Module 23 - Neuroanatomy of Motors Systems

Question 1

The starting point when considering the neuroanatomy of the motor pathways is to identify the key components, consisting of a two-neuron chain. First there are upper motor neurons (UMNs) found in the primary motor cortex located along the ________________.

Axons from the UMNs descend, forming cerebral white matter, and exit the cerebrum, travelling into the brainstem, ultimately forming the _______________. Before entering the spinal cord most (???%) of these fibres cross from one side to the other at the _________________.

The __________________ is the defining dividing line between the medulla and the spinal cord.

Answer 1

The starting point when considering the neuroanatomy of the motor pathways is to identify the key components, consisting of a two-neuron chain. First there are upper motor neurons (UMNs) found in the primary motor cortex located along the precentral gyrus (of the frontal lobe).

Axons from the UMNs descend, forming cerebral white matter, and exit the cerebrum, travelling into the brainstem, ultimately forming the pyramids of the medulla. Before entering the spinal cord most (~85%) of these fibres cross from one side to the other at the p_yramidal decussation_.

The pyramidal decussation is the defining dividing line between the medulla and the spinal cord.

Question 2

Below the decussation, the fibres descend (still white matter) to the appropriate level of the spinal cord, enter ____________ and synapse onto ________ of their target lower motor neurons (LMNs) in the anterior (ventral) horn.

Axons of LMNs (GSE) form __________________ and join spinal nerves to innervate skeletal muscle of body.

Answer 2

Below the decussation, the fibres descend (still white matter) to the appropriate level of the spinal cord, enter gray matter and synapse onto dendrites of their target lower motor neurons (LMNs) in the anterior (ventral) horn.

Axons of LMNs (GSE) form anterior (ventral) roots and join spinal nerves to innervate skeletal muscle of body.

Question 3

Axons of LMNs (GSE) form _______________ and join spinal nerves to innervate skeletal muscle of body.

These projections from UMNs to LMNs is called the ______________. The corresponding white matter tract is called the ______________.

Answer 3

Axons of LMNs (GSE) form anterior (ventral) roots and join spinal nerves to innervate skeletal muscle of body.

These projections from UMNs to LMNs is called the corticospinal pathway. The corresponding white matter tract is called the corticospinal tract.

Question 4

The axons of UMN exit the cerebrum, entering the brainstem, via a white matter region called the _______________– specifically the ______________________________.

Answer 4

The axons of UMN exit the cerebrum, entering the brainstem, via a white matter region called the internal capsule – specifically the posterior limb of the internal capsule.

Question 5

Identify the image.

Answer 5

Check your answers.

Question 6

This slide shows a horizontal section through the inferior portion of the cerebral hemispheres to illustrate the internal capsule. The internal capsule is divided into three zones:__________, _______________, ___________. The posterior limb contains (among other things) the descending axons of corticospinal UMNs.

Answer 6

This slide shows a horizontal section through the inferior portion of the cerebral hemispheres to illustrate the internal capsule. The internal capsule is divided into three zones: the anterior limb, the genu (“knee”) and posterior limb. The posterior limb contains (among other things) the descending axons of corticospinal UMNs.

Question 7

The internal capsule is surrounded by regions of _______________.

This ___________ corresponds to components of the basal ganglia, the head of the __________________ and the ______________, that consists of the _____________________and the _________________.

Realize how much the axons of the UMNs are packed into a relatively small space. This is relevant in the case of damage (we will discuss further as part of Module 28).

Answer 7

The internal capsule is surrounded by regions of gray matter (GM, labelled on the left).

This gray matter corresponds (labelled on the right side) to components of the basal ganglia (the head of the caudate nucleus (CN) and the lentiform nucleus (LN), that consists of the putamen and globus pallidus) and the thalamus (Thal).

Realize how much the axons of the UMNs are packed into a relatively small space. This is relevant in the case of damage (we will discuss further as part of Module 28).

