Laboratory Exercise 14 Nervous System Spinal Cord Spinal

Nervous System – Spinal Cord & Spinal Nerves • In this exercise, you will become familiar with the structures of the spinal cord and spinal nerves, and understand their general functions • These terms should be familiar to you: CNS, PNS, brain, spinal cord, cranial/spinal nerves, gray and white matter, rami, meninges, plexus • This laboratory exercise correlates with Chapter 12, 13 and 14 of your textbook; Exercise 14 of your laboratory manual © 2012 Pearson Education, Inc.

Laboratory Lecture © 2012 Pearson Education, Inc.

An Introduction to the Spinal Cord, Spinal Nerves and Spinal Reflexes • Spinal reflexes are rapid, automatic nerve responses triggered by specific stimuli • They are controlled by the spinal cord; they are not controlled by the brain © 2012 Pearson Education, Inc.

An Overview of the CNS and PNS The Brain (CNS) Sensory receptors Sensory input over cranial nerves Reflex centers in brain Motor output over cranial nerves (PNS) Effectors Muscles The Spinal Cord (CNS) Glands Sensory receptors Sensory input over spinal nerves Motor output over Reflex spinal nerves (PNS) centers in spinal cord Adipose tissue What is part of the CNS? What is part of the PNS? © 2012 Pearson Education, Inc.

Spinal Cord Structure and Function Gross Anatomy of the Spinal Cord • The cord is about 18 inches (45 cm) long and a 1/2 inch (14 mm) wide • It ends between the first and second lumbar vertebrae (L 1 and L 2 ) • It has bilateral symmetry (grooves divide the spinal cord into left and right sides) • • Posterior median sulcus – on posterior side Anterior median fissure – on anterior side • Cord enlargements are caused by the amount of gray matter in the segment, and the involvement with sensory and motor nerves of the limbs • The cervical enlargement serves nerves of the shoulders and upper limbs • The lumbar & sacral enlargements serve nerves of the pelvis and lower limbs © 2012 Pearson Education, Inc.

• There are 30 vertebrae with 31 pairs of spinal nerves in the PNS. Spinal nerves are based on the vertebrae where the spinal nerves originate • Cervical nerves are named for the vertebra superior to the nerve’s exit (except for the 8 th nerve; it is inferior). • All other nerves are named for the vertebra inferior to the nerve’s exit • The first pair of spinal nerves pass between the skull and C 1. The rest pass through the intervertebral foramina © 2012 Pearson Education, Inc.

Mention that Cranial section has 7 bones and 8 nerves Cranial - superior radiation of nerves Thoracic - inferior radiation of nerves C 7, T 12, L 5, S 5, C 1 = 30 bones C 8, T 12, L 5, S 5, C 1 = 31 nerves Ha. Ha! © 2012 Pearson Education, Inc.

On the distal end of the cord… • The Conus medullaris is the tapered terminus of the cord (connective tissue), located below the lumbar enlargement (L 1 -L 2) • The Filum terminale is a thin thread of fibrous tissue (pia mater) at the end of the conus medullaris. It helps anchor the conus medullaris to the coccyx (attaches to coccygeal ligament) • The Cauda equina are nerve roots extending inferiorly below the conus medullaris (L 2 -S 5) that innervates the pelvic organs and lower limbs © 2012 Pearson Education, Inc.

• Roots are the two branches of spinal nerves • • The ventral root (anterior side) contains axons of motor neurons (efferents) The dorsal root (posterior side) contains axons of sensory neurons (afferents) • Dorsal root ganglion contains many cell bodies of sensory neurons • Spinal nerves are on each side of the spine; the dorsal and ventral roots join to form a spinal nerve • Mixed Nerves carry both afferent (sensory) and efferent (motor) fibers © 2012 Pearson Education, Inc.

