Chapter 4: The Motor and Somatosensory Pathways and Approach to Weakness and Sensory Loss

The cerebral hemispheres are where the motor pathways originate, and where the somatosensory pathways terminate. The left cerebral hemisphere controls the motor functions of the right side of the body, and the right cerebral hemisphere controls the motor functions of the left side of the body. This crossed system is maintained in other modalities including the somatosensory and visual pathways: If the left hemisphere controls the right side of the body, it makes sense that it would need somatosensory information about the right side of the body and visual information about the right side of the world.

There are three main clinically relevant tracts for motor and somatosensory function for the body—one motor and two sensory—that span the brain, brainstem, and spinal cord:

• Motor: The corticospinal tracts send motor information from the cortex to the spinal cord as the name suggests.

• Sensory: The anterolateral (or spinothalamic) tracts and dorsal (or posterior) column pathways bring sensory input from the spinal cord to the brain by way of the brainstem. The names of these pathways refer to their anatomic positions within the spinal cord.

In this chapter, the anatomy of these pathways will be described, providing a foundation for localizing symptoms of weakness and sensory changes, and also laying out anatomic landmarks in the brain, brainstem, and spinal cord that will serve as points of orientation as additional pathways are described in subsequent chapters.

The corticospinal tracts are the final output of the motor system, conveying the action plan to the alpha motor neurons of the anterior horns of the spinal cord, which, in turn, relay the signals to move the muscles by way of peripheral nerves (Fig. 4–1). The corticospinal tracts are sometimes referred to as the pyramidal system (since they travel for part of their course in the medullary pyramids). The extrapyramidal system includes the basal ganglia and cerebellum, which participate in circuits with the motor cortex and are involved in action initiation and coordination (see Chs. 7–8).

Each corticospinal tract begins in the motor cortex, which is located in the precentral gyrus (immediately anterior to the central sulcus). The motor cortex is organized by the region of the body it controls: face lateral, hand and arm superior to this, and leg and foot most medial (with arm and leg representations connected at the shoulder/hip such that the hand is most lateral and the foot most medial with the arm and leg between).

The cell bodies from layer 5 of the motor cortex give rise to axons that travel in the subcortical white matter (specifically in the posterior limb of the internal capsule), then in the ventral/anterior brainstem (cerebral peduncles of the midbrain, basis pontis of the pons, and medullary pyramids of the medulla). At the junction of the medulla and the cervical spinal cord (cervicomedually junction), the corticospinal tracts cross (decussate), such that the corticospinal tract that began in the left hemisphere descends on the right side of the spinal cord and the corticospinal tract that began in the right hemisphere descends on the left side of the spinal cord.

Once in the spinal cord, the main clinically relevant corticospinal tracts are situated posterolaterally. In the spinal cord, the axons that have traveled all the way from the contralateral motor cortex synapse on alpha motor neurons in the anterior horns of the spinal cord gray matter. Axons of the alpha motor neurons leave the ventral/anterior spinal cord via ventral roots and enter peripheral nerves to travel to muscles.

For clinical purposes, the pyramidal motor system can be considered as a two-neuron system. First-order neurons have their cell bodies in the motor cortex (precentral gyrus), and their axons travel through the internal capsule, brainstem, and spinal cord (the corticospinal tract). Second-order neurons have their cell bodies in the anterior horn of the spinal cord, and their axons travel through nerve roots and peripheral nerves. The neurons of the central nervous system (CNS) component in the brain/brainstem/spinal cord (i.e., the corticospinal tract) are referred to as upper motor neurons, and the neurons of the peripheral nervous system (PNS) component (anterior horn of the spinal cord through the ventral roots into peripheral nerves) are referred to as lower motor neurons.

Because of the crossing (decussation) of the corticospinal tracts at the cervicomedullary junction, unilateral corticospinal tract lesions in the brain or brainstem cause contralateral weakness, whereas unilateral spinal cord lesions cause ipsilateral weakness. Lesions in individual roots and nerves cause weakness in the particular muscle(s) that they supply.