Question 8

Passing through the posterior limb of the internal capsule, corticospinal fibres enter and travel through the upper part of the brainstem (midbrain) via the anterior portion of the ________(another white matter region, sometimes referred to as the _______________). The most anterior of these fibres can be seen on the surface of the _________. These fibres then pass into the pons, deep to the surface, in the______ (anterior) part of the pons called the _______________. After passing through the pons the fibres then become visible on the anterior surface again, forming the _________________________.

***NOTICE AUTERIOR

Answer 8

Passing through the posterior limb of the internal capsule, corticospinal fibres enter and travel through the upper part of the brainstem (midbrain) via the anterior portion of the crus cerebri (another white matter region, sometime referred to as the cerebral peduncles). The most anterior of these fibres can be seen on the surface of the crus cerebri. These fibres then pass into the pons, deep to the surface, in the basilar (anterior) part of the pons called the basis pontis. After passing through the pons the fibres then become visible on the anterior surface again, forming the pyramids of the medulla.

Question 9

Here is another way to look at the corticospinal fibres at different levels of the brainstem. The drawing shows cross-sectional views at 6 different levels of the brainstem. The position of the corticospinal fibres are shown in red. Note that at each level the fibres are positioned relatively _________ sometimes visible on the ________ surface (midbrain, medulla) sometimes __________ (pons).

Answer 9

Here is another way to look at the corticospinal fibres at different levels of the brainstem. The drawing shows cross-sectional views at 6 different levels of the brainstem. The position of the corticospinal fibres are shown in red. Note that at each level the fibres are positioned relatively anterior, sometimes visible on the anterior surface (midbrain, medulla) sometimes deep (pons).

Question 10

The crossed fibres form the lateral corticospinal tract – located within the lateral region of spinal cord white matter called the _________________.

Answer 10

The crossed fibres form the lateral corticospinal tract – located within the lateral region of spinal cord white matter called the lateral funiculus (LF).

Question 11

What about the ~15% of corticospinal fibres that do not cross? These remain on the same side (left pyramid to the left spinal cord, right pyramid to the right spinal cord) and descend in the _________________________ (within the ________ or ___________ ____________ of spinal cord white matter).

Answer 11

What about the ~15% of corticospinal fibres that do not cross? These remain on the same side (left pyramid to left spinal cord, right pyramid to right spinal cord) and descend in the anterior corticospinal tract (within the anterior or ventral funiculus (VF) of spinal cord white matter).

Question 12

The fibres in the lateral corticospinal tract eventually exit and innervate (make synapses with) ipsilateral LMNs located in the __________ portion of the _______ horn. LMNs in the lateral portion of the anterior horn innervate limb muscles.

Answer 12

The fibres in the lateral corticospinal tract eventually exit and innervate (make synapses with) ipsilateral (same side) LMNs located in the lateral portion of the anterior (ventral) horn.

Question 13

LMNs in the lateral portion of the anterior horn innervate _________________.

Answer 13

LMNs in the lateral portion of the anterior horn innervate limb muscles.

Question 14

The fibres in the anterior corticospinal tract exit and innervate (some _______, but mostly ________) LMNs located in the ________ portion of the anterior (ventral) horn.

Answer 14

The fibres in the anterior corticospinal tract exit and innervate (some ipsilateral, but mostly contralateral) LMNs located in the medial portion of the anterior (ventral) horn.

Question 15

Therefore, even though the fibres that descended in the ________________ did not cross at the pyramidal decussation, most of the fibres eventually cross, but only once they ___________________________________.

Answer 15

Therefore, even though the fibres that descended in the anterior corticospinal tract did not cross at the pyramidal decussation, most of the fibres eventually cross, but only once they reach their target level of spinal cord.

Question 16

LMNs in the medial portion of the anterior horn innervate ________________.

Answer 16

LMNs in the medial portion of the anterior horn innervate trunk muscles.

Question 17

The __________________ (receive mostly contralateral innervation from fibres in the anterior corticospinal tract) innervate trunk muscles.