The Spinal Meninges • The spinal meninges are specialized membranes that isolate the spinal cord from their surroundings • They protect the spinal cord, provide continuity within the CNS (brain and spinal cord) and channel the vasculature (blood supply) • Meningitis is a deadly viral or bacterial infection of these tissues (inflammation and damage will kill the nervous system!) © 2012 Pearson Education, Inc.

There are three meningeal layers of the spinal cord 1. The dura mater is the dense, outer layer, composed of collagen. It is contiguous with the cranial dura mater, and serves as a longitudinal anchor, terminating with the filum terminale (coccygeal ligament – shown in previous slide) 2. The arachnoid mater is the middle, avascular layer 3. The pia mater is the delicate, inner vascular layer, composed of elastin. Dendiculate ligaments are a latitudinal anchor (next slides) © 2012 Pearson Education, Inc.

• The epidural space is between the dura mater and the walls of the vertebral body. It contains blood vessels, loose connective and adipose tissue; this is the site of epidural injection of anesthetic – commonly, it is done wrong (damage)! • The subdural space is between the dura mater and the arachnoid mater (little or no space) • The subarachnoid space is between the arachnoid mater and pia mater. It is filled with cerebrospinal fluid (CSF) that carries dissolved gases, nutrients and wastes; this is the site of lumbar puncture (spinal tap) to withdraw CSF for diagnostic purposes (L 3 to L 5) – this is a very delicate procedure! © 2012 Pearson Education, Inc.

• Paired denticulate ligaments are found along the length of the spinal cord • They extend from the pia mater to the dura mater • They help to laterally suspend anchor the spinal cord © 2012 Pearson Education, Inc.

Gray Matter and White Matter Spinal tissue is structurally divided into two types of matter • Inside gray matter (shaped like H) is the location of cell bodies and unmyelinated axons. There is a greater amount of gray matter in the spinal cord segments serving the limbs. It also has projections (horns) • Outside white matter is the location of myelinated axons organized in ascending and descending columns (funiculi) © 2012 Pearson Education, Inc.

Posterior white column Posterior gray horn Dorsal root ganglion Lateral white column Lateral gray horn Anterior white column a The left half of this sectional view shows important anatomical landmarks, including the three columns of white matter. The right half indicates the functional organization of the nuclei in the anterior, lateral, and posterior gray horns. © 2012 Pearson Education, Inc.

POSTERIOR Posterior median sulcus Structural Organization of Gray Matter The projections of gray matter toward the outer surface of the spinal cord are called horns. Posterior gray horn Lateral gray horn Dorsal root Anterior gray horn ANTERIOR Dorsal root ganglion Ventral root A micrograph of a section through the spinal cord, showing major landmarks in and surrounding the cord. © 2012 Pearson Education, Inc.

Gray Matter is Functionally Organized in Anatomical Horns • Posterior gray horns contain somatic and visceral sensory nuclei • Anterior gray horns contain somatic motor nuclei • Lateral gray horns are in thoracic and lumbar segments; they contain visceral motor nuclei • Commissures are axons that cross laterally from one side of the cord to the other before reaching gray matter (anterior and posterior sides) © 2012 Pearson Education, Inc.

We can pinpoint damage in the spinal cord to functional deficits which are observed…

Functional Organization of White Matter • White matter is organized in tracts (fasciculi); these are bundles of myelinated axons that relay information • Ascending tracts carry information to the brain • Descending tracts carry motor commands to spinal cord © 2012 Pearson Education, Inc.

Spinal Nerves and Plexuses Anatomy of Spinal Nerves • Every spinal cord segment is connected to a pair of spinal nerves • Every spinal nerve is surrounded by three connective tissue layers that support structures and contain blood vessels © 2012 Pearson Education, Inc.

Three Connective Tissue Layers of Spinal Nerves 1. Epineurium • Outer layer • Dense network of collagen fibers 2. Perineurium • Middle layer • Divides the nerve into fascicles (axon bundles) 3. Endoneurium • Inner layer • Surrounds Schwann cells that envelope individual axons in the fascicle • Is continuous onto peripheral nerves Hmmm, looks a lot like muscle tissue, eh? © 2012 Pearson Education, Inc.

