Chapters 13 & 16: Spinal Cord; Sensory and Motor Pathways

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1 Chapters 13 & 16: Spinal Cord; Sensory and Motor Pathways Chapter Objectives SPINAL CORD ANATOMY 1. Describe the external features of the spinal cord and the Cauda equina. Discuss its relative length within the vertebral column. 2. Describe the characteristics and purpose of the three layers and spaces of the meningeal structures. 3. Match each horn of gray matter with the type of cell body it contains. 4. Match each column of white matter and the type of tracts it contains. 5. Describe where sensory information enters the spinal cord and where motor information leaves the spinal cord. SPINAL NERVES 6. Define a nerve and discuss the connective tissue layers that surround and protect nerves. 7. Discuss the naming and numbering of spinal nerves, the arrangement of spinal nerves relative to the vertebrae, and the attachment of the spinal nerves to the spinal cord. 8. Discuss the branching of the spinal nerves once they emerge from the vertebral column. 9. Define a plexus; then list the names of major nerves of the cervical, brachial, lumbar, and sacral plexuses. 10. Define a dermatome. SOMATIC SENSATIONS 11. Identify the sensations and the receptors involved. 12. Describe the types of receptors in terms of microscopic features, location, and stimulus type. SOMATIC SENSORY PATHWAYS 13. Discuss the location of first order, second order and third order neuons. 14. Discuss the functions of the anterolateral pathway and where its ascending tracts are located. 15. Discuss the functions of the posterior column-medial lemniscus pathway and where its ascending tracts are located. 16. Discuss the functions of the cerebellar pathway and where its ascending tracts are located. SOMATIC MOTOR PATHWAYS 17. List the neural circuits that are part of the somatic motor pathways. 18. Describe the origin and destination for the descending, motor tracts.

2 REFLEXES AND REFLEX ARC 19. Define a reflex. 20. Describe the components of a reflex arc and their specific functions. Chapter Lecture Notes Spinal Cord Spinal cord - extends from foramen magnum to the level of the second lumbar vertebra (Fig 13.2) Shorter than vertebral column because vertebral column grows faster than spinal cord Cauda equina - nerves from lower cord don't leave vertebral column immediately, but instead look like coarse hairs of a horse tail Has grooves on surface anterior median fissure and posterior median sulcus Central canal passageway for cerebrospinal fluid; flows from ventricles in brain (Fig 13.3) Lined with ependymal cells that help circulate cerebrospinal fluid The spinal cord is surrounded by meninges (Fig 13.1) Dura mater outermost layer which is single layered and not attached to the bony vertebrae epidural space space between dura mater and vertebrae filled with adipose and blood vessels Arachnoid mater middle layer inside of dura mater Pia mater innermost layer bound very tightly to surface of spinal cord Subarachnoid space space between arachnoid mater and pia mater contains cerebrospinal fluid lumbar puncture into subarachnoid space between L3 and L4 or L4 and L5 for cerebrospinal fluid diagnosis or to introduce anesthetics Gray Matter of the Spinal Cord Gray matter (cell bodies and dendrites) - organized into horns and commissures

3 Posterior (dorsal) gray horn - contains cell bodies of interneurons which have synapsed with sensory neurons (Fig 13.3 & 13.4) Lateral gray horn - contains cell bodies of neurons from autonomic nervous system Anterior (ventral) gray horn - contains cell bodies of motor neurons Anterior gray commissure - gray communication between right and left section of cord anterior to the central canal Posterior gray commissure - gray communication between right and left section of cord posterior to the central canal White Matter of the Spinal Cord White matter (myelinated axons) - organized into columns and commissures (tracts travel in columns) (Fig 13.3 & 13.12) Posterior white column - has ascending tracts only Lateral white column - has both ascending and descending tracts Anterior white column - has both ascending and descending tracts Anterior white commissure Posterior white commissure Nerves Nerves bundles of axons in the PNS (Fig 13.5) Surrounded by connective tissue Epineurium around whole nerve Perineurium around a fascicle of nerve fibers Endoneurium around a single axon Mixed nerves contain both afferent and efferent neurons Ganglia cell bodies of neurons in the PNS Spinal Nerves