Upper Motor Neuron Lesions Versus Lower Motor Neuron Lesions

Understanding the differences in the clinical signs caused by upper motor neuron lesions (CNS) versus lower motor neuron lesions (PNS) is an essential component of the assessment of weakness. With upper motor neuron lesions, all of the signs go up: reflexes are “up” (increased: hyperreflexia), tone is “up” (increased: spasticity), and the big toe may go up when stroking the bottom of the foot (Babinski sign).

With lower motor neuron lesions, nearly all of the signs are lowered: diminished or absent reflexes (hyporeflexia or areflexia), decreased (flaccid) tone, decreased muscle bulk (atrophy), toes downgoing (no Babinski sign). Abnormal muscle twitches called fasciculations can occur with diseases affecting lower motor neurons. Fasciculations are an example of a sign of increased activity due to lower motor neuron dysfunction, and are therefore an exception to the “up” and “low” mnemonic for signs associated with pathology of upper and lower motor neurons (Table 4–1).

Another upper motor neuron sign is the Hoffmann sign, which can be thought of as a Babinski sign for the upper extremity. To see if a Hoffmann sign is present, the examiner holds the patient’s hand by the middle finger with one hand, and uses the other hand to quickly flick the tip of the middle finger (as if snapping one’s fingers, but with the patient’s finger between). If a Hoffmann sign is present, the fingers and thumb will flex. As with the Babinski sign, this sign is indicative of upper motor neuron (CNS) dysfunction.

A very important clinical point is that some of the classic upper motor neuron findings such as increased tone and hyperreflexia are often not present acutely after a CNS insult and take time to emerge. For example, with weakness caused by acute stroke or acute spinal cord trauma, the affected limb(s) will usually be flaccid and areflexic in the acute setting. Upper motor neuron signs emerge over time. Therefore, it may be more challenging to distinguish between upper and lower motor neuron causes of weakness in the acute setting. An upgoing toe (Babinski sign) may be present acutely (although not always), and when it is, can help point toward a CNS etiology of weakness.

Pronator drift is another indication of upper motor neuron pattern weakness: when the arms are held outstretched with the palms up and fingers spread (as if holding a tray), the hand may begin to close and the arm may begin to pronate and drift downward if upper extremity weakness is due to CNS/upper motor neuron pathology. (With parietal lesions, the affected arm may drift upward due to impaired proprioception.) Unlike the upper motor neuron signs described above that may not be present acutely (since it is a reflection of the pattern of weakness).

When an upper motor neuron lesion causes weakness without causing complete paralysis, a distinct pattern of weakness may be seen in which the upper extremity extensors are weaker than the flexors, and the lower extremity flexors are weaker than the extensors. In other words, the arm is stronger when flexing the elbow compared to extending the elbow, and the leg is stronger when extending the knee compared to flexing the knee. This pattern can be remembered by recalling the posture of a patient with long-standing upper motor neuron injury (e.g., prior stroke): the arm, wrist, and fingers are flexed and pronated close to the body, whereas the lower extremity is extended at the knee with the foot plantarflexed and needs to be circumducted when the patient walks. This posture demonstrates that the stronger flexors of the arm have overcome the weaker extensors, and the stronger extensors of the leg have overcome the weaker flexors. As with pronator drift, this upper motor neuron pattern of weakness can be present acutely (as opposed to hyperreflexia and increased tone, which generally take time to emerge, as described above). Note that a radial nerve palsy will affect the triceps, wrist/finger extensors, and supinator, and can thus mimic an upper motor neuron lesion and vice versa (see Ch. 16).