Blood vessels Endoneurium Perineurium (around one fascicle) (around myelin sheath) © 2012 Pearson Education,

Peripheral Distribution of Spinal Nerves • Spinal nerves form lateral to intervertebral foramen, where dorsal and ventral roots unite, and then branch, to form pathways leading to and from their destination • There are two classified branches; sensory or motor © 2012 Pearson Education, Inc.

Sensory From interoceptors of back From exteroceptors, proprioceptors of back The dorsal ramus carries sensory information from the skin and skeletal muscles of the back. Somatic sensory nuclei The ventral ramus carries sensory information from the ventrolateral body surface, structures in the body wall, and the limbs. Dorsal root ganglion From exteroceptors, proprioceptors of body wall, limbs From interoceptors of body wall, limbs Rami communicantes Ventral root Somatic sensations Visceral sensations © 2012 Pearson Education, Inc. The dorsal root of each spinal nerve carries sensory information to the spinal cord. The sympathetic nerve carries sensory information from the visceral organs. Visceral sensory nuclei From interoceptors of visceral organs

Motor To skeletal muscles of back The spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite. Dorsal root ganglion Postganglionic fibers to smooth muscles, glands, etc. , of back The dorsal ramus contains somatic motor and visceral motor fibers that innervate the skin and skeletal muscles of the back. The axons in the relatively large ventral ramus supply the ventrolateral body surface, structures in the body wall, and the limbs. The ventral root of each spinal nerve contains the axons of somatic motor and visceral motor neurons. To skeletal muscles of body wall, limbs Visceral motor nuclei Somatic motor nuclei Rami communicantes Somatic motor commands Visceral motor commands Postganglionic fibers to smooth muscles, glands, visceral organs in thoracic cavity Preganglionic fibers to sympathetic ganglia innervating abdominopelvic viscera © 2012 Pearson Education, Inc. Sympathetic ganglion Postganglionic fibers to smooth muscles, glands, etc. , of body wall, limbs The white ramus is the first branch from the spinal nerve and carries visceral motor fibers to a nearby sympathetic ganglion. Because these preganglionic axons are myelinated, this branch has a light color and is therefore known as the white ramus. A sympathetic nerve contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity. The gray ramus contains postganglionic fibers that innervate glands and smooth muscles in the body wall or limbs. These fibers are unmyelinated and have a dark gray color.

C 2 C 3 • A specific bilateral region of the skin surface monitored by a single pair of spinal nerves is known as a dermatome (The pair of spinal nerves are sensory and motor) NV C 2 C 3 T 2 C 6 L 1 L 2 C 8 C 7 T 1 L 4 • Shingles is caused by a herpes virus that attacks dorsal roots of spinal nerves and cranial ganglia L 5 S 2 C 3 T 2 T 3 T 4 T 5 T 6 T 7 T 8 T 9 T 10 T 11 T 12 L 1 L 2 L 4 L 3 L 5 C 4 C 5 T 2 C 6 T 1 C 7 SS 4 3 L 1 S 1 L 5 S 5 C 8 L 2 S 2 L 3 S 1 L 4 ANTERIOR © 2012 Pearson Education, Inc. L 3 C 4 C 5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 T 9 T 10 T 11 T 12 POSTERIOR

• If you ever had chicken pox, beware! The virus that caused it is within you and can express itself (unknown why) at a later stage in life • It follows a dermatome pattern of expression © 2012 Pearson Education, Inc.

Nerve Plexuses • Nerve plexuses (trunks) are a complex, interwoven, branching network of nerve fibers (afferent and efferent; sensory and motor) • They are formed from blended fibers of ventral rami of adjacent spinal nerves • The trunks form cords that survey and control the skeletal muscles of the neck and limbs (arms and legs) © 2012 Pearson Education, Inc.

