Anatomy and Physiology Marieb’s Human Anatomy and Physiology Ninth Edition Marieb w Hoehn Chapter 12 The Central Nervous System Spinal Cord Reflexes Plexuses 94 min, 51 slides

Lecture Overview The spinal cord Spinal cord structure Spinal meninges Cross-sectional anatomy of the spinal cord Ascending and descending spinal tracts Reflexes Spinal Nerves Nerve plexuses

Spinal Cord Structure extends from the foramen magnum to 2nd lumbar vertebra cervical and lumbar enlargements cauda equina (horse’s tail) – thin nerve fibers that exit at different level than they arise (note that spinal cord does not extend into this area of the lumbar spine). Begins around L2 and extends to S5. Good area for lumbar puncture and collection of CSF. Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007 Spinal cord extends entire length of vertebral column early in fetal development, to L3 at birth, and to L1 in an adult. Thus, it occupies only the upper two thirds of the vertebral column in an adult. The lower third of the vertebral column is occupied by the cauda equina.

Overview of the Spinal Cord What type of vertebra is shown above?

Meninges of the Spinal Cord Figure from: Marieb Human Anatomy & Physiology, Pearson 2013 Dura mater – thick, collagenous membrane (about thickness of rubber kitchen glove) Arachnoid mater – simple squamous epithelium; loose meshwork of collagenous and elastic fibers spanning the gap (subarachnoid space) Pia mater – delicate membrane, close follows contours of nervous tissue that it covers. - dura mater – outer, tough (anchoring dural folds) - arachnoid mater – web-like - pia mater – inner, delicate - Subdural space – like interstitial fluid - Subarachnoid space – CSF

Organization of Spinal Cord Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Organization of Spinal Gray Matter Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Posterior gray horn = sensory Lateral gray horn = visceral motor Anterior gray horn = somatic motor Anterior root Posterior root Gray matter = dendrites and unmyelinated axons

Spinal Cord and Nerve Roots Ventral root - axons of motor neurons whose cell bodies are in spinal cord Dorsal root - axons of sensory neurons in the dorsal root ganglion Dorsal root ganglion - cell bodies of sensory neurons

Organization of Spinal White Matter Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 White matter = Myelinated axons

Functions of the Spinal Cord a. is a conduit for nerve impulses to and from the brain (nerve tracts) b. is a center for spinal reflexes

Tracts of the Spinal Cord Ascending tracts conduct sensory impulses to the brain Descending tracts conduct motor impulses from the brain to motor neurons reaching muscles and glands All the axons in a tract share a common origin and destination Tracts are usually named for their place of origin (1st) and termination (2nd) Most axons cross over during their travel. What will this mean clinically?

Ascending Tracts fasciculus cuneatus/gracilis - fine touch, pressure, body movement - cross (decussate) in medulla spinothalamic - crude pain, temperature, pressure, and touch - cross in spinal cord spinocerebellar - subconscious coordination of muscle movements (1st and 2nd order neurons) - ipsilateral 3 2 1 Decussation (crossing over)

1st, 2nd, and 3rd Order Sensory Neurons 1st order neuron – from receptor to the spinal cord (cell bodies are located in the dorsal root ganglion) 2nd order neuron – from spinal cord to thalamus 3rd order neuron – from thalamus to sensory cerebral cortex - terminate in the cerebral cortex 3 2 1 Decussation

Descending Tracts corticospinal (direct, pyramidal) - voluntary movement of skeletal muscles - lateral cross in medulla - contralateral reticulospinal (indirect, extrapyramidal) - subconscious muscle tone, sweat glands - some lateral cross, anterior do not cross rubrospinal (indirect, extrapyramidal) - subconscious regulation of upper limb tone/movement - cross in brain (less important in humans) Upper motor – begin in precentral gyrus of cortex Decussation Lower Upper MN – Cerebral cortex to spinal cord Lower MN – Spinal cord to effector

Somatic Reflex Arcs Reflexes – automatic, subconscious, quick, stereotyped responses to stimuli either within or outside the body They occur in both the somatic and autonomic divisions

Knee-jerk (Stretch) Reflex helps maintain posture Monosynaptic, Ipsilateral

Withdrawal Reflex protective Polysynaptic, Ipsilateral, Intersegmental

Crossed-Extensor Reflex flexor muscles contract flexor muscles on opposite side inhibited extensor muscles on opposite side contract for balance Polysynaptic, Contralaterial, Intersegmental

Spinal Nerves mixed nerves 31 pairs 8 cervical (C1 to C8) 12 thoracic (T1 to T12) 5 lumbar (L1 to L5) 5 sacral (S1 to S5) 1 coccygeal (Co) THIRTY ONEderful flavors of spinal nerves! Below cervical spine, each spinal nerve leaves inferior to the same numbered vertebra Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Spinal Nerves These are ‘mixed’ nerves (sensory and motor nerve fibers) Ventral (anterior) ramus leads to formation of plexuses Spinal nerves are named according to the level of the spinal cord from which they exit.

Cervical Plexus Nerve plexus – complex network formed by anterior (ventral) branches of spinal nerves; fibers of various spinal nerves are sorted and recombined Contains both sensory and motor fibers Cervical Plexus C1-C4 lies deep in the neck supplies muscles and skin of the neck contributes to phrenic nerve (diaphragm) Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Brachial Plexus C5-T1 lies deep within shoulders supplies shoulder and upper limbs musculocutaneous nerves supply muscles of anterior arms and skin of forearms ulnar nerves supply muscles of forearms and hands supply skin of hands radial nerves supply posterior muscles of arms and skin of forearms and hands axillary nerves supply muscles and skin of superior, lateral, and posterior arms

Lumbosacral Plexus T12 – S5 supplies pelvis and lower limbs extend from lumbar region into pelvic cavity obturator nerves supply adductors of thighs femoral nerves supply muscles and skin of thighs and legs sciatic nerves supply muscles and skin of thighs, legs, and feet May be separated into lumbar, sacral, pudendal, and coccygeal plexuses

Classification of Nerve Fibers SAME Sensory = Afferent Motor = Efferent SOMAtic - Skin - BOnes - Muscles - Articulations Table from: Saladin, Anatomy & Physiology, McGraw Hill, 2007