Introduction Millions of sensory neurons are delivering information to the CNS all the time Millions of motor neurons are causing the body to respond in a variety of ways Sensory and motor neurons travel by different tracts within the spinal cord 2

Organization and Patterns of Spinal Cord Tracts Communication involves sensory and motor tracts Ascending tract Sensory (delivers information to the brain) Descending tract Motor (delivers information to the periphery) All tracts involve the spinal cord and the brain 3

Organization and Patterns of Spinal Cord Tracts Naming a tract often involves its origin and destination Spinocerebellar tract Origin is spinal cord and destination is cerebellum Spinothalamic tract Origin is spinal cord and destination is the thalamus 4

Organization and Patterns of Spinal Cord Tracts Naming a tract often involves its origin and destination Corticospinal tract Origin is cerebral cortex and destination is spinal cord Vestibulospinal tract Origin is vestibular nuclei and destination is spinal cord 5

Sensory Tracts There are three major sensory tracts The posterior column The spinothalamic tract The spinocerebellar tract All three of these tracts are also called somatosensory tracts 6

Posterior spinocerebellar tract Figure 15.1 A Cross-Sectional View Indicating the Locations of the Major Ascending (Sensory) Tracts in the Spinal Cord Fasciculus gracilis Posterior columns Fasciculus cuneatus Dorsal root Dorsal root ganglion Posterior spinocerebellar tract Anterior spinocerebellar tract Ventral root Lateral spinothalamic tract Anterior spinothalamic tract © 2015 Pearson Education, Inc.

Sensory Tracts Posterior Column consists of: Fasciculus gracilis Transmits information to the cerebrum coming from areas inferior to T6 Fasciculus cuneatus Transmits information to the cerebrum coming from areas superior to T6 8

Sensory Tracts Posterior Columns (dorsal columns) Transmits information such as: Proprioception Fine touch Pressure Vibration sensations 9

© 2015 Pearson Education, Inc. Figure 15.3a The Posterior Column, Spinothalamic, and Spinocerebellar Sensory Tracts Posterior Columns Ventral nuclei in thalamus Midbrain Medial lemniscus Nucleus gracilis and nucleus cuneatus Medulla oblongata Fasciculus cuneatus and fasciculus gracilis Dorsal root ganglion Fine-touch, vibration, pressure, and proprioception sensations from right side of body a The posterior columns deliver fine-touch, vibration, and proprioception information to the primary sensory cortex of the cerebral hemisphere on the opposite side of the body. The crossover occurs in the medulla, after a synapse in the nucleus gracilis or nucleus cuneatus. © 2015 Pearson Education, Inc.

Sensory Tracts The Spinothalamic Tract Transmits information such as: Pain Temperature Crude sensations of touch and pressure 11

Anterior Spinothalamic Tract Figure 15.3b The Posterior Column, Spinothalamic, and Spinocerebellar Sensory Tracts Anterior Spinothalamic Tract A Sensory Homunculus A sensory homunculus (“little human”) is a functional map of the primary sensory cortex. The proportions are very different from those of the individual because the area of sensory cortex devoted to a particular body region is proportional to the number of sensory receptors it contains. Midbrain Medulla oblongata Anterior spinothalamic tract KEY Axon of first-order neuron Second-order neuron Crude touch and pressure sensations from right side of body Third-order neuron b The anterior spinothalamic tract carries crude touch and pressure sensations to the primary sensory cortex on the opposite side of the body. The crossover occurs in the spinal cord at the level of entry. © 2015 Pearson Education, Inc.

Lateral Spinothalamic Tract Figure 15.3c The Posterior Column, Spinothalamic, and Spinocerebellar Sensory Tracts Lateral Spinothalamic Tract Midbrain Medulla oblongata Lateral spinothalamic tract Spinal cord Axon of first-order neuron Second-order neuron Pain and temperature sensations from right side of body Third-order neuron c The lateral spinothalamic tract carries sensations of pain and temperature to the primary sensory cortex on the opposite side of the body. The crossover occurs in the spinal cord, at the level of entry. © 2015 Pearson Education, Inc.