• There are four major Plexuses of Ventral Rami 1. Cervical plexus (C 1 – C 5) 2. Brachial plexus (C 4 – T 2) 3. Lumbar plexus (T 12 – L 4) 4. Sacral plexus (L 4 – S 4) https: //en. wikipedia. org/wiki/Nerve_plexus © 2012 Pearson Education, Inc.

There is no Thoracic Plexus There are Thoracic Nerves, though © 2012 Pearson Education,

Cervical plexus Brachial plexus C 1 C 2 C 3 C 4 C 5 C 6 C 7 C 8 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 T 9 T 10 T 11 © 2012 Pearson Education, Inc. Lesser occipital nerve Great auricular nerve Transverse cervical nerve Supraclavicular nerve Phrenic nerve Axillary nerve Musculocutaneous nerve Thoracic nerves

T 12 L 1 Lumbar plexus Sacral plexus Radial nerve L 2 L 3 L 4 L 5 S 1 S 2 S 3 S 4 S 5 Co 1 Ulnar nerve Median nerve Iliohypogastric nerve Ilioinguinal nerve Lateral femoral cutaneous nerve Genitofemoral nerve Femoral nerve Obturator nerve Superior Inferior Gluteal nerves Pudendal nerve Saphenous nerve Sciatic nerve © 2012 Pearson Education, Inc.

The Cervical Plexus • Includes ventral rami of spinal nerves C 1–C 5 • It innervates the neck, thoracic cavity and diaphragmatic muscles • Major nerves: • Phrenic n. (diaphragm mvt) • Great auricular n. (ear mvt) • Cervical n. (neck mvt) © 2012 Pearson Education, Inc.

Redundant slide… Cranial Nerves Accessory nerve (XI) Hypoglossal nerve (XII) Lesser occipital nerve Nerve Roots of Cervical Plexus C 1 C 2 C 3 C 4 C 5 Supraclavicular nerves Clavicle © 2012 Pearson Education, Inc.

The Brachial Plexus • Includes ventral rami of spinal nerves C 4–T 2 • It innervates the pectoral girdle and upper limbs • Nerves that form the brachial plexus originate from: • Superior, middle, and inferior trunks; these are large bundles of axons from several spinal nerves • Lateral, medial, and posterior cords; these are smaller branches that originate at trunks © 2012 Pearson Education, Inc.

Major nerves of the Brachial Plexus • Musculocutaneous (lateral cord) and Ulnar n. (medial cord) flexes the arm • Median n. (lateral and medial cords) flexes and pronates the arm • Radial n. (posterior cord) elongates and supinates the arm © 2012 Pearson Education, Inc.

Major nerves of the Brachial Plexus • Pectoral n. pectoralis • Thoracic n. serratus

Major nerves of the Brachial Plexus • Axillary n. deltoid and teres minor • Thoracodorsal n. latissimus dorsi • Subscapular n. subscapular and teres major • Suprascapular n. supraspinatus and infraspinatus • Dorsal Scapular n. rhomboid and levator scapulae © 2012 Pearson Education, Inc.

Iliohypogastric nerve Ilioinguinal nerve Genitofemoral nerve The Lumbar Plexus • Includes ventral rami of the spinal nerves T 12–L 4 • Major nerves: • Genitofemoral n. skin of genitals • Lateral femoral cutaneous n. skin of upper pelvic area Lateral femoral cutaneous nerve Femoral nerve Obturator nerve Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve (cut) Sciatic nerve Saphenous nerve • Femoral n. flex the thigh and hip • Sciatic n. flex the knee • Fibular n’s (from Sciatic branch) flex the knee (to walk) © 2012 Pearson Education, Inc. Common fibular nerve Superficial fibular nerve Deep fibular nerve Nerves of the lumbar and sacral plexuses, anterior view

