White matter

The gray matter is composed of nerve cell bodies and unmyelinated interneuron fibers. The location of the gray matter in the spinal cord is opposite to that of the brain. In the brain, the gray matter of the cerebrum and the cerebellum is found externally forming a cortex, or covering, over the internally located white matter. In the spinal cord, the gray matter is found internally and is surrounded by the white matter.

The white matter is composed of myelinated axons of neurons. These axons are grouped together according to function to form tracts. Neurons transmitting impulses toward the brain in the ascending tracts carry sensory information. Those transmitting impulses away from the brain in the descending tracts carry motor information.

Dorsal horn

Dorsal horn

Ventral horn

GRAY MATTER WHITE MATTER SPINAL NERVES

Figure 7.1 Cross-sectional view of the spinal cord. In contrast to the brain, the gray matter of the spinal cord is located internally, surrounded by the white matter. The gray matter consists of nerve cell bodies and unmyelinated interneuron fibers. This component of the spinal cord is divided into three regions: the dorsal, lateral, and ventral horns. The white matter consists of bundles of myelinated axons of neurons, or tracts. Each segment of the spinal cord gives rise to a pair of spinal nerves containing afferent and efferent neurons. Afferent neurons enter the spinal cord through the dorsal root and efferent neurons exit it through the ventral root.

Ventral horn

GRAY MATTER WHITE MATTER SPINAL NERVES

Figure 7.1 Cross-sectional view of the spinal cord. In contrast to the brain, the gray matter of the spinal cord is located internally, surrounded by the white matter. The gray matter consists of nerve cell bodies and unmyelinated interneuron fibers. This component of the spinal cord is divided into three regions: the dorsal, lateral, and ventral horns. The white matter consists of bundles of myelinated axons of neurons, or tracts. Each segment of the spinal cord gives rise to a pair of spinal nerves containing afferent and efferent neurons. Afferent neurons enter the spinal cord through the dorsal root and efferent neurons exit it through the ventral root.

7.3.1 Gray matter

A cross-sectional view of the spinal cord reveals that the gray matter has a butterfly or "H" shape (see Figure 7.1). As such, on each side of the spinal cord the gray matter is divided into three regions:

• Dorsal horn (posterior, toward the back)

• Ventral horn (anterior)

Each spinal segment contains millions of neurons within the gray matter. Functionally, four types of neurons exist:

• Second-order sensory neurons

• Somatic motor neurons

• Visceral motor neurons

• Interneurons

The cell bodies of second-order sensory neurons are found in the dorsal horn. These neurons receive input from afferent neurons (first-order sensory neurons) entering the CNS from the periphery of the body through the dorsal root of the spinal nerve. The function of the second-order sensory neuron is to transmit nerve impulses to higher levels in the CNS. The axons of these neurons leave the gray matter and travel upward in the appropriate ascending tracts of the white matter.

The cell bodies of somatic motor neurons are found in the ventral horn. The axons of these neurons exit the CNS through the ventral root of the spinal nerve and innervate skeletal muscles. The two types of motor neurons located in the ventral horn are:

• Alpha motor neurons innervate skeletal muscle fibers to cause contraction.

• Gamma motor neurons innervate intrafusal fibers of the muscle spindle, which monitors muscle length.

The spatial organization of the cell bodies of the motor neurons follows a proximal-distal rule. Motor neurons that innervate the most proximal muscles (axial muscles of the neck and trunk) lie most medially in the gray matter. Motor neurons innervating the most distal muscles (wrists, ankles, digits) lie most laterally in the gray matter.

The cell bodies of visceral motor neurons are found in the lateral horn. The axons of these neurons form efferent nerve fibers of the autonomic nervous system (ANS). The ANS innervates cardiac muscle, smooth muscle and glands (see Chapter 9). The axons of these neurons exit the spinal cord by way of the ventral root.

Interneurons are found in all areas of the spinal cord gray matter. These neurons are quite numerous, small, and highly excitable; they have many interconnections. They receive input from higher levels of the CNS as well as from sensory neurons entering the CNS through the spinal nerves. Many interneurons in the spinal cord synapse with motor neurons in the ventral horn. These interconnections are responsible for the integrative functions of the spinal cord including reflexes.

Afferent neurons that transmit sensory information toward the spinal cord are referred to as first-order sensory neurons. The cell bodies of these neurons are found in the dorsal root ganglia. These ganglia form a swelling in each of the dorsal roots just outside the spinal cord. The portion of the axon between the distal receptor and the cell body is referred to as the peripheral axon and the portion of the axon between the cell body and the axon terminal within the CNS is referred to as the central axon.