4 Spinal nerve - has bundles of axons of both motor and sensory neurons, therefore it is a mixed nerve The anterior root and the posterior root combine to form the spinal nerve Posterior (dorsal) root - connection to spinal cord from the peripheral nervous system that carries only sensory information and ends at the posterior gray horn Posterior (dorsal) root ganglia - cell bodies of sensory neurons (unipolar neurons) Anterior (ventral) root - connection from the spinal cord to the peripheral nervous system that carries only motor information Motor neurons = multipolar neurons Exits from vertebral column at the intervertebral foramen 31 pairs of spinal nerves (Fig 13.2) 8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal each has 3 branches (rami) (Fig 13.6) Visceral (Rami communicantes, part of autonomic nervous system) Posterior (dorsal) Anterior (ventral) branch (anterior rami) divides into plexus (braids) except thoracic (T1 and possibly T2 exception) Cervical plexus (C1-C5) - Phrenic nerve (phren = diaphragm) (Fig 13.7) Brachial plexus (C5-C8, T1) (Fig 13.8) Radial Ulnar Median nerve

5 Median nerve is compressed in carpal tunnel syndrome causing numbness, tingling, and pain in the palm and fingers Lumbar plexus - (L1-L4) femoral nerve (Fig 13.9) Sacral plexus - (L4-S4) sciatic nerve (Fig 13.10) Dermatomes Dermatomes - letter and numbers indicating spinal nerve innervating a given region of skin (area of the skin that supplies sensory input to the dorsal roots of one pair of spinal nerves) (Fig 13.11) Sensations Crude touch can tell something has contacted skin but where and what can t be determined Fine touch can tell where and what has contacted skin Meissner s corpuscles Pressure deformation of deeper tissues felt over larger area than touch Pacinian corpuscles Vibration Itch Tickle Thermal sensations warm and cold Pain nociceptors Proprioceptive Sensory Receptors Classified by Microscopic Features Free (unencapsulated) nerve endings (Fig 16.1 & Table 16.1) Bare dendrites associated with pain, thermal, tickle, itch, and some touch sensations First-order neurons conduct impulses from the PNS into the CNS Encapsulated nerve endings Dendrites enclosed in a connective tissue capsule

6 Meissner s corpuscle or Pacinian corpuscle First-order neuron Receptor cell synapses with first-order neuron Photoreceptors of the eye Inner ear hair cells Taste buds of the tongue Sensory Receptors Classified by Location and Activating Stimuli Exteroceptors located near or at the external surface of the body Hearing, vision, smell, taste, touch, pressure, vibration, temperature, and pain Interoceptors located in blood vessels, visceral organs and nervous system Proprioceptors located in muscles, tendons, joints, and inner ear Body position, muscle length and tension, and the position and movement of joints Sensory Receptors Classified by Stimulus Detected Mechanoreceptors mechanical pressure Touch, pressure, vibration, proprioception, hearing and equilibrium, and stretch of blood vessels and internal organs Thermoreceptors changes in temperature Nociceptors physical or chemical damage to tissues Photoreceptors light detection in retina Chemoreceptors taste and smell Osmoreceptors osmotic pressure of body fluids Somatic Sensory Pathways Ascending pathways Neuronal composition First-order neurons body to spinal cord Second-order neurons spinal cord and brain stem to thalamus

7 Third-order neurons thalamus to primary somatosensory of cerebral cortex Pathways Anteriolateral pain, thermal, tickle, itch, crude touch, and pressure (Fig 16.6) Uses spinothalamic tracts Posterior column-medial lemniscus (lemniscus = ribbon) (Fig 16.5) Uses spinothalamic and spinoreticular tracts fine touch, stereognosis (recognize objects by touch) proprioception, and vibratory sensations Cerebellar posture, balance, and coordination of skilled movements (Table 16.3 & 16.12) Uses spinocerebellar tracts Somatic Motor Pathways Upper motor neurons located in cerebral cortex (Fig 16.9) Basal ganglia neurons Cerebellar neurons (Fig 16.12) Local circuit neurons interneurons synapse on lower motor neurons in brain stem and spinal cord coordination of rhythmic activity Descending tracts (Table 16.4 & Fig 16.10) Corticospinal cerebral cortex to spinal cord (crosses in decussation of pyramids) Rubrospinal red nucleus to spinal cord Reticulospinal reticular formation to spinal cord Tectospinal Corpora quadragemini to spinal cord Reflexes Reflex - quick involuntary response to an internal or external stimulus (results in either somatic or visceral reflex) Parts (Fig )

8 Receptor Sensory neuron CNS integration center Motor neuron Effector - muscle or gland that responds to motor neuron impulse