Chapter 12 The Central Nervous System Part D Shilla Chakrabarty, Ph.D.

The Spinal Cord: Embryonic Development Develops from caudal portion of embryonic neural tube By week 6 two clusters of neuroblasts that have migrated from the original neural tube can be recognized: Alar plate—will become interneurons; axons form white matter of cord Basal plate—will become motor neurons; axons will grow to effectors Neural crest cells that come to lie alongside the cord form the dorsal root ganglia sensory neurons; axons grow into the dorsal aspect of the cord White matter Neural tube cells Central cavity Alar plate: interneurons Dorsal root ganglion: sensory neurons from neural crest Basal plate: motor neurons

Spinal Cord Location Enclosed in the vertebral column, begins from the foramen magnum of skull Ends as conus medullaris at L1 or L2 vertebra just inferior to the ribs Functions Provides two-way communication to and from the brain Contains spinal reflex centers

Spinal Cord: Protection Protected by bone, meninges, and CSF Single layer of dura mater is not attached to bony walls of vertebral column Cushion of fat and a network of veins in the epidural space between the vertebrae and spinal dura mater CSF in subarachnoid space Denticulate ligaments: extensions of pia mater that secure cord to dura mater Filum terminale: fibrous extension from conus medullaris; anchors the spinal cord to the coccyx

Ligamentum flavum Lumbar puncture needle entering subarachnoid space Supra- spinous ligament L5 Filum terminale S1 Inter- vertebral disc Cauda equina in subarachnoid space Arachnoid matter Dura mater

Spinal Cord About the width of a thumb for most of its length, but has enlargements in cervical and lumbar regions Spinal nerves 31 pairs attach to the cord by paired roots Cervical and lumbar enlargements The nerves serving the upper and lower limbs emerge here Cauda equina The collection of nerve roots at the inferior end of the vertebral canal that resemble a horse’s tail

Cross-Sectional Anatomy Two lengthwise grooves divide cord into right and left halves Ventral (anterior) median fissure Dorsal (posterior) median sulcus Gray commissure—connects masses of gray matter; encloses central canal

Cross-Sectional Anatomy Two lengthwise grooves divide cord into right and left halves Ventral (anterior) median fissure Dorsal (posterior) median sulcus Gray commissure—connects masses of gray matter; encloses central canal (a) Cross section of spinal cord and vertebra Epidural space (contains fat) Pia mater Spinal meninges Arachnoid mater Dura mater Bone of vertebra Subdural space Subarachnoid space (contains CSF) Dorsal root ganglion Body of vertebra

(b) The spinal cord and its meningeal coverings Dorsal median sulcus Gray commissure Dorsal funiculus Dorsal horn White columns Ventral funiculus Gray matter Ventral horn Lateral funiculus Lateral horn Dorsal root ganglion Spinal nerve Central canal Dorsal root (fans out into dorsal rootlets) Ventral median fissure Ventral root (derived from several ventral rootlets) Pia mater Arachnoid mater Spinal dura mater (b) The spinal cord and its meningeal coverings Figure 12.31b

Gray Matter Dorsal horns—interneurons that receive somatic and visceral sensory input Ventral horns—somatic motor neurons whose axons exit the cord via ventral roots Lateral horns (only in thoracic and lumbar regions) – autonomic or sympathetic neurons Dorsal root (spinal) ganglia—contain cell bodies of sensory neurons Somatic sensory neuron Dorsal root (sensory) Dorsal root ganglion Visceral motor neuron Spinal nerve Ventral root (motor) Ventral horn (motor neurons) Dorsal horn (interneurons) motor Interneurons receiving input from somatic sensory neurons Interneurons receiving input from visceral sensory neurons Visceral motor (autonomic) neurons Somatic motor neurons

White Matter Composed of myelinated and unmyeinated nerve fibers Fibers allow communication between different parts of the spinal cord and between the cord and brain Fibers run in three directions: Ascending- up to higher centers (sensory) Descending- down to cord from brain, or within the cord to lower levels (motor tracts) Transverse tracts- cross from one side to the other (commissural fibers) White matter on each side is divided into three white columns or funiculi, named according to their position as dorsal (posterior), lateral, and ventral (anterior) funiculi Each funiculus contains several fiber tracts Each spinal tract is composed of axons with similar functions

Pathway Generalizations Decussation: Most pathways decussate (cross over)from one side of the CNS to the other Relay: Most pathways consist of a chain of two or three neurons (a relay) that contribute to successive tracts of the pathway Somatotopy: Most pathways exhibit somatotopy, a precise spatial relationship among tract fibers that reflect orderly mapping of the body Symmetry: All pathways are paired symmetrically (one on each side of the spinal cord or brain)

Ventral corticospinal tract Ventral spinothalamic tract Ascending tracts Descending tracts Fasciculus gracilis Ventral white commissure Dorsal white column Fasciculus cuneatus Lateral reticulospinal tract Dorsal spinocerebellar tract Lateral corticospinal tract Rubrospinal tract Ventral spinocerebellar tract Medial reticulospinal tract Lateral spinothalamic tract Ventral corticospinal tract Ventral spinothalamic tract Vestibulospinal tract Tectospinal tract Figure 12.33

