖
• Other disorders: Athetosis, Chorea,
Hemiballismus
66. Parkinsonism
• due to destruction of dopamine secreting pathways from
substantia nigra to caudate and putamen.
– also called ―paralysis agitans‖ or ―shaking palsy‖
– first described by Dr. James Parkinson in 1817.
• In the west, it affects 1% of individuals after 60 yrs
Classical Clinical features:
• Tremor, resting
• Rigidity of all the muscles
• Akinesia (bradykinesia): very slow movements
• Postural instability
67. – expressionless face
– flexed posture
– soft, rapid, indistinct speech
– slow to start walking
– rapid, small steps, tendency to run
– reduced arm swinging
– impaired balance on turning
– resting tremor (3-5 Hz) (pill-rolling tremor)
• diminishes on action
– cogwheel rigidity
– lead pipe rigidity
– impaired fine movements
– impaired repetitive movements
71. Dynamic vs static nature of motor
control
• Static stability
– is dependent on the position of the centre of gravity
with respect to the base of support
• whereas dynamic stability
– is dependent more on the moment of inertia of the
body
72. Adult vs child
• In normal standing, a tall adult will have
a much larger moment of inertia than a
toddler
• Once the centre of gravity moves
outside the base of support the body
will begin to fall
– The adult with the large moment of inertia
will fall much more slowly and will therefore
have a longer time to react to prevent the
fall
– This is one of the reasons that young
children fall more often than adults.
74. maintenance of posture
• mainly to maintain the static
posture
• necessary for the stability of
movements
• involve a set of reflexes
• integrated at spinal cord, brain
stem and cortical level
75. normal postural control
• three inputs are required
– Vision
– Proprioception (joint position sense)
– Vestibular Mechanism (balance mechanisms)
– Cutaneous sensations
78. • these reflexes are under higher centre inhibition
• transection of spinal cord or brain stem at
different levels release this inhibition
• then the relevant reflexes are seen
80. cerebellum
• centre of motor coordination
• cerebellar disorders cause
–incoordination or ataxia
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85. structure
• Cerebellum is divided into 3 lobes by 2
transverse fissures
– anterior lobe
– posterior lobe
– flocculonodular lobe
86.
87. structure
– anterior lobe (paleocerebellum)
– large posterior lobe (neocerebellum)
– flocculonodular lobe (archicerebellum is the
oldest lobe)
88. • Anterior cerebellum and part of posterior
cerebellum
– receives information from the spinal cord
• Rest of the posterior cerebellum
– receives information from the cortex
• Flocculonodular lobe
– involved in controlling the balance through
vestibular apparatus
89. • Functionally cerebellum is divided into 3
areas medial to lateral
– lateral zone
– intermediate zone
– vermis
92. Neuronal circuitry of the cerebellum
• Main cortical cells in cerebellum are known
as Purkinje Cells (large cells).
• There are about 30 million such cells.
• These cells constitute a unit which repeats
along the cerebellar cortex.
93.
94.
95.
96. Functions of cerebellum
• planning of movements
• timing & sequencing of movements
• particularly during rapid movments such as
during walking, running
• from the peripheral feedback & motor cortical
impulses, cerebellum calculates when does a
movement should begin and stop
98. ‘Error correction’
• cerebellum receives two types of information
– intended plan of movement
• direct information from the motor cortex
– what actual movements result
• feedback from periphery
– these two are compared: an error is calculated
– corrective output signals goes to
• motor cortex via thalamus
• brain stem nuclei and then down to the anterior horn cell
through extrapyramidal tracts
99. • ‘Prevention of overshoot’
– Soon after a movement has been initiated
– cerebellum send signals to stop the movement at
the intended point (otherwise overshooting occurs)
• Ballistic movements
– rapid movements of the body, eg. finger movements
during typing, rapid eye movements (saccadic eye
movements)
– movements are so rapid it is difficult to decide on
feedback
– therefore the movement is preplanned
• Cerebellum perform motor learning (memory)
100. planning of movements
• mainly performed by lateral zones
• sequencing & timing
– lateral zones communicate with premotor areas,
sensory cortex & basal ganglia to receive the plan
– next sequential movement is planned
– predicting the timings of each movement
101. features of cerebellar disorders
• ataxia
– incoordination of movements
– ataxic gait
• broad based gait
• leaning towards side of the lesion
• dysmetria
– cannot plan movements
• past pointing & overshoot
• decomposition of movements
• intentional tremor
102. features of cerebellar disorders
• Dysdiadochokinesis (adiadochokinesis)
– unable to perform rapidly alternating movements
• dysarthria
– slurring of speech
• nystagmus
– oscillatory movements of the eye
103. features of cerebellar disorders
• hypotonia
– reduction in tone
• due to reduction in excitatory influence on gamma motor
neurons by cerebellum (through vestibulospinal tracts)
• decreased reflexes
• head tremor
• head tilt
• In unilateral cerebellar lesions, incoordination
occurs in the ipsilateral side
104. • But what finally drives us to action???
•perhaps motivation
•motivation is controlled by limbic
system and hypothalamus
108. limbic cortex
• consist of 3 layered cortex (in contrast to 6
layered cortex of the neocortex)
109. • Limbic system is a link between the
brain stem and neocortex
• Limbic structures are connected to
each other and with the association
cortex and the brain stem
110. • Medial forebrain bundle is a major efferent
connection of the limbic system:
• projected to the hypothalamus, reticular formation. Influence on
autonomic and endocrine activity
• Amygdala receives inputs from olfactory pathways
• Connections with the neocortex provide a
synthesis of emotional and rational thought
111. Functions
Limbic system is also referred to as the
‘emotional brain’
• Emotional (include motor activity)
• Behavioural (Motivations, Drives: appetite,
thirst, sexual behaviour, Reward system)
• Memory
– Utilizes the hypothalamus to effect the physical manifestations
associated with emotions, etc.
112. Complex role of the limbic system
• as an intermediary between
– external events (carried to the CNS via afferents)
– our processing of those events (involving cortical
and subcortical brain areas)
– our responses to those events (both behavioral and
autonomic)
113. Role in memory storage
• Working memory—short term
– cortical phenomenon
• Explicit (declarative)—factual knowledge
– temporal events, stored in hippocampus
• Examples: what innervates biceps femoris m.?
• Implicit (procedural)—learned skills
– unconsciously recalled—includes emotional
responses—stored in amygdala (at least in part)
• Examples: writing, playing a musical instrument
114. Hippocampus
• is a part of the brain located
inside the temporal lobe
• plays a major role memory
consolidation
• responsible for spatial memory
• might act as a cognitive map —
a neural representation of the
layout of the environment.
• In Alzheimer's disease, the
hippocampus becomes one of
the first regions of the brain to
suffer damage