Answer 17

The medial motor nuclei (receive mostly contralateral innervation from fibres in the anterior corticospinal tract) innervate trunk muscles.

Question 18

The crossed projections from the anterior corticospinal tract travel from one side of the spinal cord to the other (“decussate”) via the ________________.

Answer 18

The crossed projections from the anterior corticospinal tract travel from one side of the spinal cord to the other (“decussate”) via the anterior white commissure.

Question 19

The motor pathways show somatotopic organization: a point-for-point correspondence of an area of the body to a specific region of CNS.
The primary motor cortex (pre-central gyrus) shows somatotopic organization: the UMNs responsible for motor function of specific body regions are organized systematically along the length of the precentral gyrus.
•UMN for lower limb are along the _______ and _______ surface
•UMN for upper limb are next, lower down (inferior) on the upper part of the ______________
•UMN for neck and head are on the most _________________

Answer 19

The motor pathways show somatotopic organization: a point-for-point correspondence of an area of the body to a specific region of CNS.
The primary motor cortex (pre-central gyrus) shows somatotopic organization: the UMNs responsible for motor function of specific body regions are organized systematically along the length of the precentral gyrus.
•UMN for lower limb are along the medial and superior surface
•UMN for upper limb are next, lower down (inferior) on the upper part of the precentral gyrus
•UMN for neck and head are on the most inferolateral surface

Question 20

There is also somatotopic organization at each level where corticospinal fibers are found:

Posterior limb of the internal capsule
Direction?

Organization?

Answer 20

There is also somatotopic organization at each level where corticospinal fibres are found:

Posterior limb of the internal capsule
Posterior → Anterior

Leg-Trunk-Arm-Face

Question 21

There is also somatotopic organization at each level where corticospinal fibres are found:

Central portion of crus cerebri

Direction?

Organization?

Answer 21

There is also somatotopic organization at each level where corticospinal fibres are found:

Central portion of crus cerebri
Lateral → Medial

Leg-Trunk-Arm-Face

Question 22

There is also somatotopic organization at each level where corticospinal fibres are found:

Lateral corticospinal tract

Direction?

Organization?

Anterior corticospinal tract

Direction?

Organization?

Answer 22

There is also somatotopic organization at each level where corticospinal fibres are found:

Lateral corticospinal tract
Lateral → Medial

Leg-Trunk-Arm

Anterior corticospinal tract

Trunk

Question 23

How do UMNs reach the LMNs that innervate the muscles of the head and neck?

Answer 23

The answer to the first question is that there are projections from UMNs (primary motor cortex, precentral gyrus) associated with motor control of the head and neck that form the corticobulbar (sometimes called corticonuclear) tract. Initially (within the cerebral white matter) the corticobulbar tract travels with the corticospinal tract.

Question 24

At lower levels corticobulbar fibres are still nearby to corticospinal fibres, but have distinct locations. In the internal capsule, corticobulbar fibres are found within the ________ of the internal capsule. In the crus cerebri they are ________ to the corticospinal fibres. Only a few corticobulbar fibres pass into the spinal cord, and these are associated with the ______________________.

Answer 24

At lower levels corticobulbar fibres are still nearby to corticospinal fibres, but have distinct locations. In the internal capsule, corticobulbar fibres are found within the genu of the internal capsule. In the crus cerebri they are medial to the corticospinal fibres. Only a few corticobulbar fibres pass into the spinal cord, and these are associated with the lateral corticospinal tract.

