The dorsal column system begins with the axon of a dorsal root ganglion neuron entering the dorsal root and joining the dorsal column white matter in the spinal cord. The sensory pathways in each of these systems are composed of three successive neurons. The dorsal column system (sometimes referred to as the dorsal column–medial lemniscus) and the spinothalamic tract are two major pathways that bring sensory information to the brain ( Figure 14.5.1). Somatosensory stimuli from below the neck pass along the sensory pathways of the spinal cord, whereas somatosensory stimuli from the head and neck travel through the cranial nerves-specifically, the trigeminal system. However, the somatosensory pathways are divided into two separate systems on the basis of the location of the receptor neurons. Tactile and other somatosensory stimuli activate receptors in the skin, muscles, tendons, and joints throughout the entire body. The various sensory modalities each follow specific pathways through the CNS. Spinal Cord and Brain StemĪ sensory pathway that carries peripheral sensations to the brain is referred to as an ascending pathway, or ascending tract. The important regions of the CNS that play a role in somatic processes can be separated into the spinal cord brain stem, diencephalon, cerebral cortex, and subcortical structures. More complex arrangements are possible to integrate peripheral sensory information with higher processes. A simple case is a reflex caused by a synapse between a dorsal sensory neuron axon and a motor neuron in the ventral horn.

Specific regions of the CNS coordinate different somatic processes using sensory inputs and motor outputs of peripheral nerves. The general senses of somatosensation for the face travel through the trigeminal system. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Cranial NervesĬranial nerves convey specific sensory information from the head and neck directly to the brain. Typically, spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. This will help to underscore the relationships between the different components. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons.

As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. Describe several reflex arcs and their functional roles.Explain the initiation of movement from the neurological connections.Compare different descending pathways, both by structure and function.

Describe the pathway of descending motor commands from the cortex to the skeletal muscles.

List the components of the basic processing stream for the motor system.

Explain topographical representations of sensory information in at least two systems.

Describe the pathway of somatosensory input from the face and compare it to the ascending pathways in the spinal cord.

Differentiate between the two major ascending pathways in the spinal cord.

Describe the pathways that sensory systems follow into the central nervous system.

By the end of this section, you will be able to:

Example of somatic nervous system