The muscles of the face, tongue, larynx, and pharynx also have upper motor neurons and lower motor neurons. The upper motor neurons arise from the motor cortex and travel with the corticospinal tracts as the corticobulbar tracts. Instead of continuing to the spinal cord like the corticospinal tracts do, the corticobulbar fibers terminate in their respective cranial nerve nuclei in the brainstem. The lower motor neurons arise in the cranial nerve nuclei and travel in the cranial nerves. For example, corticobulbar fibers (upper motor neurons) synapse on the contralateral cranial nerve 7 nucleus in the pons for movements of the face, and the contralateral cranial nerve 12 nucleus for movements of the tongue. These pathways are discussed further in Chapters 13 and 14.

Overview of the Somatosensory Pathways for the Body

In contrast to the motor pathways that begin in the brain and end in the periphery, the sensory pathways begin in the periphery and end in the brain (Fig. 4–2). Sensory information from the skin (light touch, pressure, pain, temperature, vibration) and from the muscle spindles and the Golgi tendon organs (proprioception) travels from the periphery via the peripheral nerves to arrive in the dorsal root ganglia. The dorsal root ganglia are cell bodies of pseudounipolar neurons, each of which has one process in a peripheral nerve and the other traveling in the dorsal roots to the spinal cord.

At the entrance to the spinal cord, somatosensory information is “sorted” into two pathways: pain and temperature information enters the anterolateral tracts (also known as the spinothalamic tracts), whereas proprioception and vibration information enters the dorsal columns. Light touch information travels to some extent in both pathways and, therefore, has less localizing value. Both sensory pathways ascend through the spinal cord and brainstem to arrive at the ventral posterior lateral (VPL) nucleus of the thalamus, and the thalamus transmits this somatosensory information to the somatosensory cortex in the postcentral gyrus, just posterior to the motor strip. Further details of these pathways are described below.

The anterolateral and dorsal column systems both cross to transmit sensory information from one side of the body to the contralateral cerebral hemisphere, but the sites of crossing differ. The crossing of the dorsal column system occurs in the medulla, just superior to the crossing of the corticospinal tracts (which cross at the cervicomedullary junction). Therefore, unilateral lesions of the dorsal column pathway from the upper medulla and superiorly (i.e., pons, midbrain, thalamus, subcortical white matter, somatosensory cortex) affect contralateral sensation, whereas unilateral lesions from the lower medulla through the spinal cord affect ipsilateral sensation. This pattern essentially mimics that of the corticospinal tracts.

Unlike the corticospinal tracts and dorsal column pathways, the anterolateral tracts cross immediately after entering the spinal cord (they actually cross over the course of a few spinal levels). Therefore, unilateral lesions anywhere in this pathway (spinal cord, brainstem, thalamus, brain) affect contralateral sensation, since it crosses almost immediately after entering the CNS. (Since the pathway crosses over the course of a few spinal levels, a small patch of ipsilateral pain/temperature loss involving the spinal levels over which the tract crosses may also be observed.)

The Dorsal Column–Medial Lemniscus Pathways

The dorsal columns travel dorsally (posteriorly) in the spinal cord (Fig. 4–3). They are divided into the fasciculus cuneatus and fasciculus gracilis (one of each on each side). The fasiculi cuneatus run laterally in the dorsal columns and carry upper extremity sensory information, whereas the fasciculi gracilis run medially in the dorsal columns and carry lower extremity sensory information. The columns rise all the way to the lower medulla where they synapse in the nucleus cuneatus and nucleus gracilis on each side. The axons exiting the nuclei cuneatus and gracilis on each side (second-order neurons in the system) cross by way of the internal arcuate fibers to become the contralateral medial lemniscus. Each medial lemniscus ascends through the brainstem to synapse in the ipsilateral VPL nucleus of the thalamus, which provides the third-order neurons in the system that synapse in the somatosensory cortex in the postcentral gyrus (just posterior to the central sulcus).

