Neuroscience/Objectives/Lectures 13-14

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Contents 1 Spinal cord: Segmental spinal circuitry and segmental motor control 1.1 Predict the deficits you would see following each of the following lesions: dorsal root, ventral root, spinal nerve. 1.2 Describe the relationship of the spinal cord, its roots, and its coverings to the bony vertebral canal in the adult. Discuss how this relationship changes during development. 1.3 List the number of spinal cord segments at the cervical, thoracic, lumbar, sacral, and coccygeal levels of the spinal cord. Describe how and where the spinal nerves for each region exit the vertebral canal. Differentiate between a spinal cord segmental level and a vertebral segmental level. 1.4 Draw and label a typical cross section of spinal cord. 1.5 Describe the somatotopic organization of the spinal cord using the ventral horn gray matter and dorsal column white matter as specific examples. 1.6 Define lower motor neuron and motor unit. 1.7 Describe the trophic influence of an α-motor neuron on the muscle it innervates and the consequences of motor neuron death or axon injury on the muscle. 1.8 Describe the structure and function of muscle spindles, motor neurons, Renshaw cells, and Golgi tendon organs. 1.9 Define a reflex and list the components in a spinal reflex circuit. Draw and label a reflex arc for a knee jerk. 1.10 List the elements of and discuss the functional role of the following structures. 1.11 Describe the contribution of static and dynamic γ-motor neurons to adjusting the sensitivity of muscle spindles.

Spinal cord: Segmental spinal circuitry and segmental motor control

Predict the deficits you would see following each of the following lesions: dorsal root, ventral root, spinal nerve.

Dorsal root lesion

Lesioning the dorsal root will result in a loss of afferent (sensory) input from parts of the body innervated by that segment of spinal cord.

Ventral root lesion

Ventral root lesions result in loss of efferent (motor) output to the parts of the body innervated by that segment of spinal cord.

Spinal nerve lesion

The spinal nerve is the combination of ventral efferents and dorsal afferents. Lesioning the spinal nerve eliminates both motor and sensory innervation at that segment of spinal cord.

Note that although each of these lesions involves the spinal cord, all are considered peripheral nervous system injuries. This is because the spinal nerve and its ventral/dorsal roots are peripheral, not central nervous system components. One piece of evidence for this is that these structures are myelinated by Schwann cells, not oligodendrocytes.

Describe the relationship of the spinal cord, its roots, and its coverings to the bony vertebral canal in the adult. Discuss how this relationship changes during development.

The spinal cord occupies the upper two-thirds of the vertebral canal, with its caudal ending at the level of the vertebral disc between L1 and L2. Each spinal nerve exits at the appropriate intervertebral foramen, even if the actual spinal cord segment is a distance from its appropriate foramen. For example, the S1 segment of the spinal cord is located near the 12th thoracic vertebra, yet S1's spinal nerves still exit through the intervertebral foramina between sacral vertebrae 1 and 2.

List the number of spinal cord segments at the cervical, thoracic, lumbar, sacral, and coccygeal levels of the spinal cord. Describe how and where the spinal nerves for each region exit the vertebral canal. Differentiate between a spinal cord segmental level and a vertebral segmental level.

Number of spinal segments:

• Cervical: 8
• Thoracic: 12
• Lumbar: 5
• Sacral: 5
• Coccygeal: 1

With the exception of the cervical spinal cord segments, all spinal nerves exit through intervertebral foramina directly below the corresponding vertebral body. For example, L1's spinal nerves exit through intervertebral foramina between lumbar vertebrae 1 and 2. The exception for cervical segments exists because there are eight cervical spinal cord segments, but only seven cervical vertebral bodies. Therefore C1-C7 exit above their corresponding cervical vertebral bodies, while C8 exits below the 7th cervical vertebra.

Both the spinal cord and vertebral column are segmental structures. It is common to speak of (for example) spinal nerve 7 which refers to the spinal nerve that emerges from the seventh segment of the cervical spinal cord. Similarly, one could speak of the 5th lumbar vertebra, in reference to the most caudal segment of the lumbar vertebrae.