Superior gluteal nerve Inferior gluteal nerve The Sacral Plexus • Includes ventral rami of spinal nerves L 4–S 4 • Major nerves: • • • Pudendal n. controls urinary and anal sphincters (control urination and defacation) Sciatic n. Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Tibial nerve Common fibular nerve Two branches of the sciatic nerve 1. Fibular n. flex the knee (to walk) 2. Tibial n. flex the knee, extend the ankle, extend the toes (to walk) © 2012 Pearson Education, Inc. Sural nerve Nerves of the sacral plexus, posterior view

Saphenous nerve Sural nerve Tibial nerve Fibular nerve Saphenous nerve Tibial nerve Sural nerve

Somatic and Visceral Sensory Pathways • Most somatic sensory information is relayed to the thalamus for processing • Only a very small fraction of the arriving information (1%) is projected to the cerebral cortex and reaches our awareness © 2012 Pearson Education, Inc.

• Sensory pathways have ordered, labeled lines: • A first-order general sensory neuron delivers sensations to the CNS. The cell body of a first-order sensory neuron is located in the dorsal root ganglion or cranial nerve ganglion • A second-order neuron is a decussating interneuron within the CNS; the axon of the first-order sensory neuron synapses on the second-order interneuron, which may be located in the spinal cord or brain stem • If the sensation is to reach our awareness, the secondorder neuron synapses with a third-order neuron in the thalamus Hang on guys, you will see this very soon! © 2012 Pearson Education, Inc.

Somatic Sensory Pathways • Somatic sensory pathways carry sensory information from the skin and musculature of the body wall, head, neck and limbs • There are three major somatic sensory pathways 1. The Spinothalamic Pathways 2. The Posterior Column Pathways 3. The Spinocerebellar Pathways © 2012 Pearson Education, Inc.

Locations of Sensory Pathways and Ascending Tracts in the Spinal Cord Dorsal root ganglion Dorsal root Posterior column pathway Fasciculus gracilis Fasciculus cuneatus Spinocerebellar pathway Posterior spinocerebellar tract Anterior spinocerebellar tract Spinothalamic pathway Ventral root Lateral spinothalamic tract Anterior spinothalamic tract You know where the tracts are and where they go because of their names! © 2012 Pearson Education, Inc.

Observe the “precise-order” of ascending and descending fibers in the Spinal Cord NOTHING IS

1. The Spinothalamic Pathway • The spinothalamic pathway provides conscious sensations of poorly localized (“crude”) touch, pressure, pain and temperature • Highlights: • Axons of first-order sensory neurons enter the spinal cord and synapse on second-order neurons within posterior gray horns • Second-order neurons cross to the opposite side of the spinal cord before ascending within the anterior or lateral spinothalamic tracts • Third-order neurons synapse in the ventral nucleus group of the thalamus. After the sensations have been sorted and processed, they are relayed to the primary sensory cortex © 2012 Pearson Education, Inc.

Somatic Sensory Pathways SPINOTHALAMIC PATHWAY KEY Axon of firstorder neuron Second-order neuron Third-order neuron Anterior Spinothalamic Tract crude touch & pressure sensations Midbrain Medulla oblongata Anterior spinothalamic tract Crude touch and pressure sensations from right side of body © 2012 Pearson Education, Inc.

Somatic Sensory Pathways SPINOTHALAMIC PATHWAY KEY Axon of firstorder neuron Second-order neuron Third-order neuron Lateral Spinothalamic Tract pain & temperature sensations Midbrain Medulla oblongata Spinal cord Lateral spinothalamic tract Pain and temperature sensations from right side of body © 2012 Pearson Education, Inc.

Table 15 -1 Principal Ascending (Sensory) Pathways (Part 1 of 3). © 2012 Pearson

2. The Posterior Column Pathway • The posterior column pathway carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception • Highlights • Axons of first-order sensory neurons enter the spinal cord, ascend within the gracilis and cuneatus fasciculi, and synapse on second-order neurons within the medulla oblongata • Second-order neurons cross to the opposite side of the medulla oblongata before ascending within the medial lemniscus • Third-order neurons synapse in the ventral nucleus group of the thalamus. After the sensations have been sorted and processed, they are relayed to the primary sensory cortex © 2012 Pearson Education, Inc.

