chapter 1
Clinically Oriented Neuroanatomy
‘MERIDIANS OF LONGITUDE AND PARALLELS OF LATITUDE’
Although most textbooks on clinical neurology begin with a chapter on history taking, there is a very good reason for placing neuroanatomy as the initial chapter. It is because clinical neurologists use their detailed knowledge of neuroanatomy not only when examining a patient but also when obtaining a neurological history in order to determine the site of the problem within the nervous system. This chapter not only describes the neuroanatomy but attempts to place it in a clinical context.
The ‘student of neurology’ cannot be expected to remember all of the detail but needs to understand the basic concepts. This understanding, combined with the correct technique when taking the neurological history (see Chapter 2, ‘The neurological history’) and performing the neurological examination (see Chapter 3, ‘Neurological examination of the limbs’, Chapter 4, ‘The cranial nerves and understanding the brainstem’, and Chapter 5, ‘The cerebral hemispheres and cerebellum’), together with the illustrations in this chapter will enable the ‘non-neurologist’ to localise the site of the problem in most patients almost as well as the neurologist. It is intended that this chapter serve as a resource to be kept on the desk or next to the examination couch.
It is also crucial to understand the difference between upper and lower motor neurons. The terms are more often (and not unreasonably) used to refer to the central and peripheral nervous systems, CNS and PNS, respectively. More specifically, upper motor neuron refers to motor signs that result from disorders affecting the motor pathway above the level of the anterior horn cell, i.e. within the CNS, while lower motor neuron refers to motor symptoms and signs that relate
to disorders of the PNS, the anterior horn cell, motor nerve root, brachial or lumbrosacral plexus or peripheral nerve (see Table 1.1). The alterations in strength, tone, reflexes and plantar responses (scratching the lateral aspect of the sole of the foot to see which way the big toe points) are different in upper and lower motor neuron problems.
TABLE 1.1
Upper and lower motor neuron signs
Upper motor neuron signs | Lower motor neuron signs | |
Weakness | The UMN pattern∗ | Specific to a nerve or nerve root |
Tone | Increased | Decreased |
Reflexes | Increased | Decreased or absent |
Plantar response | Up-going | Down-going |
∗The muscles that abduct the shoulder joint and extend the elbow and wrist joints are weak in the arms while the muscles that flex the hip and knee joints and the muscles that dorsiflex the ankle joint (bend the foot upwards) are weak in the legs.
The reason why this is so important is highlighted in Case 1.1.
CONCEPT OF THE MERIDIANS OF LONGITUDE AND PARALLELS OF LATITUDE
• The descending motor pathway from the cortex to the muscle
• The ascending sensory pathway for pain and temperature
• The ascending sensory pathway for vibration and proprioception
The ascending sensory pathways extend from the peripheral nerves to the cortex.1
The parallels of latitude
If the patient has weakness the pathological process must be affecting the motor pathway somewhere between the cortex and the muscle while, if there are sensory symptoms, the pathology must be somewhere between the sensory nerves in the periphery and the cortical sensory structures. The presence of motor and sensory symptoms/signs together immediately rules out conditions that are confined to muscle, the neuromuscular junction, the motor nerve root and anterior horn cell.
It is the pattern of weakness and sensory symptoms and/or signs together with the parallels of latitude that are used to determine the site of the pathology.
The following examples combine weakness with various parallels of latitude to help explain this concept. The parallels of latitude follow the + sign.
• Weakness + marked wasting – the peripheral nervous system, as marked wasting does not occur with central nervous system problems
• Weakness + cranial nerve involvement – brainstem
• Weakness + visual field disturbance (not diplopia) or speech disturbance (i.e. dysphasia) – cortex
• Weakness in both legs + loss of pain and temperature sensation on the torso – spinal cord
• Weakness in a limb + sensory loss in a single nerve (mononeuritis) or nerve root (radiculopathy) distribution – peripheral nervous system
THE MERIDIANS OF LONGITUDE: LOCALISING THE PROBLEM ACCORDING TO THE DESCENDING MOTOR AND ASCENDING SENSORY PATHWAYS
The descending motor pathway (also referred to as the corticospinal tract) and the ascending sensory pathways represent the meridians of longitude. The dermatomes, myotomes, reflexes, brainstem cranial nerves, basal ganglia and the cortical signs represent the parallels of latitude. The motor pathways and dorsal columns both cross at the level of the foramen magnum, the junction between the lower end of the brainstem and the spinal cord, while the spinothalamic tracts cross soon after entering the spinal cord.
If there are left-sided upper motor neuron signs or impairment of vibration and proprioception, the lesion is either on the left side of the spinal cord below the level of the foramen magnum or on the right side of the brain above the level of the foramen magnum. If there is impairment of pain and temperature sensation affecting the left side of the body, the lesion is on the opposite side either in the spinal cord or brain. If the face is also weak the problem has to be above the mid pons.
Cases 1.2 and 1.3 illustrate how to use the meridians of longitude.
The motor pathway
The motor pathway (see Figure 1.2) refers to the corticospinal tract within the central nervous system that descends from the motor cortex to lower motor neurons in the ventral horn of the spinal cord and the corticobulbar tract that descends from the motor cortex to several cranial nerve nuclei in the pons and medulla that innervate muscles plus the motor nerve roots, plexuses, peripheral nerves, neuromuscular junction and muscle in the peripheral nervous system.
• arises in the motor cortex in the pre-central gyrus (see Figure 1.5) of the frontal lobe
• descends in the cerebral hemispheres through the corona radiata and internal capsule
• passes into the brainstem via the crus cerebri (level of midbrain) and descends in the ventral and medial aspect of the pons and medulla
• descends in the lateral column of the spinal cord to the anterior horn cell where it synapses with the lower motor neuron
• leaves the spinal cord through the anterior (motor) nerve root
• passes through the brachial plexus to the arm or through the lumbosacral plexus to the leg and via the peripheral nerves to the neuromuscular junction and muscle.
The sensory pathways
There are two sensory pathways: one conveys vibration and proprioception and the other pain and temperature sensation and both convey light touch sensation.
PROPRIOCEPTION AND VIBRATION
• arises in the peripheral sensory receptors in the joint capsules and surrounding ligaments and tendons (proprioception) or in the pacinian corpuscles in the subcutaneous tissue (vibration) [1]
• ascends up the limb in the peripheral nerves
• traverses the brachial or lumbosacral plexus
• enters the spinal cord through the dorsal (sensory) nerve root
• ascends in the ipsilateral dorsal column of the spinal cord with the sacral fibres most medially and the cervical fibres lateral
• ascends in the medial lemniscus in the medial aspect of the brainstem via the thalamus to the sensory cortex in the parietal lobe.

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