Pathways of the Nervous System CNS communicates with body structures via pathways. sensory or motor information
processing and integration occur continuously Pathways travel through the white matter of the spinal cord. Connect various CNS regions with peripheral nerves. 17-1 Pathways of the Nervous System
Consists of a tract and a nucleus. Tracts are groups or bundles of axons that travel together in the CNS and connects two masses of gray matter. Each tract may work with multiple nuclei groups in the CNS. A nucleus is a collection of neuron cell bodies located within the CNS. 17-2
Nervous System Pathways Ascending pathways carry sensory information from the peripheral body to the brain
Descending pathways transmit motor information from the brain or brainstem to muscles or glands Pathway crosses over from one side of the body to the other side at some point in its travels. The left side of the brain processes information from the right side of the body, and vice versa. 17-3 Nervous System Pathways Most exhibit a precise correspondence
between a specific area of the body and a specific area of the CNS. . 17-4 Nervous System Pathways
All pathways are composed of paired tracts. A pathway on the left side of the CNS has a matching tract on the right side of the CNS. Both left and right tracts are needed to innervate both the left and right sides of the body. Pathways are composed of a series of two or three neurons that work together. 17-5 Nervous System Pathways
Sensory pathways have primary neurons, secondary neurons, and sometimes tertiary neurons that facilitate the pathways functioning Motor pathways
use an upper motor neuron and a lower motor neuron the cell bodies are located in the nuclei associated with each pathway 17-6 Nervous System Pathways Sensory pathways
Somatosensory pathways conduct information about limb position and the sensations of touch, temperature, pressure, and pain process stimuli received from receptors within the skin, muscles, and joints Viscerosensory pathways
process stimuli received from the viscera 17-7 Sensory Receptors Detect stimuli and then conduct nerve impulses to the CNS Sensory pathway centers within either the spinal cord or brainstem process and filter the
incoming sensory information. They determine whether the incoming sensory stimulus should be transmitted to the cerebrum or terminated. More than 99% of incoming impulses do not reach the cerebral cortex and our conscious awareness. 17-8
Primary (First-Order) Neuron Sensory pathways utilize a series of two or three neurons to transmit stimulus information from the body periphery to the brain. The first order neuron has the cell bodies reside in the posterior root ganglia of spinal nerves or the sensory ganglia of cranial nerves. 17-9
Secondary (Second-Order) Neuron Formed by the axon of the cells in the spinal gray matter. The cell body resides within either the posterior horn of the spinal cord or a brainstem nucleus. The axon projects to the thalamus, where it synapses
with the tertiary neuron. 17-10 Tertiary (Third-Order) Neuron Located in the thalamus. Carry the sensations to the cerebral cortex. 17-11 Lateral Spinothalamic
Tract Red Blue 1
2 Green 3 17-12 Lateral Spinothalamic Tract Red
Blue 1 2 Green
3 17-13 Anterior Spinothalamic Tract Red:
1 Blue 2 17-14 Somatic Pathways 17-15midline Figure 10-9: Sensory pathways cross the bodys
Medulla and Pons Olive 17-16 = 17-17 17-18 17-19 17-20
Demonstrates that the area of the cortex dedicated to the sensations of various body parts is proportional to how sensitive that part of the body is. 17-23 17-24
25 Anterolateral Pathway Located in the anterior and lateral white funiculi of the spinal cord.
anterior spinothalamic tract lateral spinothalamic tract Axons projecting from primary neurons enter the spinal cord and synapse on secondary neurons within the posterior horns. Axons entering these pathways conduct stimuli related to crude touch and pressure as well as pain and temperature. Axons of the secondary neurons cross over and relay stimulus information to the opposite side of the spinal cord before ascending toward the brain. 17-26
mechanosensory system 17-34 Dorsal Column Ascending Pathway Carries sensations related to
discriminative touch, visceral pain, vibration, and proprioception 1st order neuron - detects stimulus Fasciculus gracilis Fasciculus cuneatus
Carries sensation from below T6 Carries sensation from T6 or higher 2nd order neuron synapses with 1st in medulla and decussates 3rd order neuron synapses with 2nd in thalamus and carries signal to cerebral cortex (postcentral gyrus) System is contralateral
17-35 Spinocerebellar Pathway Conducts proprioceptive information to the cerebellum for processing to coordinate body movements. Composed of anterior and posterior spinocerebellar tracts.
the major routes for transmitting postural input to the cerebellum Sensory input is critical for regulation of posture and balance and coordination of skilled movements. These are different from the other sensory pathways in that they do not use tertiary neurons. they only have primary and secondary neurons
17-36 Spinocerebellar Pathway 1st order neurons originate in muscles and tendons 2nd order neurons ascend in ipsilateral lateral column
Terminate in cerebellum (a large motor control are of the brain) Transmit proprioceptive signals from limbs and trunk 17-37 38
39 Motor Pathways Descending pathways in the brain and spinal cord that control the activities of skeletal muscle. Formed from the cerebral nuclei, the cerebellum, descending projection tracts, and motor neurons. Regulate the activities of skeletal muscle.
