Chapter 14 The Spinal Cord & Spinal Nerves PDF

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Dr. Michael Kirifides, Drexel University ANAT 102...

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Chapter 14: The Spinal Cord & Spinal Nerves ● Spinal cord extends inferiorly from the medulla oblongata through the vertebral canal ● The spinal cord is associated with 31 pairs of spinal nerves ● Each side of the spinal cord contains 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves and 1 coccygeal nerve Spinal roots & spinal nerve ● Each spinal nerve anchors to the spinal cord by 2 roots; posterior root and anterior root, each is composed of multiple rootlets ● Posterior root houses sensory neurons that extend from the sensory receptors. Critical role that the cell bodies of these sensory neurons are located external to the spinal cord and from the posterior root ganglion ● Anterior root contains motor neurons that extend to effectors (muscles and glands). These motor neurons relay nerve signals from the spinal cord and controls muscles and glands. Motor axons exit from the spinal cord within the anterior root and extend through the spinal nerve to their terminal ends, which innervate an effector ● Each spinal nerve forms where the posterior root and the anterior root join. Thus, both sensory and motor neurons compose each spinal nerve, and it is classified as a mixed nerve Spinal meninges & the structure of the spinal cord ● Vertebral column ○ Bony structure formed by 26 stacked vertebrae ● Meninges ○ Pia mater ○ Arachnoid mater ○ Dura mater ● Cerebrospinal fluid ○ Central canal ○ Subarachnoid space Cross section of spinal cord & spinal roots ● Central canal ○ Contains cerebrospinal fluid ● White matter ○ Posterior funiculus ○ Lateral funiculus ○ Anterior funiculus ● Gray matter ○ Posterior horn ○ Lateral horn ○ Anterior horn ○ Gray commissure ● The collections of neuron cell bodies within the CNS from specific nuclei ● The collections of neuron cell bodies in PNS form ganglia Cross Sections of the Spinal Cord ● Cervical ● Thoracic

● Lumbar ● Sacral Difference between sensory and motor ● Sensory pathways transmit nerve signals from sensory receptors that ascend from the anterior, posterior, and lateral funiculi of the spinal cord to the brain. These pathways use up to three neurons to transmit this information (primary, secondary, and tertiary neurons) ● Motor pathways transmit nerve signals from the brain and descend to effectors. These pathways typically travel through the anterior and lateral funiculi of the spinal cord, and they use two neurons (upper and lower motor neurons) Sensory and motor pathways ● Two pathways transmit sensory information from the lower body including lower and upper limbs. These two pathways convey separate modalities through a chain of three neurons ○ The posterior funiculus- medial lemniscal pathway- transmits tactile information (discriminatory touch, precise pressure, and vibration) from the skin and proprioceptive information from joints, muscles and tendons. ○ The anterolateral pathway transmits tactile information of pain, temperature, pressure, and crude touch from the skin and mucous membranes. ● Two pathways transmit motor information from the primary motor cortex of the frontal lobe or the brainstem to the lower body. Tracts and fasciculi within the spinal cord ● The axons within each funiculus are organized into smaller structural units (bundles of myelinated axons) called fasciculi ○ White matter on each side of the cord can also be referred to as tracts, which have common functions. Individual tracts are either ■ Sensory (or ascending) tracts, which conduct nerve signals from the spinal cord to the brain ■ Motor ( or descending) tracts, which conduct nerve signals from the brain to the spinal cord ○ All three funiculus regions Posterior Funiculus- Medial Lemniscal pathway ● This pathway transmits sensory information about discriminative touch, precise pressure, vibration and proprioception (limb position) ● Three neurons in this serial pathway ○ The primary neuron is purple ○ The secondary neuron is blue ○ The tertiary neuron is green ● Decussation of axons occurs just prior to the medial- lemniscus in the medulla oblongata Anterolateral Pathway ● This pathway conducts crude touch, pressure, pain, and temperature sensations toward the brain ● Three neurons in this serial pathway ○ The primary neuron is purple ○ The secondary neuron is blue

○ The tertiary neuron is green ● Decussation of axons occur at the level where the primary neuron axon enters the spinal cord Corticospinal tracts ● These are two primary (anterior and lateral) corticospinal tracts. Each is formed by a chain of two motor neurons ● Voluntary motor control originates in the precentral gyrus forming upper motor neurons ● Upper motor neurons synapse directly onto lower motor neurons in the brainstem or spinal cord ● Upper motor neurons decussate in the pyramids (lateral tract) or the spinal cord (anterior tract) and synapse on lower motor neurons within the anterior horns of the spinal cord ● Axons of lower motor neurons leave the brainstem or spinal cord and form the anterior roots of the spinal nerves and cranial nerves innervating skeletal muscles Upper Motor Neuron Lesions ● Damage or lesions to the UMN include ○ Stroke ○ Multiple sclerosis (MS) ○ Traumatic brain injury ○ Cerebral palsy (CP) ● Symptoms include ○ Spastic paralysis/paresis- loss of motor function along with increased muscle tone ○ Hypertonia- increase in muscle tone in extensor muscles of lower limbs and flexor muscles of upper limbs ○ Hyperreflexia- exaggerated tendon reflex ○ Clasp knife- response where initial resistance is followed by relaxation ○ Babinski reflex Lower Motor Neuron Lesions ● Lesion or damage to the LMS may soan from the anterior horn of the spinal cord to muscles. ● Symptoms include ○ Flaccid paralysis/paresis- weakness or paralysis and reduced muscle tone ○ Fasciculation- a brief, spontaneous contraction affecting a small number of muscle fibers, often causing a flicker of movement under the skin ○ Hypotonia- a state of low muscle tone (the amount of tension or resistance to stretch in a muscle) , often involving reduced muscle strength and atrophy ○ Areflexia- absence of reflexes ALS Lou Gehrig’s Disease ● Degeneration of the primary motor cortex, UMN and cell bodies of LMN ● Symptoms include stiff muscles and muscle twitching ● Causes progressive muscle weakness and atrophy resulting in difficulty speaking, swallowing and eventually breathing ● 85% is idiopathic and approx. 15% is genetic ● Two types, fast and slow progression. Typical survival 2-4 years after diagnosis of fast type