Draw and label a typical cross section of spinal cord.

α-Motor neurons (in the ventral horn gray matter) receive inputs from three sources:

1. Muscle spindle afferents
2. Upper motor neurons from motor cortex and brainstem
3. Interneurons within spinal cord (this is the largest input to α-motor neurons)

Describe the somatotopic organization of the spinal cord using the ventral horn gray matter and dorsal column white matter as specific examples.

The ventral horn gray matter generally consists of somatic motor neurons that innervate muscles belonging to the spinal cord segment's myotome. In general, motor neurons innervating proximal muscles are located medially, while those that innervate distal muscles are located more laterally.

Axons from upper motor neurons that synapse on lower motor neurons are also somatotopically arranged. In particular, UMN axons to LMNs subserving distal muscles are carried in the dorsolateral white matter, while axons of UMNs that innervate proximal muscle LMNs are located in the ventromedial white matter.

Define lower motor neuron and motor unit.

Lower motor neuron

Lower motor neurons are the motor neurons found in the ventral horn of the spinal cord gray matter. They come in two flavors based on the type of muscle fibers they innervate. α-Motor neurons innervate extrafusal muscle fibers, whereas γ-motor neurons innervate intrafusal fibers associated with muscle spindles.

Motor unit

A motor unit is an α-motor neuron and all the muscle fibers it innervates. Most mature skeletal muscle fibers in mammals are innervated by a single nerve fiber.

Motor neuron (or unit) pool

A motor pool is all the motor neurons (or units) that innervate a single muscle.

Describe the trophic influence of an α-motor neuron on the muscle it innervates and the consequences of motor neuron death or axon injury on the muscle.

Muscles and the motor neurons that innervate them provide each other with reciprocal trophic stimuli. Muscles provide motor neurons with neurotrophic factors. Likewise, motor neuron firing and stimulation of a muscle maintains it.

Describe the structure and function of muscle spindles, motor neurons, Renshaw cells, and Golgi tendon organs.

Muscle spindle

Muscle spindles are specialized structures that detect muscle stretch. They are composed of 8-10 intrafusal muscle fibers and lie in parallel with the extrafusal muscle fibers comprising a single skeletal muscle. These intrafusal muscle fibers are innervated by three types of neurons: group Ia sensory afferents, group II sensory afferents, and γ-motor neurons.

Group Ia (myotatic) sensory afferents are large, heavily myelinated, rapidly conducting fibers that are highly sensitive to stretch. Group II sensory afferents are smaller, less myelinated, slower conducting, and are less sensitive to stretch. Both groups of afferents innervate the nuclear bag and chain fibers, specific populations of intrafusal muscle fibers within the spindle. Stretching of the skeletal muscle activates mechanoreceptors in the terminals of the sensory neurons resulting in action potential firing. In the spinal cord, group Ia and group II fibers synapse directly on α-motor neurons (and γ-MNs—see below) of homonymous and synergistic muscles, and indirectly on antagonistic α-motor neurons via inhibitory interneurons. Thus afferent input via these fibers causes the contraction of synergistic and homonymous muscles and relaxation of antagonistic muscles.

Intrafusal muscle fibers are not innervated by α-motor neurons. Thus, when the extrafusal muscle fibers are contracted, the intrafusal muscle fibers would be lax and unable to respond to stretch (as only a taut intrafusal fiber can detect stretch). Therefore, intrafusal muscle fibers are also innervated by γ-motor neurons, a population of smaller neurons whose cell bodies lie in the ventral horn of the spinal cord gray matter. γ-Motor neurons modulate the sensitivity of muscle spindles by stimulating intrafusal fibers to contract in response to activation by group Ia and II afferent fibers. In this way, γ-motor neurons "change the gain" of the muscle fiber in response to spindle stretch.