Question 25

The corticobulbar fibres innervate LMNs associated with motor nuclei of certain cranial nerves: - the\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (SVE, located at the _______ level) that contributes fibres to CN V that innervate muscles of mastication - the ________________ (SVE, located within \_\_\_\_\_\_\_\_\_) that contributes fibres to CN VII that innervate muscles of facial expression - the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_(SVE, ________ \_\_\_\_\_\_\_\_\_) that contributes fibres to CN IX and X to innervate muscles of the pharynx and larynx - the ___________ (SVE, ________ \_\_\_\_\_\_ of cervical spinal cord) the sole source of fibres for CN XI that innervates sternocleidomastoid and trapezius muscles. The corticobulbar fibres that innervate these LMNs come via the lateral corticospinal tract. - the __________________ (GSE, _______ \_\_\_\_\_\_\_\_\_) that is the sole source of fibres for CN XII that innervates muscles of the tongue.

Answer 25

The corticobulbar fibres innervate LMNs associated with motor nuclei of certain cranial nerves: - the **trigeminal motor nucleus** (SVE, located at the **midpons** level) that contributes fibres to CN V that innervate muscles of mastication - the **facial motor nucleus** (SVE, located within **lower pons**) that contributes fibres to CN VII that innervate muscles of facial expression - the **Nucleus ambiguus** (SVE, **upper medulla**) that contributes fibres to CN IX and X to innervate muscles of the pharynx and larynx - the spinal accessory nucleus (SVE, anterior horn of cervical spinal cord) the sole source of fibres for CN XI that innervates sternocleidomastoid and trapezius muscles. The corticobulbar fibres that innervate these LMNs come via the lateral corticospinal tract. - the hypoglossal nucleus (GSE, upper medulla) that is the sole source of fibres for CN XII that innervates muscles of the tongue.

Question 26

The innervation of these cranial nerve motor nuclei (LMNs) by corticobulbar fibres is \_\_\_\_\_\_\_\_\_\_\_\_\_\_. An exception: ?????????

Answer 26

The innervation of these cranial nerve motor nuclei (LMNs) by corticobulbar fibres is bilateral (crossed and uncrossed). An exception: innervation of neurons in CN VII (facial) motor nuclei that innervate muscles of facial expression for the lower face are typically unilateral and crossed.

Question 27

Thus far I have emphasized the importance of UMNs located in the primary motor area (in the _________ \_\_\_\_\_\_\_) and the corticospinal & corticobulbar pathways. There are four other sets of nuclei and associated tracts that serve other spinal motor functions. WHAT ARE THEIR NAMES? PYRAMIDAL OR EXTRAPYRAMIDAL SYSTEM?

Answer 27

Thus far I have emphasized the importance of UMNs located in the primary motor area (in the **precentral gyrus**) and the corticospinal & corticobulbar pathways. There are four other sets of nuclei and associated tracts that serve other spinal motor functions. These other tracts are not found with the corticospinal tracts (PYRAMIDAL) and are, therefore, often referred to as EXTRAPYRAMIDAL. They are: RUBROSPINAL TRACT VESTIBULOSPINAL TRACT RETICULOSPINAL TRACT TECTOSPINAL TRACT

Question 28

The pyramidal system subserves ___________ movements, whereas most (3 of 4) of the extrapyramidal systems are \_\_\_\_\_\_\_\_, concerned with maintaining \_\_\_\_\_\_\_, ___________ and some ___________ of \_\_\_\_\_\_\_\_\_\_\_\_.

Answer 28

The pyramidal system subserves **volitional** movements, whereas most (3 of 4) of the extrapyramidal systems are **automatic**, concerned with maintaining **balance**, **posture and some coordination of movement.**

Question 29

The origin of the rubrospinal tract are neurons located in the ____________ in the \_\_\_\_\_\_\_\_\_\_\_\_\_. The rubrospinal tract travels through _______ and \_\_\_\_\_\_\_, ultimately innervates \_\_\_\_\_\_\_\_\_\_\_\_motor nuclei. Functions: ?????????????

Answer 29

The origin of the rubrospinal tract are neurons located in the **red nucleus in the midbrain.** The rubrospinal tract travels through **brainstem** and **spinal cord,** ultimately innervates **lateral** motor nuclei. Functions: **crawling in babies, arm-swinging in adults.**

Question 30

The vestibular nuclei (in the pons and medulla) receive sensory information about body position in space from the vestibular system (via CN #####????, ___________________ nerve).