Sensory Tracts The Spinocerebellar Tracts Transmits information such as: Proprioception concerning muscles, tendons, and joints 14

Spinocerebellar Tracts Figure 15.3d The Posterior Column, Spinothalamic, and Spinocerebellar Sensory Tracts Spinocerebellar Tracts PONS Cerebellum Medulla oblongata Spinocerebellar tracts Anterior spinocerebellar tract Posterior spinocerebellar tract Spinal cord Proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsules d The spinocerebellar tracts carry proprioceptive information to the cerebellum. (Only one tract is detailed on each side, although each side has both tracts.) © 2015 Pearson Education, Inc.

Sensory Tracts The three major sensory tracts involve chains of neurons First-order neuron Delivers sensations from the periphery to the CNS Second-order neuron Decussates to the contralateral side and ascends to the thalamus Third-order neuron Transmits information from the thalamus to the cerebral cortex 16

Figure 15.2 Organization of Spinal Cord Tracts (1 of 4) First-Order, Second-Order, and Third-Order Neurons in Ascending Tracts Sensory tracts that deliver somatic sensory information to the sensory cortex of the cerebral or cerebellar hemispheres involve a chain of neurons. Sensory homunculus on primary sensory cortex of left cerebral hemisphere Cerebral Sensory Cortex KEY First-order neuron Second-order neuron Third-order neuron Thalamus 3 Third-Order Neuron In tracts ending at the cerebral cortex, the second-order neuron synapses on a third-order neuron in the thalamus. The axon of the third-order neuron carries the sensory information from the thalamus to the appropriate sensory area of the cerebral cortex. In most cases, the axon of either the first-order or second-order neuron crosses over to the opposite side of the spinal cord or brain stem as it ascends. As a result of this crossover, or decussation, sensory information from the left side of the body is delivered to the right side of the brain, and vice versa. Brain stem 2 Second-Order Neuron The axon of the first-order neuron synapses on a second-order neuron. The second-order neuron’s cell body may be located in either the spinal cord or the brain stem. Sensory tract in spinal cord 1 First-Order Neuron Dorsal root ganglion A first-order neuron is the sensory neuron that delivers the sensations to the CNS; its cell body is in a dorsal root ganglion or a cranial nerve ganglion. Somatic sensations from right side of the body © 2015 Pearson Education, Inc.

Sensory Tracts Neurons in the sensory tracts are arranged according to three anatomical principles Sensory modality Medial–lateral rule Somatotropic 18

Sensory Tracts Sensory Modality Medial–Lateral Rule Somatotopic Fine touch sensations are carried in one sensory tract Medial–Lateral Rule Sensory neurons that enter a low level of the spinal cord are more medial within the spinal cord Sensory neurons that enter at a higher level of the spinal cord are more lateral within the spinal cord Somatotopic Ascending tracts are arranged according to the site of origin 19

Figure 15.2 Organization of Spinal Cord Tracts (2 of 4) Neuron Arrangement within Sensory Tracts Neurons within the sensory tracts are not randomly arranged. Rather they are segregated, or arranged according to at least three anatomical principles: MEDIAL LATERAL 2 Medial-lateral rule Most sensory nerves entering the spinal cord at more inferior levels travel more medially within a sensory tract than sensory nerves entering the cord at a more superior level. For instance, a sensory nerve that enters the cord at T11 (11th) thoracic spinal nerve) is located more medially within a sensory tract than a nerve that enters at C4. Lower limb Upper limb Hip Trunk 1 Sensory modality arrangement Sensory fibers are arranged within the spinal cord according to the type of sensory information carried by the individual neurons. In other words, information dealing with fine touch will be carried within one sensory tract, while information dealing with pain will be carried within another. Sensory fibers carrying fine touch, pressure, proprioception, and vibration Sensory fibers carrying pain and temperature 3 Somatotopic arrangement Ascending sensory fibers are arranged within individual tracts according to their site of origin within the body. Sensory fibers coming from a particular region of the body, such as the upper limb, form a tract containing organized bundles of sensory fibers from the hand, wrist, forearm, and arm. Sensory fibers carrying crude touch © 2015 Pearson Education, Inc.