α- and γ-motor neurons

Both α- and γ-motor neurons are lower motor neurons and, as such, have their cell bodies in the ventral horn of the spinal cord gray matter. α-Motor neurons innervate extrafusal muscle fibers and are the typical motor neurons one thinks of when considering the innervation of a skeletal muscle. γ-Motor neurons are smaller than α-motor neurons and innervate intrafusal muscle fibers of muscle spindles. Both types of neurons receive afferent input from muscle spindles, and both can be modulated independently by upper motor neurons in the cerebral cortex and braintem.

Renshaw cells

Renshaw cells are inhibitory interneurons of the spinal cord that limit the strength and duration of activation of α-motor neurons. They receive excitatory inputs from recurrent axonal collaterals of α-motor neurons and send their own axons to synapse on the same α-motor neuron. In this way, Renshaw cells allow the α-motor neuron can indirectly feedback-inhibit its own activity, helping to prevent hyperactivation that may lead to tetanus.

Golgi tendon organ

Like the muscle spindle, the Golgi tendon organ (GTO) is a proprioceptive organ associated with muscles. In contrast to the spindle, the GTO is located in tendons, and is therefore arranged in series with the muscle rather than in parallel. The function of the GTO is to maintain a steady muscle force via a negative feedback system, whereas the muscle spindle acts to maintain constant length.

Mechanoreceptors embedded in the tendon capsule respond to muscle contraction by firing action potentials via sensory Ib afferents. These afferents have excitatory synapses with Ib inhibitory interneurons that themselves synapse on α-motor neurons of the homonymous muscle. Ib afferents also have excitatory contact with excitatory interneurons that synapse on α-motor neurons of heteronymous muscles. Thus activation of the GTO-mediated reflex results in inhibition of homonymous α-motor neurons and activation of heteronymous α-motor neurons, and is one of the reflex mechanisms by which muscle overexertion is avoided.

Ib inhibitory interneurons receive inputs not only from Ib afferents from the GTO, but also from cutaneous receptors, joint receptors, muscle spindles (Ia and II fibers), and descending efferent pathways (upper motor neurons). In this way, reflexes normally mediated by the GTO can be modulated by other pathways.

Define a reflex and list the components in a spinal reflex circuit. Draw and label a reflex arc for a knee jerk.

A reflex is a rapid, unconscious response to a stimulus mediated by a reflex arc (or reflex circuit). The simplest reflex arc is monosynaptic, with two components: 1) sensory afferents impinging directly on 2) α-motor neurons.

The knee-jerk reflex is a stretch reflex activated by stretching of the patellar tendon. It is mediated by a reflex arc that involves spinal cord segments L2, L3, and L4. Stretching this tendon stretches the quadriceps femoris, which is detected by muscle spindles. Groups Ia and II afferent fibers send stimulatory inputs to α-motor neurons that innervate the quadriceps femoris, causing contraction of the muscle.

List the elements of and discuss the functional role of the following structures.

Stretch reflex (maintaining constant muscle length)

• Muscle spindle
  • Group Ia afferent fibers
  • Group II afferent fibers
  • γ-Motor neurons
• α-Motor neurons
• Interneurons
  • Excitatory
  • Inhibitory (including Renshaw cells; glycinergic)

Golgi tendon organ-mediated reflex (maintaining constant muscle tension)

• Golgi tendon organ
  • Group Ib afferents
• α-Motor neurons
• Interneurons
  • Excitatory
  • Inhibitory (including group Ib interneurons and Renshaw cells; glycinergic)

Flexion-withdrawal reflexes (more complex circuits)

• Somatosensory (not proprioceptive) afferents (e.g. Aδ nociceptive afferents)
• α-Motor neurons

Flexion-withdrawal reflexes include more complicated reflexes such as the crossed-extension reflex. For example, if a noxious stimulus is received by the foot, it will travel along Aδ nociceptive afferent fibers and excite a number of interneurons. The result is that ipsilateral extensors will be inhibited and ipsilateral flexors will be stimulated. Similarly, contralateral extensors will be stimulated and contralateral flexors will be inhibited.

The cutaneous flexion reflex is another example of a flexion-withdrawal reflex.

Describe the contribution of static and dynamic γ-motor neurons to adjusting the sensitivity of muscle spindles.

This was covered neither in the lecture nor syllabus.