The nervous system: Part C

Somatic nervous system Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division

Spinal Cord Location Functions Begins at the foramen magnum Ends at L1 vertebra Functions Provides two-way communication to and from the brain Contains spinal reflex centers

Spinal Cord Spinal nerves Cauda equina 31 pairs The collection of nerve roots at the inferior end of the vertebral canal

Spinal Cord Protection Bone Meninges CSF

Dura and arachnoid mater Cervical spinal nerves Dura and arachnoid mater Thoracic spinal nerves Lumbar spinal nerves Cauda equina Sacral spinal nerves Figure 7.18

Gray Matter Dorsal horns—interneurons that receive sensory input Ventral horns—somatic motor neurons whose axons exit the cord via ventral roots Dorsal root (spinal) ganglia—contain cell bodies of sensory neurons

Dorsal horn (interneurons) Dorsal root (sensory) Dorsal root ganglion Dorsal horn (interneurons) Somatic sensory neuron Visceral sensory neuron Visceral motor neuron Spinal nerve Ventral horn (motor neurons) Ventral root (motor) Somatic motor neuron Interneurons receiving input from somatic sensory neurons Interneurons receiving input from visceral sensory neurons Visceral motor (autonomic) neurons Somatic motor neurons Figure 12.32

(b) The spinal cord and its meningeal coverings Dorsal horn Gray matter White matter Ventral horn Lateral horn Dorsal root ganglion Spinal nerve Central canal Dorsal root Ventral root Pia mater Arachnoid mater Spinal dura mater (b) The spinal cord and its meningeal coverings Figure 7.19

White Matter Mainly ascending (sensory) and descending (motor) tracts Tracts are located in three white columns on each side—posterior, lateral, and anterior columns

Spinal Cord Trauma Flaccid paralysis—severe damage to the ventral root or ventral horn cells Impulses do not reach muscles; there is no voluntary or involuntary control of muscles Muscles atrophy

Structure of a Nerve Cordlike organ of the PNS Bundle of myelinated and unmyelinated peripheral axons enclosed by CT

Connective tissue coverings include: Structure of a Nerve Connective tissue coverings include: Endoneurium—loose CT; encloses axons and their myelin sheaths Perineurium—coarse CT ; bundles fibers into fascicles Epineurium—tough fibrous sheath around a nerve

Axon Myelin sheath Endoneurium Perineurium Epineurium Fascicle Blood vessels Figure 7.20

Classifying Nerves Mixed nerves carry both sensory and motor fibers 4/23/2017 Classifying Nerves Mixed nerves carry both sensory and motor fibers Afferent (sensory) nerves carry impulses toward CNS Efferent (motor) nerves carry impulses away from CNS 20

4/23/2017 Spinal Nerves A spinal nerve is formed where ventral and dorsal roots fuse After spinal nerve is formed it divides into dorsal and ventral rami Ventral Rami form intercostal nerves (T1-T12) and networks of nerves called plexuses Plexuses serve limbs: Brachial plexus Lumbar plexus Sacral plexus 23

Extend from base of brain 4/23/2017 Cranial Nerves 12 pairs Extend from base of brain Primarily innervate head and neck (except Vagus N.) 21

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Remaining cranial nerves are motor or mixed 4/23/2017 Cranial Nerves Sensory only CN I (Olfactory) -smell CN II (Optic) -vision CN VIII (Vestibulocochlear) -Hearing and balance Remaining cranial nerves are motor or mixed 11

Somatic & Autonomic Nervous System Compared 4/23/2017 Somatic & Autonomic Nervous System Compared Somatic Nervous System Autonomic Nervous System The axon of ONE motor neuron extends all the way to skeletal muscle A chain of TWO motor neurons

Somatic & Autonomic Nervous System Compared Somatic nervous system All somatic motor neurons release acetylcholine (ACh) ANS Preganglionic fibers release acetylcholine (Ach) Postganglionic fibers release norepinephrine or ACh at effectors

Figure 7.24 Neuro- transmitter at effector Cell bodies in central nervous system Effector organs Peripheral nervous system Effect Single neuron from CNS to effector organs SOMATIC NERVOUS SYSTEM ACh Skeletal muscle Two-neuron chain from CNS to effector organs ACh NE SYMPATHETIC Ganglion Epinephrine and norepinephrine ACh AUTONOMIC NERVOUS SYSTEM Adrenal medulla Blood vessel ACh ACh Smooth muscle (e.g., in gut), glands, cardiac muscle PARASYMPATHETIC Ganglion Acetylcholine (ACh) Norepinephrine (NE) Figure 7.24

Parasympathetic division Dual innervation Divisions of the ANS Sympathetic division Parasympathetic division Dual innervation Most visceral organs are served by both divisions, but they cause opposite effects

Role of the Sympathetic Division Mobilizes the body during activity; “fight-or-flight” system Promotes adjustments during exercise, or when threatened Blood flow directed to skeletal muscles & heart Bronchioles dilate Liver releases glucose

Role of the Parasympathetic Division Promotes maintenance activities, conserves body energy Its activity is illustrated in a person who relaxes, reading, after a meal Blood pressure, heart rate, and respiratory rates are low GI tract activity is high

Parasympathetic Sympathetic Eye Eye Brain stem Salivary glands Skin* Cranial Salivary glands Sympathetic ganglia Heart Cervical Lungs Lungs T1 Heart Stomach Thoracic Stomach Pancreas Liver and gall- bladder Pancreas L1 Liver and gall- bladder Adrenal gland Lumbar Bladder Bladder Genitals Genitals Sacral Figure 7.25

Sympathetic Division Preganglionic fibers pass through white rami communicantes and enter sympathetic trunk

Spinal cord Dorsal root Ventral root Sympathetic trunk ganglion Sympathetic trunk Ventral ramus of spinal nerve Gray ramus communicans White ramus communicans (a) Location of the sympathetic trunk

Sympathetic Trunks and Pathways Upon entering a sympathetic trunk ganglion a preganglionic fiber may do one of the following: Synapse with a ganglionic neuron at the same level Ascend or descend the sympathetic trunk to synapse at another level Pass through trunk ganglion and emerge without synapsing

Lateral horn (visceral motor zone) Dorsal root Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Skin (arrector pili muscles and sweat glands) Gray ramus communicans Ventral root White ramus communicans Sympathetic trunk ganglion Sympathetic trunk To effector 1 Synapse at the same level Blood vessels (b) Three pathways of sympathetic innervation Figure 7.26

Synapse at a higher or lower level Skin (arrector pili muscles and sweat glands) To effector Blood vessels 2 Synapse at a higher or lower level (b) Three pathways of sympathetic innervation Figure 7.26

Synapse in a distant collateral ganglion Splanchnic nerve Collateral ganglion Target organ in abdomen Synapse in a distant collateral ganglion anterior to the vertebral column 3 (b) Three pathways of sympathetic innervation Figure 7.26

(b) The spinal cord and its meningeal coverings Dorsal horn Gray matter White matter Ventral horn Lateral horn Dorsal root ganglion Spinal nerve Central canal Dorsal root (fans out into dorsal rootlets) Ventral root (derived from several ventral rootlets) Pia mater Arachnoid mater Spinal dura mater (b) The spinal cord and its meningeal coverings Figure 7.19