Motor Tracts Motor Tracts CNS transmits motor commands in response to sensory information These are descending tracts Motor commands are delivered by the: Somatic nervous system (SNS) Directs contraction of skeletal muscles Autonomic nervous system (ANS) Directs the activity of glands, smooth muscles, and cardiac muscle 21

Figure 15.4a Motor Tracts in the CNS and PNS Upper-motor neurons in primary motor cortex BRAIN Somatic motor nuclei of brain stem Skeletal muscle SPINAL CORD Lower-motor neurons Somatic motor nuclei of spinal cord Skeletal muscle a In the somatic nervous system (SNS), an upper-motor neuron in the CNS controls a lower-motor neuron in the brain stem or spinal cord. The axon of the lower-motor neuron has direct control over skeletal muscle fibers. Stimulation of the lower-motor neuron always has an excitatory effect on the skeletal muscle fibers. © 2015 Pearson Education, Inc.

Figure 15.4b Motor Tracts in the CNS and PNS Visceral motor nuclei in hypothalamus BRAIN Preganglionic neuron Visceral Effectors Smooth muscle Autonomic nuclei in brain stem Autonomic ganglia Glands Ganglionic neurons Cardiac muscle SPINAL CORD Adipocytes Autonomic nuclei in spinal cord Preganglionic neuron b In the autonomic nervous system (ANS), the axon of a preganglionic neuron in the CNS controls ganglionic neurons in the periphery. Stimulation of the ganglionic neurons may lead to excitation or inhibition of the visceral effector innervated. © 2015 Pearson Education, Inc.

Motor Tracts There are two principal motor tracts Corticospinal tract: Conscious control of skeletal muscles Corticobulbar, lateral corticospinal, and anterior corticospinal tracts Subconscious tract: Subconscious regulation of balance, muscle tone, eye, hand, and upper limb position Vestibulospinal, tectospinal, reticulospinal, and rubrospinal tracts 24

Motor Tracts The Corticospinal Tracts Consists of three pairs of descending tracts Corticobulbar tracts Conscious control over eye, jaw, and face muscles Lateral corticospinal tracts Conscious control over skeletal muscles Anterior corticospinal tracts 25

Figure 15.5 The Corticospinal Tracts Motor homunculus on primary motor cortex of left cerebral hemisphere KEY Cerebral Sensory Cortex Axon of upper-motor neuron Lower-motor neuron Corticobulbar tract To skeletal muscles Mesencephalon Cerebral peduncle Motor nuclei of cranial nerves To skeletal muscles Medulla oblongata Decussation of pyramids Pyramids Lateral corticospinal tract Anterior corticospinal tract To skeletal muscles Spinal cord © 2015 Pearson Education, Inc.

Figure 15.2 Organization of Spinal Cord Tracts (2 of 4) Neuron Arrangement within Sensory Tracts Neurons within the sensory tracts are not randomly arranged. Rather they are segregated, or arranged according to at least three anatomical principles: MEDIAL LATERAL 2 Medial-lateral rule Most sensory nerves entering the spinal cord at more inferior levels travel more medially within a sensory tract than sensory nerves entering the cord at a more superior level. For instance, a sensory nerve that enters the cord at T11 (11th) thoracic spinal nerve) is located more medially within a sensory tract than a nerve that enters at C4. Lower limb Upper limb Hip Trunk 1 Sensory modality arrangement Sensory fibers are arranged within the spinal cord according to the type of sensory information carried by the individual neurons. In other words, information dealing with fine touch will be carried within one sensory tract, while information dealing with pain will be carried within another. Sensory fibers carrying fine touch, pressure, proprioception, and vibration Sensory fibers carrying pain and temperature 3 Somatotopic arrangement Ascending sensory fibers are arranged within individual tracts according to their site of origin within the body. Sensory fibers coming from a particular region of the body, such as the upper limb, form a tract containing organized bundles of sensory fibers from the hand, wrist, forearm, and arm. Sensory fibers carrying crude touch © 2015 Pearson Education, Inc.