Dysfunction in the dorsal column pathway leads to deficits in vibration sense and proprioception (joint position sense). Isolated dorsal column pathway dysfunction is most commonly due to large fiber peripheral neuropathy (see Ch. 27), dorsal root ganglionopathy (see Ch. 15), or spinal cord disease (see Ch. 5). Vibration sense is tested by placing a vibrating 128-Hz tuning fork on a joint, and comparing the length of time the patient feels the vibration with the time the examiner can feel it. Proprioception is tested by moving a particular joint up or down and asking the patient to determine if the joint has been moved up or down without looking. Evaluation of vibration sense is a more sensitive test for dorsal column pathway dysfunction than evaluation of proprioception.

Assessing for the Romberg sign is another test of dorsal column pathway dysfunction. Patients are asked to stand with their feet together, and once balance is achieved, they are asked to close their eyes. Patients with a deficit in proprioception can stand with their feet together when using vision to compensate, but closing their eyes removes this cue and requires them to rely exclusively on proprioception. If the dorsal column pathway is not working properly due to large fiber peripheral neuropathy, ganglionopathy, or spinal cord disease, patients will sway and take a step to maintain balance, which is Romberg’s sign. If patients do not take a step when testing for Romberg’s sign, but exhibit involuntary piano playing–like movements of the toes (pseudoathetosis), this is a sign of proprioceptive dysfunction (a sort of pre–Romberg sign).

The Anterolateral (Spinothalamic) Tracts

The dorsal root neurons carrying pain and temperature sensation enter the posterior spinal cord and go directly into the posterior gray matter via Lissauer’s tract (Fig. 4–4). In the dorsal gray matter, these fibers synapse, and the second-order neurons travel anteriorly and cross in the anterior commissure of the spinal cord to arrive in the anterior lateral spinal cord. The axons of these second-order neurons then ascend through the spinal cord as the anterolateral (spinothalamic) tracts to the brainstem, where they eventually join the medial lemniscus in the pons to travel together to the VPL nucleus of the thalamus. After synapsing in the thalamus, the third-order neurons of the pathway travel to the somatosensory cortex just like in the dorsal column pathways.

Pain sensation is generally assessed by determining if the patient can feel a pin as sharp, and if the pin feels of equal sharpness in different locations. Isolated diminished pain sensation can be due to peripheral neuropathy (affecting small fibers), radiculopathy, or dysfunction in the anterolateral system in the spinal cord. When sensory loss is due to spinal cord pathology, a distinct spinal level may be detected when pin prick is tested along the spine: patients will note a discrete level above which the pin is felt as sharp and below which it is felt as dull (or not felt). Temperature can be assessed by using the side of a metal tuning fork to see if it feels equally cold in different locations.

Facial sensation is carried in the trigeminal nerve (cranial nerve 5) to the brainstem, from which facial sensory information is transmitted to the contralateral ventral posterior medial (VPM) nucleus of the thalamus and then to the somatosensory cortex. This pathway is discussed in further detail in Chapter 13.

Weakness can occur due to lesions anywhere in the motor pathways: brain, brainstem, spinal cord, ventral roots, peripheral nerves, neuromuscular junction, and/or muscles. Sensory changes (loss of sensation, paresthesias, proprioceptive deficits) can occur due to lesions anywhere along the sensory pathways: peripheral nerves, dorsal root ganglia, dorsal roots, spinal cord, brainstem, or brain.

The clinical evaluation of weakness or sensory disturbances requires determining the distribution of the patient’s symptoms (i.e., unilateral, bilateral, proximal, distal) and the time course of the onset and evolution of the patient’s symptoms (i.e., sudden, acute, subacute, chronic, and whether there has been progression and/or fluctuation).

For weakness, the examination must also determine whether the weakness is consistent with localization to the CNS (brain, brainstem, or spinal cord; i.e., upper motor neuron signs on examination), PNS (roots, peripheral nerves; i.e., lower motor neuron signs on examination), neuromuscular junction (i.e., fatigability on examination; see Ch. 29), or muscles (often proximal and symmetric, although not always; see Ch. 30).

