Midbrain, hindbrain, spinal cord

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3 Midbrain, hindbrain, spinal cord

Study Guidelines

2 Box 3.1 deserves special attention, because it indicates why certain pathways cross the midline and others do not. The brainstem crossings are formally addressed in Chapters 15 and 16.

Box 3.1 Four decussations

image

Figure Box 3.1.1 (B) Afferents. The left parietal lobe constructs a map of the right hand in relation to the mouse, based on information sent to the left somatic sensory cortex (postcentral gyrus) from the skin and deep tissues. The information is relayed by three successive sets of neurons from the skin and by another set of three from the deep tissues. The first set in each case is composed of first-order or primary afferent neurons. These neurons are called unipolar, because each axon emerges from a single point (or pole) of the cell body and divides in a T-shaped manner to provide continuity of impulse conduction from tissue to central nervous system. The primary afferent neurons terminate by forming contacts known as synapses on the multipolar (more or less star-shaped) cells of the second-order (secondary) set. The axons of the second-order neurons project across the midline before turning up to terminate on third-order (tertiary) multipolar neurons projecting to the postcentral gyrus.

Primary afferents activated by contacts with the skin of the hand (S1) terminate in the posterior horn of the gray matter of the spinal cord. Second-order cutaneous afferents (S2) cross the midline in the anterior white commissure and ascend to the thalamus within the spinothalamic tract (STT), to be relayed by third-order neurons to the hand area of the sensory cortex.

The most significant deep tissue sensory organs are neuromuscular spindles (muscle spindles) contained within skeletal muscles. The primary afferents supplying the muscle spindles of the intrinsic muscles of the hand belong to large unipolar neurons whose axons (labeled M1) ascend ipsilaterally (on the same side of the spinal cord) within the posterior funiculus, as already seen in Figure 3.5. They synapse in the nucleus cuneatus in the medulla oblongata. The multipolar second-order neurons send their axons across the midline in the sensory decussation (seen in Figure 3.6).The axons ascend (M2) through pons and midbrain before synapsing on third-order neurons (M3) projecting from thalamus to sensory cortex.

PCML, posterior column–medial lemniscal pathway.

The midbrain connects the diencephalon to the hindbrain. As explained in Chapter 1, the hindbrain is made up of the pons, medulla oblongata, and cerebellum. The medulla oblongata joins the spinal cord within the foramen magnum of the skull.

In this chapter, the cerebellum (part of the hindbrain) is considered after the spinal cord, for the sake of continuity of motor and sensory pathway descriptions.

Brainstem

Ventral view (Figures 3.1 and 3.2A)

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