Upon entering the spinal cord, the first-order sensory neurons may enter the gray matter and may then synapse with one or more of the following neurons:

• Second-order sensory neuron that transmits impulses to higher levels of the CNS

• Alpha motor neuron that transmits impulses to skeletal muscles

• Interneurons that transmit impulses to motor neurons

Synapses between first-order sensory neurons and alpha motor neurons, either directly or by way of interneurons, result in spinal cord reflexes. Reflexes are discussed in more detail in a subsequent section in this chapter.

Alternatively, the first-order sensory neurons may initially enter the white matter of the spinal cord. In this case, the axons of these neurons may ascend the cord to the medulla or travel up or down the cord to a different spinal segment. Upon reaching its destination, the axon then enters the gray matter of the spinal cord and synapses with one or more of the neurons discussed previously.

7.3.2 White matter

The white matter of the spinal cord consists of myelinated axons of neurons. These axons may travel up the spinal cord to a higher spinal segment or to the brain. On the other hand, they may travel down the spinal cord to a lower spinal segment. The axons of neurons that carry similar types of impulses are bundled together to form tracts. Ascending tracts carry sensory information from the spinal cord toward the brain. Descending tracts carry motor impulses from the brain toward the motor neurons in the lateral or ventral horns of the spinal cord gray matter. In general, these tracts are named according to their origin and termination. For example, the ventral spinocerebellar tract is an ascending tract carrying information regarding unconscious muscle sense (proprioception) from the spinal cord to the cerebellum. On the other hand, the ventral corticospinal tract is a descending tract carrying information regarding voluntary muscle control from the cerebral cortex to the spinal cord.

Ascending tracts. These tracts contain three successive neurons:

• First-order neurons

• Second-order neurons

• Third-order neurons

As discussed, the first-order neuron is the afferent neuron that transmits impulses from a peripheral receptor toward the CNS. Its cell body is located in the dorsal root ganglion. This neuron synapses with the second-order neuron whose cell body is located in the dorsal horn of the spinal cord or in the medulla of the brainstem. The second-order neuron travels upward and synapses with the third-order neuron, whose cell body is located in the thalamus. Limited processing of sensory information takes place in the thalamus. Finally, the third-order neuron travels upward and terminates in the soma-tosensory cortex where more complex, cortical processing begins.

All ascending tracts cross to the opposite side of the CNS. For example, sensory input entering the left side of the spinal cord ultimately terminates on the right side of the cerebral cortex. These tracts may cross — at the level of entry into the spinal cord; a few segments above the level of entry; or within the medulla of the brainstem. The locations of specific ascending tracts are illustrated in Figure 7.2 and a summary of their functions is found in Table 7.1.

DORSAL SURFACE

ASCENDING TRACTS

Fasciculus gracilis Fasciculus cuneatus Dorsal spinocerebellar Ventral spinocerebellar Lateral spinothalamic Ventral spinothalamic

DORSAL SURFACE

ASCENDING TRACTS

Fasciculus gracilis Fasciculus cuneatus Dorsal spinocerebellar Ventral spinocerebellar Lateral spinothalamic Ventral spinothalamic

DESCENDING TRACTS

Lateral corticospinal Rubrospinal Ventral corticospinal Vestibulospinal

VENTRAL SURFACE

Figure 7.2 Ascending and descending tracts in white matter of the spinal cord. Tracts are formed of bundles of neuronal axons that transmit similar types of information.

DESCENDING TRACTS

Lateral corticospinal Rubrospinal Ventral corticospinal Vestibulospinal

VENTRAL SURFACE

Figure 7.2 Ascending and descending tracts in white matter of the spinal cord. Tracts are formed of bundles of neuronal axons that transmit similar types of information.

Table 7.1 Ascending and Descending Tracts in White Matter of the Spinal Cord

Ascending pathway

Fasciculus gracilis

Fasciculus cuneatus

Dorsal spinocerebellar

Ventral spinocerebellar Lateral spinothalamic Ventral spinothalamic

Descending pathway

Lateral corticospinal Rubrospinal

Ventral corticospinal Vestibulospinal

Function

Fine touch discrimination (ability to recognize size, shape, and texture of objects and their movement across the skin); proprioception; vibration from legs and lower trunk; crossed Fine touch discrimination; proprioception; vibration from neck, arms, upper trunk; crossed Proprioception (important for muscle tone and posture); uncrossed Proprioception; crossed Pain; temperature; crossed Light touch; pressure; crossed

Function

Voluntary control of skeletal muscles; crossed Originates in brainstem; subconscious control of skeletal muscle (muscle tone, posture); crossed Voluntary control of skeletal muscles; uncrossed Originates in brainstem; subconscious control of skeletal muscle (muscle tone, balance, equilibrium); uncrossed

Pharmacy application: spinal anesthesia

Injecting a local form of anesthetic into the cerebrospinal fluid surrounding the spinal cord causes spinal anesthesia. This injection is made below the level of the second lumbar vertebra in order to minimize direct nerve trauma. Spinal anesthesia is effective in the control of pain during lower body surgical procedures, such as knee surgery. Currently, the drugs most commonly used in the U.S. include lidocaine, bupivacaine, and tetracaine. The choice of anesthetic is determined by the duration of anesthesia required: lidocaine is used for short procedures; bupivacaine is chosen for procedures of intermediate length; and tetracaine is used for long-duration procedures.