Figure 15 -5 Somatic Sensory Pathways POSTERIOR COLUMN PATHWAY Posterior Column Pathway Fine touch Vibration Pressure Proprioception Ventral nuclei in thalamus Midbrain Nucleus gracilis and nucleus cuneatus Medial lemniscus Medulla oblongata Fasciculus gracilis and fasciculus cuneatus Dorsal root ganglion Fine-touch, vibration, pressure, and proprioception sensations from right side of body © 2012 Pearson Education, Inc.

Table 15 -1 Principal Ascending (Sensory) Pathways (Part 2 of 3). © 2012 Pearson

3. The Spinocerebellar Pathway • The cerebellum receives proprioceptive information about the position of skeletal muscles, tendons and joints • Highlights (a little bit more complicated): • The posterior spinocerebellar tracts contain second-order axons that do not cross over to the opposite side of the spinal cord. The axons reach the cerebellar cortex via the inferior cerebellar peduncle of that side • The anterior spinocerebellar tracts are dominated by second-order axons that have crossed over to the opposite side of the spinal cord. Sensations reach the cerebellar cortex via the superior cerebellar peduncle. Many axons that cross over and ascend to the cerebellum then cross over again within the cerebellum, synapsing on the same side as the original stimulus! © 2012 Pearson Education, Inc.

Figure 15 -5 Somatic Sensory Pathways SPINOCEREBELLAR PATHWAY Spinocerebellar Pathway Proprioceptive input (Golgi tendon organs) (Muscle spindles) (Joint capsules) PONS Cerebellum Medulla oblongata Spinocerebellar pathway Spinal cord Posterior spinocerebellar tract Anterior spinocerebellar tract © 2012 Pearson Education, Inc. Proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsules

Table 15 -1 Principal Ascending (Sensory) Pathways (Part 3 of 3). © 2012 Pearson

Visceral Sensory Pathways • Visceral sensory information is collected by interoceptors; they survey visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities • These interoceptors include nociceptors, thermoreceptors, tactile receptors, baroreceptors and chemoreceptors • Cranial Nerves V, VII, IX, and X carry visceral sensory information from the mouth, palate, pharynx, larynx, trachea, esophagus, and associated vessels and glands • The solitary nucleus (a large nucleus in the medulla oblongata) is a major processing and sorting center for visceral sensory information; it has extensive connections with the various cardiovascular and respiratory centers, and the reticular formation © 2012 Pearson Education, Inc.

Somatic Motor Pathways • The somatic nervous system (SNS) is also called the somatic motor system. It controls contractions of skeletal muscles (discussed next) • The autonomic nervous system (ANS) is also called the visceral motor system. It controls visceral effectors, such as smooth muscle, cardiac muscle, and glands (covered in Ch. 16) © 2012 Pearson Education, Inc.

• Somatic motor pathways always involve at least two motor neurons in series (a labeled line) • The upper motor neuron cell body lies in a CNS processing center in the cortex; it synapses on the lower motor neuron (activity in the upper motor neuron may facilitate or inhibit the lower motor neuron) • The lower motor neuron cell body lies in a nucleus of the brain stem or spinal cord; it triggers a contraction in an innervated muscle. (Destruction of, or damage to, the lower motor neuron eliminates voluntary and reflex control over the innervated motor unit) © 2012 Pearson Education, Inc.

Somatic Motor Pathways • Conscious and subconscious motor commands control skeletal muscles by traveling over three integrated motor pathways • The three major somatic motor pathways are: 1. The Corticospinal Pathway 2. The Medial Pathway 3. The Lateral Pathway © 2012 Pearson Education, Inc.

Locations of Descending (Motor) Tracts in the Spinal Cord Corticospinal pathway Lateral corticospinal tract Anterior corticospinal tract Lateral pathway Reticulospinal tract (lateral) Rubrospinal tract Vestibulospinal tract (lateral) Medial pathway Reticulospinal tract (medial) Tectospinal tract Vestibulospinal tract (medial) © 2012 Pearson Education, Inc.