Motor Tracts The Subconscious Motor Tracts Consists of four tracts involved in monitoring the subconscious motor control Vestibulospinal tracts Tectospinal tracts Reticulospinal tracts Rubrospinal tracts 28

Motor Tracts The Subconscious Motor Tracts Vestibulospinal tracts Send information from the inner ear to monitor position of the head Vestibular nuclei respond by altering muscle tone, neck muscle contraction, and limbs for posture and balance 29

Motor Tracts The Subconscious Motor Tracts Tectospinal tracts Send information to the head, neck, and upper limbs in response to bright lights and sudden movements and loud noises The tectum area consists of superior and inferior colliculi Superior colliculi: receives visual information Inferior colliculi: receives auditory information 30

Motor Tracts The Subconscious Motor Tracts Reticulospinal tracts Send information to cause eye movements and activate respiratory muscles Rubrospinal tracts Send information to the flexor and extensor muscles 31

Figure 15.6 Nuclei of Subconscious Motor Pathways Motor cortex Basal Nuclei Thalamus Caudate nucleus Superior colliculus Putamen Globus pallidus Inferior colliculus Red nucleus Cerebellar nuclei Tectum Reticular formation Pons Vestibular nucleus Medulla oblongata © 2015 Pearson Education, Inc.

Figure 15.2 Organization of Spinal Cord Tracts (4 of 4) Descending Motor Tracts Conscious and subconscious motor commands control skeletal muscles through descending motor tracts within the spinal cord. Motor Tracts Carrying Subconscious Motor Commands Motor Tracts Carrying Conscious Motor Commands The rubrospinal tract automatically adjusts upper limb position and muscle tone during voluntary movements. The anterior corticospinal tract contains axons that do not cross over (decussate) in the brain stem. The reticulospinal tract originates in the reticular formation, a diffuse network of neurons in the brain stem. The functions of the tract vary depending on which area of the reticular formation is stimulated. The lateral corticospinal tract contains the motor fibers that decussate within the brain stem. The tectospinal tract controls reflexive changes in position in response to auditory or visual stimuli. The vestibulospinal tract carries motor commands that reflexively control posture and balance. © 2015 Pearson Education, Inc.

Levels of Somatic Motor Control Somatic motor control involves: Cerebral cortex Basal nuclei Cerebellum Hypothalamus Pons Medulla oblongata Brain stem and spinal cord Thalamus 34

Figure 15.7a Somatic Motor Control BASAL NUCLEI CEREBRAL CORTEX Modify voluntary and reflexive motor patterns at the subconscious level Plans and initiates voluntary motor activity HYPOTHALAMUS THALAMUS AND MESENCEPHALON Controls stereotyped motor patterns related to eating, drinking, and sexual activity; modifies respiratory reflexes Control reflexes in response to visual and auditory stimuli PONS AND SUPERIOR MEDULLA OBLONGATA CEREBELLUM Coordinates complex motor patterns Control balance reflexes and more-complex respiratory reflexes BRAIN STEM AND SPINAL CORD INFERIOR MEDULLA OBLONGATA Control simple cranial and spinal reflexes Controls basic respiratory reflexes a Somatic motor control involves a series of levels, with simple spinal and cranial reflexes at the bottom and complex voluntary motor patterns at the top. © 2015 Pearson Education, Inc.

Levels of Somatic Motor Control Summary of Somatic Motor Control Cerebral cortex initiates voluntary movement Information goes to the basal nuclei and cerebellum These structures modify and coordinate the movements so they are performed in a smooth manner 36

Figure 15.7b Somatic Motor Control Motor association areas Cerebral cortex Decision in frontal lobes Basal nuclei Cerebellum b The planning stage: When a conscious decision is made to perform a specific movement, information is relayed from the frontal lobes to motor association areas. These areas in turn relay the information to the cerebellum and basal nuclei. © 2015 Pearson Education, Inc.

Levels of Somatic Motor Control Summary of Somatic Motor Control Information goes from: The basal nuclei and cerebellum constantly monitor position and muscle tone Information goes back to: The cerebral cortex Reason: Constantly monitor position and muscle tone 38