The mechanism of action of these anesthetics involves the blockade of sodium channels in the membrane of the second-order sensory neuron. The binding site for these anesthetics is on a subunit of the sodium channel located near the internal surface of the cell membrane. Therefore, the agent must enter the neuron in order to block the sodium channel effectively. Without the influx of sodium, neurons cannot depolarize and generate an action potential, so the second-order sensory neuron cannot be stimulated by impulses elicited by pain receptors associated with the first-order sensory neuron. In other words, the pain signal is effectively interrupted at the level of the spinal cord and does not travel any higher in the CNS. In this way, the brain does not perceive pain.

Interestingly, second-order sensory neurons are neurons of the spinal cord gray matter most susceptible to the effects of spinal anesthesia. These neurons have a small diameter and are unmy-elinated. The small diameter allows the drug to locate its binding site on the sodium channel more readily due to a smaller volume of distribution of drug within the neuron. Furthermore, unmyeli-nated neurons have a greater number of sodium channels located over a larger surface area. Alpha motor neurons in the ventral horn are susceptible to these anesthetics only at high doses because alpha motor neurons have a large diameter and are myeli-nated. The larger diameter results in a larger volume of distribution of the drug within the neuron. Myelination limits the number and availability of sodium channels upon which the anesthetic can exert its effect.

Descending tracts. Voluntary movement of skeletal muscles is controlled by two types of descending tracts. Neurons in these tracts terminate on and influence activity of alpha motor neurons in the ventral horn. The two types of tracts include:

• Corticospinal (pyramidal) tracts

• Multineuronal (extrapyramidal) tracts

The corticospinal tracts originate in the cerebral cortex. Neurons of the primary motor cortex are referred to as pyramidal cells. Most of these neurons' axons descend directly to the alpha motor neurons in the spinal cord. In other words, these are primarily monosynaptic pathways. This type of synaptic connection is particularly important for the movement of individual fingers. A primary function of these tracts is to regulate fine, discrete, voluntary movements of the hands and fingers. The multineuronal tracts originate in many regions of the brain, including the motor regions of the cerebral cortex, the cerebellum, and the basal ganglia. Impulses from these various regions are transmitted to nuclei in the brainstem, in particular the reticular formation and vestibular nuclei. The axons of neurons in these nuclei descend to the alpha motor neurons in the spinal cord. The multineuronal tracts regulate overall body posture. Specifically, these tracts control subconscious movements of large muscle groups in the trunk and limbs.

These two types of descending motor tracts do not function in isolation. They are extensively interconnected and cooperate in the control of movement. For example, in order to grasp a doorknob to open a door, there is subconscious positioning of the body to face the door and extend an arm toward the doorknob.

As with the ascending tracts, descending tracts cross from one side of the CNS to the other. Most of the tracts cross over in the medulla of the brainstem. Therefore, the right side of the brain influences the activity of the alpha motor neurons and thus the skeletal muscles on the left side of the body. The locations of specific descending tracts are illustrated in Figure 7.2 and a summary of their functions is found in Table 7.1.

Pharmacy application: epidural anesthesia

Epidural anesthesia is administered by injecting local anesthetic into the epidural space. Located outside the spinal cord on its dorsal surface, the epidural space contains fat and is highly vascular. Therefore, this form of anesthesia can be performed safely at any level of the spinal cord. Furthermore, a catheter may be placed into the epidural space, allowing for continuous infusions or repeated bolus administrations of anesthetic.

The primary site of action of epidurally administered agents is on the spinal nerve roots. As with spinal anesthesia, the choice of drug to be used is determined primarily by the duration of anesthesia desired. However, when a catheter has been placed, short-acting drugs can be administered repeatedly. Bupivacaine is typically used when a long duration of surgical block is needed. Lidocaine is used most often for intermediate length procedures; chloroprocaine is used when only a very short duration of anesthesia is required.

An important difference between epidural anesthesia and spinal anesthesia is that agents injected into the epidural space may readily enter the blood due to the presence of a rich venous plexus in this area. This is an important consideration when epidural anesthesia is used to control pain during labor and delivery. The agents used are able to cross the placenta, enter the fetal circulation, and exert a depressant effect on the neonate.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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