1. The Corticospinal Pathways • The corticospinal pathway is sometimes called the pyramidal system. It provides voluntary skeletal muscle control, employing three pairs of descending tracts • Highlights: • The system begins at pyramidal cells of the primary motor cortex. Axons of these upper motor neurons descend into the brain stem and spinal cord to synapse on lower motor neurons (anterior gray horns) that control skeletal muscle function • Corticobulbar tracts provide conscious control over skeletal muscles that move the eye, jaw, face and some muscles of the neck and pharynx (decussation in brain stem) • Lateral and anterior corticospinal tracts provide conscious control over skeletal muscles that move the remaining axial and appendicular regions of the body (decussation varies) © 2012 Pearson Education, Inc.

Figure 15 -9 The Corticospinal Pathway. Motor homunculus on primary motor cortex of left cerebral hemisphere KEY Decussation Notes: Corticobulbar tract nerves decussate in the midbrain to synapse with cranial nerves Lateral Corticospinal tract nerves (majority of axons) decussate in the medulla before synapsing on lower motor neurons in anterior gray horns* Anterior Corticospinal tract nerves decussate in the spinal cord before synapsing on lower motor neurons in anterior gray horns* *same gray horn nuclei! Axon of uppermotor neuron Lower-motor neuron Corticobulbar tract To skeletal muscles Cerebral peduncle Motor nuclei of cranial nerves To skeletal muscles Decussation of pyramids Medulla oblongata Pyramids Lateral corticospinal tract Anterior corticospinal tract To skeletal muscles © 2012 Pearson Education, Inc. Midbrain Decussation of anterior commisure Spinal cord

Table 15 -2 Principal Descending (Motor) Pathways (Part 1 of 2). © 2012 Pearson

2. & 3. The Medial and Lateral Pathways • Several centers in the cerebrum, diencephalon, and brain stem may issue somatic motor commands as a result of processing that has been performed at the subconscious level (a. k. a. extrapyramidal system) • These nuclei and tracts are grouped by their primary functions: • Components of the medial pathway help control gross movements of the neck, trunk and proximal limb muscles (muscle tone and balance; eye, head, neck, trunk and upper limb movement) • Components of the lateral pathway help control precise movements of distal limb muscles of the upper extremity (muscle tone and upper limb movement) © 2012 Pearson Education, Inc.

A Summary of the Medial and Lateral Pathways • Once again, the medial pathway is concerned with the control of muscle tone and gross movements of the neck, trunk and proximal limb muscles. • Cell bodies of the Vestibular Nuclei (VN) of the Pons receive information from the vestibular branch of the vestibulocochlear nerve (CN VIII) via receptors that monitor the position and movement of the head. Axons of these upper motor neurons (VN) descend in the vestibulospinal tract. Their primary goal is to subconsciously regulate body posture, balance and tone • Cell bodies of the Superior and Inferior Colliculi (SC & IC) of the Midbrain (tectum) receive visual (SC) and auditory (IC) sensations from the eyes and ears. Axons of these upper motor neurons (SC & IC) cross to the opposite side of the midbrain before descending in the tectospinal tract to synapse on lower motor neurons in the brain stem or spinal cord. Their primary goal is to subconsciously regulate eye, head, neck and upper limb position in response to visual and auditory stimuli • Cell bodies of the Reticular Formation (RF) are a loosely organized network of neurons that extend throughout the brain stem (Pons and Medulla). Axons of these upper motor neurons (RF) descend in the reticulospinal tract. Their primary goal is to subconsciously regulate reflex activity • The lateral pathway is primarily concerned with the control of muscle tone and more precise movements of the distal limb muscles. Axons of upper motor neurons in the red nuclei cross to the opposite side of the Midbrain and descend in the rubrospinal tract © 2012 Pearson Education, Inc.