● No known cure Integration of Motor Systems ● Somatic motor pathways provide the input into lower motor neurons and are divided into four distinct circuits ○ Local circuit neurons are located close to LMNs in the brainstem and spinal cord. Involved with reflexes and rhythmic activity ○ Upper motor neurons- input to both lower circuit neurons and directly onto LMNs. essential for execution of voluntary muscles ○ Basal nuclei neurons- assist movement by providing indirect input to UMNs via the thalamus. These circuits initiate and terminate movement and suppress unwanted movements. ○ Cerebellar neurons- assist movement via control of activity of UMNs Functions of the Basal Nuclei ● Basal nuclei play a major role in the initiation and termination of movements ○ Caudate and putamen receive inputs from the substantia nigra and cortex ○ Output from the globus pallidus goes primarily to the thalamus ● Basal nuclei suppress unwanted movements ● Involved with reducing muscle tone ● Involved with cognitive functions such as attention, memory, and planning Disorders of the Basal Nuclei ● Parkinson’s Disease (Reduction of dopamine input from the substantia nigra of the midbrain) ○ Difficulty starting and stopping movement ○ Disturbs transitional muscle movement ○ Resting tremor- uncontrollable rolling of the wrist when still ○ Bradykinesia- decreases movement ● Huntington’s disease (Reduction of acetylcholine due to casudate & putamen degeneration) ○ Rapid jerky uncontrollable movements ○ Symptoms occur between the ages of 30 to 40 ● Tourette syndrome ○ Involuntary body movements (motor tics) ○ Inappropriate sounds Functions of the Cerebellum ● Monitoring intentions for movement (pre movements) ○ Cortec ○ Basal nuclei ● Monitoring actual movement (real time) ○ Proprioceptors (muscles, tendons, and joints) ○ Vestibular apparatus (inner ear) ○ Eyes (retina) ● Comparing command signals with sensory information ○ Compares intentions with actual movements ● Sending our corrective feedback ○ Determines if there is a discrepancy between intended and actual movement

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Sends corrective information to the premotor cortex via the thalamus Premotor cortex processes the information and regulates the upper motor neurons in the primary Disorders of the Cerebellum ● Include ○ Trauma ○ Tumor ○ Stroke ○ Multiple sclerosis ○ Toxicity ● Symptoms of cerebellar disorders include ○ Ataxia- of the ability to coordinate muscle movements ○ Intention tremor- the amplitude of a cerebellar tremor increases as the extremity approaches the endpoint of deliberate and visually guided movement ○ Dysmetria- the lack of coordination of movement (overshoot & undershoot of intended target Spinal nerve branches ● Each spinal nerve (except C1 and Co1) extends through an intervertebral foramen to exit the vertebral column ● Each spinal nerve splits almost immediately into two primary branches, termed rami. ○ The posterior (dorsal) ramus- is the smaller of the two main branches. It innervates the deep muscles of the back and the skin of the back. ○ The anterior (ventral) ramus- is the larger of the two main branches. The anterior ramus splits into multiple other branches, which innervates skin and skeletal muscles of the anterior and lateral portions Dermatome Maps ● A specific segment of skin innervated by a single nerve. ● Dermatomes are clinically important because they can indicate potential potential damage to one or more spinal nerves ● Dermatomes are also involved in referred visceral pain or discomfort from one organ is mistakenly referred to a dermatome Intercostal nerves ● The anterior rami of spinal nerves T1-T11 are called intercostal nerves because they are located within the intercostal space sandwiched between two adjacent ribs ● T12 is called a subcostal nerve because it arises inferior to the ribs, not between two ribs ● With the exception of T1, the intercostal nerves do no form plexuses ● The intercostal nerves innervate much of the torso wall and portions of the upper limb Cervical Plexus (C1-C5) ● One important branch of the cervical plexus is the phrenic nerve, which is formed primarily from the C4 nerve and some contributing from C3 and C5 ● The phrenic nerve extends through the thoracic cavity to innervate the thoracic diaphragm which is the primary skeletal muscle of breathing Upper limb nerve supply ● Anterior compartment innervated by the musculocutaneous nerve ○ Biceps brachii

○ Brachialis ○ Coracobrachialis ● Posterior compartment innervated by the radialis nerve ○ Triceps brachii Lumbar plexus (L1-L4) ● The lumbar plexus has two main divisions ● The femoral nerve is the main nerve of the posterior division. The nerve innervates the anterior thigh muscles, such as the quadriceps femoris ● The obturator nerve is the main nerve of the anterior division. The nerve innervates the medial thigh muscles which adduct the thigh Sacral plexus (L4-S4) ● The sciatic nerve is the largest and longest nerve in the body. It projects from the pelvis through the greater sciatic notch of the os coxae and extends into the posterior region of the thigh ● Sciatic nerve is composed of two divisions ○ Tibial (anterior) division ○ Common fibular (posterior) division ● The tibial nerve is formed from the anterior division of the sciatic nerve. In the posterior thigh, the tibial division innervates the hamstrings. Extends within the posterior...