Answer 30

The vestibular nuclei (in the pons and medulla) receive sensory information about body position in space from the vestibular system (via **CN VIII, the vestibulocochlear nerve**).

Question 31

The vestibular nuclei give rise to two descending tracts: the ______ and ________ vestibulospinal tracts.

Answer 31

The vestibular nuclei give rise to two descending tracts: the **lateral** and **medial** vestibulospinal tracts.

Question 32

Medial VST – innervation + function? lateral VST – innervation + function?

Answer 32

medial VST – cervical cord, medial nuclei, stabilization of head position lateral VST – entire cord interneurons, balance, posture

Question 33

Neurons found in the pons and medulla give rise to the \_\_\_\_\_\_\_\_\_\_\_\_reticulospinal and ___________ reticulospinal tracts. Ultimately innervate _________ \_\_\_\_\_\_\_nuclei or __________ in the spinal cord and contribute to _______________ and ____________ movements.

Answer 33

Neurons found in the pons and medulla give rise to the pontine reticulospinal and medullary reticulospinal tracts. Ultimately innervate medial motor nuclei or interneurons in the spinal cord and contribute to automatic posture and gait-related movements.

Question 34

The origin of the tectospinal tract are neurons found in the ______________ in the upper midbrain. The \_\_\_\_\_\_\_\_\_\_\_\_\_\_receives direct visual input from the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Answer 34

The origin of the tectospinal tract are neurons found in the superior colliculus in the upper midbrain. The superior colliculus receives direct visual input from the retina.

Question 35

The axons from the superior colliculus neurons descending through the brainstem and ultimately innervate _____________ nuclei in the cervical spinal cord. Involved with ______________ movements.

Answer 35

The axons from the superior colliculus neurons descending through the brainstem and ultimately innervate medial motor nuclei in the cervical spinal cord. Involved with visually-guided head movements.

Question 36

There are three cranial nerves (and corresponding nuclei) with GSE motor function involved with eye movements: CN III oculomotor nerve (exits from ????????????) CN IV trochlear nerve (exits from ?????????????) CN VI abducens nerve (exits from \_\_\_\_\_\_\_?????\_\_\_ junction)

Answer 36

There are three cranial nerves (and corresponding nuclei) with GSE motor function involved with eye movements: CN III oculomotor nerve (exits from upper midbrain) CN IV trochlear nerve (exits from lower midbrain) CN VI abducens nerve (exits from pontomedullary junction)

Question 37

Most cranial nerve nuclei, including the nuclei of CN III and VI, give rise to \_\_\_\_\_\_\_\_\_\_\_nerves. One exception: the ________________ gives rise to projections that decussate (cross). Also different from other cranial nerves, that exit on the anterior or lateral surface of the brainstem, CN ###??? emerges from the posterior \_\_\_\_\_\_\_\_\_, and then travel __________ (\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_).

Answer 37

Most cranial nerve nuclei, including the nuclei of CN III and VI, give rise to ipsilateral nerves. One exception: the trochlear nucleus gives rise to projections that decussate (cross). That is, the left trochlear nucleus give rise to the right trochlear nerve; the right trochlear nucleus gives rise to the left trochlear nerve. Also different from other cranial nerves, that exit on the anterior or lateral surface of the brainstem, CN IV emerges from the posterior brainstem, and then travel anteriorly (wraps around outside the brainstem).

Question 38

The innervation of the eye movement cranial nerve nuclei depends on the direction of eye movement. The _____________________________ is the horizontal gaze center with neurons projecting to the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_nucleus. In addition to providing innervation of the ___________ rectus (abductor) muscle, neurons in the \_\_\_\_\_\_\_\_nucleus cross and project (via a tract called the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_) to the oculomotor (CN III) nucleus, the neurons of which innervate the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_rectus (adductor).