Medial Path - Vestibulospinal Tract • Cell bodies of the Vestibular Nuclei (VN) of the Pons and Medulla Oblongata receive information from the vestibular branch of the vestibulocochlear nerve (CN VIII) via receptors that monitor the position and movement of the head • Axons of these upper motor neurons (VN) descend in the vestibulospinal tract. Their primary goal is to subconsciously regulate body posture, balance and tone • This tract is more complicated than this figure conveys (on to the next slide)! © 2012 Pearson Education, Inc.

Medial Path - Vestibulospinal Tract • As you can see from a more detailed figure, some axons decussate from the Medial Nuclei in the Pons and Medulla Oblongata (red color), and descend ipsilaterally (same side) and contralaterally (opposite side) to each other • These axons terminate at the 3 rd Cervical vertebra and innervate neck muscles that support the head • Axons that descend from the Superior and Lateral Nuclei (blue color) remain ipsilaterally (same side) through the entire spinal cord • They are responsible for maintaining whole body balance • Neat, huh! © 2012 Pearson Education, Inc.

Medial Path - Tectospinal Tract (blue color) • Cell bodies of the Superior and Inferior Colliculi (SC & IC) of the Midbrain (tectum) receive visual (SC) and auditory (IC) sensations from the eyes and ears. Axons of these upper motor neurons (SC & IC) cross to the opposite side of the midbrain before descending in the tectospinal tract to synapse on lower motor neurons in the brain stem or spinal cord. Their primary goal is to subconsciously regulate eye, head, neck and upper limb position in response to visual and auditory stimuli Medial Path - Reticulospinal Tract (green color) • Cell bodies of the Reticular Formation (RF) are a loosely organized network of neurons that extend throughout the brain stem (Pons and Medulla); they receive information from a wide array of special senses (visual, auditory, taste, smell). Axons of these upper motor neurons (RF) descend in the reticulospinal tract. Their primary goal is to subconsciously regulate reflex activity © 2012 Pearson Education, Inc.

Lateral Path - Rubrospinal Tract (red color) • Cell bodies of the Red Nuclei of the Midbrain receive afferent fibers from the motor cortex and cerebellum. Axons of the upper motor neurons in the Red Nuclei cross to the opposite side of the Midbrain and descend in the rubrospinal tract • The rubrospinal tract terminates in the cervical spinal cord; it is primarily concerned with the control of voluntary muscle tone and more precise movements of the distal limb muscles of the upper extremity © 2012 Pearson Education, Inc.

Table 15 -2 Principal Descending (Motor) Pathways (Part 2 of 2). © 2012 Pearson

The Basal Nuclei and Cerebellum • The basal nuclei and cerebellum are responsible for coordination and feedback control over consciously or subconsciously directed muscle contractions • The basal nuclei provide background patterns of movement that are involved in voluntary motor activities • Some axons extend to the premotor cortex, the motor association area that directs activities of the primary motor cortex. These axons alter the pattern of instructions carried by the corticospinal tracts • Other axons alter the excitatory or inhibitory output of the reticulospinal tracts • The Cerebellum monitors proprioceptive (position) sensations, visual information from the eyes and vestibular (balance) sensations from the inner ear as movements are under way © 2012 Pearson Education, Inc.

Summary – Levels of Processing and Motor Control • All sensory and motor pathways involve a series of synapses, one after the other. This is a general pattern: • Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that preserve homeostasis over the short term; they control the most basic motor activities • Integrative centers in the brain (cerebellum) perform more elaborate processing, and as we move from the medulla oblongata to the cerebral cortex, the motor patterns become increasingly more complex and variable • The most complex and variable motor activities are directed by the primary motor cortex. Neurons of the primary motor cortex innervate motor neurons in the brain and spinal cord responsible for stimulating skeletal muscles • Higher centers in the brain can suppress or facilitate reflex responses; reflexes can complement or increase the complexity of voluntary movements © 2012 Pearson Education, Inc.