17-40 Corticobulbar Tracts Originate from the facial region of the motor homunculus within the primary motor cortex. Axons extend to the brainstem, where they synapse with lower motor neuron cell bodies
that are housed within brainstem cranial nerve nuclei. Axons of these lower motor neurons help form the cranial nerves. 17-41 Corticobulbar Tracts Transmit motor information to control:
eye movements (via CN III, IV, and VI) cranial, facial, pharyngeal, and laryngeal muscles (via CN V, VII, IX, and X) some superficial muscles of the back and neck (via CN XI) intrinsic and extrinsic tongue muscles (via CN XII) 17-42 Corticospinal Tracts
Descend from the cerebral cortex through the brainstem and form a pair of thick bulges in the medulla called the pyramids. Continue into the spinal cord to synapse on lower motor neurons in the anterior horn of the spinal cord. 17-43 44
Indirect Pathway Several nuclei within the mesencephalon initiate motor commands for activities that occur at an unconscious level. Nuclei and their associated tracts. Cell bodies of its upper motor neurons are located
within brainstem nuclei. Axons take a complex, circuitous route before finally conducting the motor impulse into the spinal cord. 17-45 46 Indirect Motor Pathways in the Spinal Cord
Originate from neurons housed within the brainstem. Muscular activity localized within the head, limbs, and trunk of the body. Multisynaptic. Exhibit a high degree of complexity. 17-47 Role of the Cerebral Nuclei
Receive impulses from the entire cerebral cortex, including the motor, sensory, and association cortical areas, as well as input from the limbic system. Most of the output goes to the primary motor cortex. Do not exert direct control over lower motor
neurons. Provide the patterned background movements needed for conscious motor activities by adjusting the motor commands issued in other nuclei. 17-48 Somatic Motor Control
Several regions of the brain participate in the control of motor activities. Motor programs require conscious directions from the frontal lobes. Movement is initiated when commands are received by the primary motor cortex from the motor association areas. The cerebellum is critically important in coordinating movements because it specifies the exact timing of control signals to different muscles. 17-49
Levels of Processing and Motor Control Simple reflexes that stimulate motor neurons represent the lowest level of motor control.
The nuclei controlling these reflexes are located in the spinal cord and the brainstem. Brainstem nuclei also participate in more complex reflexes. Initiate motor responses to control motor neurons directly. Oversee the regulation of reflex centers elsewhere in the brain. 17-50 Cerebral Cortex
Control highly variable and complex voluntary motor patterns. Occupy the highest level of processing and motor control. Motor commands may be conducted to specific motor neurons directly. May be conveyed indirectly by altering the activity of a reflex control center.
consciousness, learning, memory, and reasoning involve multiple brain regions connected by complicated networks and arrays of axons conscious and unconscious processing of information are involved in higher-order mental functions may be continually adjusted or modified 17-52 Cerebral Lateralization
Each hemisphere tends to be specialized for certain tasks. Higher-order centers in both hemispheres tend to have different but complementary functions. 17-53 Cerebral Lateralization Left hemisphere is the categorical hemisphere
and it functions in categorization and symbolization. contains Wernickes area and the motor speech area specialized for language abilities important in performing sequential and analytical reasoning tasks (science and mathematics)
appears to direct or partition information into smaller fragments for analysis Speech-dominant hemisphere. controls speech in almost all right-handed people as well as in many left-handed ones 17-54 Cerebral Lateralization Right hemisphere is called the representational
hemisphere. concerned with visuospatial relationships and analyses the seat of imagination and insight, musical and artistic skill, perception of patterns and spatial relationships, and comparison of sights, sounds, smells, and tastes Both cerebral hemispheres remain in constant communication through commissures, especially the corpus callosum, which contains hundreds of millions of
axons that project between the hemispheres. 17-55 56 57 58
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