Answer 38

The innervation of the eye movement cranial nerve nuclei depends on the direction of eye movement. The paramedian pontine reticular formation (PPRF) is the horizontal gaze center with neurons projecting to the abducens (CN VI) nucleus. In addition to providing innervation of the lateral rectus (abductor) muscle, neurons in the abducens nucleus cross and project (via a tract called the medial longitudinal fasciculus or MLF) to the oculomotor (CN III) nucleus, the neurons of which innervate the medial rectus (adductor).

Question 39

The _____________ nucleus of the MLF (riMLF) and \_\_\_\_\_\_\_nucleus of Cajal, both in the \_\_\_\_\_\_\_\_\_\_\_\_\_\_, are vertical gaze centers. These centers project to the _____________ nucleus (shown) and the \_\_\_\_\_\_\_\_\_\_\_() nucleus (not shown) thereby controlling the activity of the LMNs that innervate eye muscles involved with vertical movements.

Answer 39

The rostral interstitial nucleus of the MLF (riMLF) and interstitial nucleus of Cajal, both in the midbrain, are vertical gaze centers. These centers project to the oculomotor nucleus (shown) and the trochlear (CN IV) nucleus (not shown) thereby controlling the activity of the LMNs that innervate eye muscles involved with vertical movements.

Question 40

The neurons of the oculomotor, trochlear and abducens nuclei, are NOT innervated by \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_. Other cortical areas, the\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (in the region of the middle frontal gyrus) and the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_area, are cortical regions associated with volitional eye movements. These regions do not project directly to the eye movement nuclei (of CN III, IV and VI) but to other structures (e.g. PPRF, vestibular nuclei, cerebellum) that are involved in eye movement control. These areas are influenced heavily by projections from cortical regions involved with vision (e.g. \_\_\_\_\_\_\_\_\_\_\_).

Answer 40

The neurons of the oculomotor, trochlear and abducens nuclei, are not innervated by “UMNs” in the primary motor cortex. Other cortical areas, the frontal eye fields (in the region of the middle frontal gyrus) and the parieto-occipito-temporal area, are cortical regions associated with volitional eye movements. These regions do not project directly to the eye movement nuclei (of CN III, IV and VI) but to other structures (e.g. PPRF, vestibular nuclei, cerebellum) that are involved in eye movement control. These areas are influenced heavily by projections from cortical regions involved with vision (e.g. visual cortex).

Question 41

In fact, even in the case of the control of corticospinal and corticobulbar pathways, the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_is NOT the actual start of volitional motor control. “Motor association” areas lie just anterior to the primary motor cortex. 1- ? 2- ? These regions are involved in ____________________ and make reciprocal connections with the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ They also make reciprocal connections with sensory and association areas.

Answer 41

In fact, even in the case of the control of corticospinal and corticobulbar pathways, the **primary motor cortex** is not the actual start of volitional motor control. “Motor association” areas lie just anterior to the primary motor cortex. * *-supplementary motor area - premotor cortex** These regions are involved in higher-order motor planning and make reciprocal connections with the primary motor cortex. They also make reciprocal connections with sensory and association areas**.**

Question 42

Motor processing involves much more than just cerebral cortex, brainstem and spinal cord. The output of the motor system is modulated extensively by the \_\_\_\_\_\_\_\_\_\_\_\_\_\_and \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.

Answer 42

Motor processing involves much more than just cerebral cortex, brainstem and spinal cord. The output of the motor system is modulated extensively by the **cerebellum and basal nuclei (ganglia).**

Question 43

The autonomic nervous system is not (at least not completely) autonomous. An important source of control of pre-ganglionic neurons is the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_with a family of descending projections to pre-ganglionic autonomics in brainstem and spinal cord.

Answer 43

The autonomic nervous system is not (at least not completely) autonomous. An important source of control of pre-ganglionic neurons is the **hypothalamus** with a family of descending projections to pre-ganglionic autonomics in brainstem and spinal cord.

Question 44

Fill the table

Answer 44

Answers are in the table