For sensory disturbances, the affected sensory modalities must be determined (i.e., pain/temperature and/or vibration/proprioception). Light touch has less localizing value than vibration, proprioception, pain, and temperature.

Understanding the localization of particular distributions of motor and/or sensory symptoms rests on the foundation of the pathways discussed in this chapter.

Involvement of the Motor and Sensory Pathways in the Brain

The motor and sensory cortices are adjacent to one another, and their white matter tracts in the brain (descending from the motor cortex; ascending from the thalamus to the sensory cortex) are also close to one another. Isolated weakness or isolated sensory loss caused by a lesion of the brain can occur if there is a small lesion affecting just one of these pathways (e.g., small embolic stroke affecting just one gyrus, or a small deep [lacunar] infarct in either the posterior limb of the internal capsule [motor] or the thalamus [sensory]). Lesions in the brain will cause contralateral motor and/or sensory deficits. Since the anterolateral and dorsal column pathways are adjacent from the level of the pons until the thalamus (where both synapse in the VPL nucleus), lesions above the level of the medulla usually cause sensory loss affecting all modalities (although very small brainstem lesions affecting just one of these pathways can sometimes occur). Isolated proprioception/vibration or isolated pain/temperature deficit generally suggests a lesion in the lower medulla, spinal cord, dorsal roots/ganglia, or peripheral nerves.

Since the face and arm are adjacent on the cortical surface and share the vascular supply of the middle cerebral artery (the leg region is supplied by the anterior cerebral artery; see Ch. 7), a common pattern of motor and/or sensory changes due to a lesion in the brain is deficits in the face and hand on one side. Since the hand and the the mouth are among the most sensitive and dexterous, they have substantial cortical representation. Therefore, the symptom of sensory changes and/or weakness in the hand and ipsilateral perioral region (cheiro-oral pattern) is highly suggestive of a lesion in the contralateral cerebral hemisphere.

Isolated lesions of the somatosensory cortex may not lead to sensory loss but may lead to higher level sensory deficits. For example, the patient may be unable to recognize a number or letter drawn on the hand (agraphesthesia). If the patient’s left and right sides of the body are touched simultaneously, the patient may not note the sensation in the limb contralateral to the lesion, despite being able to detect sensation when this limb is touched in isolation (extinction to double simultaneous stimulation).

Involvement of the Motor and/or Sensory Pathways in the Brainstem

In the brainstem, the corticospinal tracts travel in the anterior (ventral) portion, whereas the sensory pathways are predominantly dorsolateral (the medial lemniscus beginning in the anterior medial medulla is an exception to this generalization). Therefore, anterior brainstem lesions can affect the motor pathways, whereas dorsolateral lesions can affect the sensory pathways. Since all of the cranial nerve nuclei are also in the brainstem, isolated motor or sensory symptoms limited to the extremities from a brainstem lesion are rare, although they can occur with small anterior lesions affecting only the corticospinal tracts, or small dorsal lesions affecting only the sensory tracts.

As will be discussed in Chapter 9, unilateral brainstem lesions can cause crossed signs: weakness and/or sensory changes in the ipsilateral face, but in the contralateral body. This is because nearly all cranial nerves control ipsilateral functions in the head, whereas the not-yet-crossed corticospinal tracts and the already-crossed ascending sensory tracts at the level of the brainstem control contralateral functions in the body.

Involvement of the Motor and/or Sensory Pathways in the Spinal Cord

The anatomic arrangement of the long tracts in the spinal cord gives rise to several possible clinical syndromes, which are discussed in detail in Chapter 5. Since the spinal cord is relatively small, it is common for disease processes affecting it to cause bilateral symptoms and signs, although small lesions within the spinal cord (intramedullary lesions) or unilateral external compression of the spinal cord (extramedullary lesions) can lead to unilateral symptoms.

Involvement of the Motor and/or Sensory Pathways in the Peripheral Nervous System

As discussed in Chapter 27, peripheral neuropathies can be motor, sensory, or mixed, and can affect an individual nerve (mononeuropathy), multiple nerves (mononeuropathy multiplex), or all nerves (polyneuropathy), leading to varying patterns of weakness and/or sensory changes. When an individual limb is weak and/or has changes in sensation such as numbness or pain, the problem may be referable to an individual root or nerve, multiple roots or nerves, or to a nerve plexus (see Chs. 16–17).

In summary (Fig. 4–5):

• If there are sensory and/or motor changes in the face (with or without deficits in the extremities), this requires a lesion at the level of the brainstem or in the cerebral hemisphere. (Evaluation of facial weakness and sensory changes is discussed in further detail in Chapter 13.)

• If the arm and leg are affected on one side without the face being involved, the lesion is most likely in the medulla or cervical spinal cord.

• If one limb is affected in isolation, a lesion at nearly any level of the nervous system is possible: a small cortical or subcortical lesion, a spinal cord lesion, or a lesion at the level of the nerve roots, plexus, or one or more peripheral nerves. A brainstem lesion causing unilateral involvement of one limb is unlikely since the fibers of the motor and sensory pathways are packed so closely in the brainstem.

• Isolated bilateral weakness suggests a process affecting the spinal cord, peripheral nerves, muscles, or neuromuscular junction. To have bilateral weakness from a brain or brainstem lesion would require bilateral lesions, which will usually cause symptoms/signs beyond just motor problems (e.g., changes in mental state if there are bilateral cerebral lesions or cranial nerve deficits if there are bilateral brainstem lesions). An exception is a very medial lesion affecting the bilateral leg areas of the motor cortices, which can lead to paraplegia mimicking a spinal cord lesion. This can be caused by a parasagittal meningioma or bilateral anterior cerebral artery strokes (which can occur when both anterior cerebral arteries arise from a common trunk, called azygous anterior cerebral arteries; see Ch. 7). In the case of bilateral anterior cerebral artery strokes, there are usually cognitive deficits in addition to the motor findings. Isolated bilateral sensory changes generally suggest a process affecting the spinal cord, dorsal roots, dorsal root ganglia, or peripheral nerves.

With respect to time course of symptom onset and evolution:

• Sudden-onset weakness and/or sensory changes. Common causes of sudden-onset weakness and/or sensory changes localizing to the brain include stroke, seizure, or migraine. The most common cause of sudden-onset weakness and/or sensory changes localizing to the brainstem is stroke. Common causes of sudden-onset weakness and/or sensory changes localizing to the spinal cord include acute disc prolapse or vertebral collapse (e.g., due to trauma or metastatic malignancy to the vertebrae), which are typically painful. Spinal cord infarct occurs rarely (see Ch. 19). Sudden-onset weakness and/or sensory changes localizing to a particular nerve may be caused by a nerve infarct (as can be seen in vasculitic neuropathies; see Ch. 15)

• Acute- to subacute-onset (over hours to days) weakness and/or sensory changes. Acute- to subacute-onset weakness and/or sensory changes at any level of the nervous system can be due to inflammatory conditions (e.g., Guillain-Barré syndrome (see Ch. 27), transverse myelitis, or an acute demyelinating lesion in multiple sclerosis; see Ch. 21) or infectious processes (e.g., cerebral or epidural abscess; see Ch. 20).

• Subacute- to chronic-onset (over weeks to months to years) weakness and/or sensory changes. Subacute- to chronic-onset weakness and/or sensory changes can be caused by neoplasm, chronic inflammatory disease (e.g., chronic inflammatory demyelinating polyneuropathy [CIDP]; see Ch. 27), or degenerative disease (e.g., in the case of isolated weakness, motor neuron disease; see Ch. 28).