Ascending Pathways Conduct sensory impulses upward, through chains of three neurons First-order neurons: Cell bodies in a ganglion; conduct impulses from cutaneous receptors and proprioceptors to spinal cord or brain stem; branches diffusely and synapse with second-order neuron Second-order neurons – Interneurons with cell bodies in dorsal horn of spinal cord or medullary nuclei; axons extend to thalamus or cerebellum where they synapse Third-order neuron – Interneuron with cell body in thalamus; axon extends to somatosensory cortex Transmitting Somatosensory Information To Sensory Cortex: Two pathways transmit somatosensory information to the sensory cortex via the thalamus for conscious interpretation: Dorsal column-medial lemniscal pathways Spinothalamic pathways These pathways collectively provide discriminative touch and conscious proprioception Third pathway, spinocerebellar pathway, terminates in cerebellum and does not contribute to sensory perception

Medial lemniscus (tract) (axons of second-order neurons) Dorsal spinocerebellar tract (axons of second-order neurons) Medial lemniscus (tract) (axons of second-order neurons) Nucleus gracilis Nucleus cuneatus Medulla oblongata Fasciculus cuneatus (axon of first-order sensory neuron) Joint stretch receptor (proprioceptor) Axon of first-order neuron Cervical spinal cord Fasciculus gracilis (axon of first-order sensory neuron) Muscle spindle (proprioceptor) Lumbar spinal cord Touch receptor (a) Spinocerebellar pathway Dorsal column–medial lemniscal pathway Figure 12.34a (2 of 2)

Primary somatosensory cortex Axons of third-order neurons Thalamus Cerebrum Midbrain Cerebellum Pons (a) Spinocerebellar pathway Dorsal column–medial lemniscal pathway Figure 12.34a (1 of 2)

Anterolateral Pathways Formed by lateral and ventral spinothalamic tracts whose fibers cross over in the spinal cord Transmit pain, temperature, and coarse touch impulses within the lateral spinothalamic tract Axons of first-order neurons Temperature receptors Lateral spinothalamic tract (axons of second-order neurons) Pain receptors Medulla oblongata Cervical spinal cord Lumbar spinal cord (b) Spinothalamic pathway

Primary somatosensory cortex Axons of third-order neurons Thalamus Cerebrum Midbrain Cerebellum Pons (b) Spinothalamic pathway Figure 12.34b (1 of 2)

Spinocerebellar Tracts Last pair or ascending pathways: ventral and dorsal tracts that terminate in the cerebellum These pathways do not contribute to conscious sensation Convey information about muscle or tendon stretch to the cerebellum Cerebellum uses this information to coordinate skeletal muscle activity

Medial lemniscus (tract) (axons of second-order neurons) Dorsal spinocerebellar tract (axons of second-order neurons) Medial lemniscus (tract) (axons of second-order neurons) Nucleus gracilis Nucleus cuneatus Medulla oblongata Fasciculus cuneatus (axon of first-order sensory neuron) Joint stretch receptor (proprioceptor) Axon of first-order neuron Cervical spinal cord Fasciculus gracilis (axon of first-order sensory neuron) Muscle spindle (proprioceptor) Lumbar spinal cord Touch receptor (a) Spinocerebellar pathway Dorsal column–medial lemniscal pathway Figure 12.34a (2 of 2)

Primary somatosensory cortex Axons of third-order neurons Thalamus Cerebrum Midbrain Cerebellum Pons (a) Spinocerebellar pathway Dorsal column–medial lemniscal pathway Figure 12.34a (1 of 2)

Descending Pathways and Tracts Deliver efferent impulses from the brain to the spinal cord Direct pathways—pyramidal tracts that regulate fast and fine (skilled movements) Indirect pathways— complex multineuronal pathways that regulate muscles for coarse movements; muscles for head, neck and eye movements, and axial muscles for balance and posture Involve two neurons: Upper motor neurons: Pyramidal cells in primary motor cortex (precentral gyrus) Axons synapse with interneurons or ventral horn motor neurons Lower motor neurons: Ventral horn motor neurons Innervate skeletal muscles

Spinal Cord Trauma Any localized damage to spinal cord or its roots leads to some functional loss Functional losses Parasthesias: Sensory loss Paralysis: Loss of motor function

Spinal Cord Trauma: Paralysis 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 Without stimulation, muscles atrophy Spastic paralysis—damage to upper motor neurons of the primary motor cortex Spinal neurons remain intact; muscles are stimulated irregularly by reflex activity No voluntary control of muscles

Spinal Cord Trauma Transection Cross sectioning of the spinal cord at any level Results in total motor and sensory loss in regions inferior to the cut Paraplegia—transection between T1 and L1 Quadriplegia—transection in the cervical region NOTE: Anyone with traumatic spinal cord injury must be watched for symptoms of spinal shock, a transient period of functional loss that follows the injury

Poliomyelitis Destruction of ventral horn motor neurons by the poliovirus Early symptoms include fever, headache, muscle pain and weakness, and loss of some somatic reflexes Later paralysis develops and muscles atrophy Death may occur due to paralysis of respiratory muscles or cardiac arrest Survivors often develop postpolio syndrome many years later, as neurons are lost

Amyotrophic Lateral Sclerosis (ALS) Also called Lou Gehrig’s disease is a devastating neuromuscular condition Involves progressive destruction of ventral horn motor neurons and fibers of the pyramidal tract Symptoms—loss of the ability to speak, swallow, and breathe Death typically occurs within five years Linked to glutamate excitotoxicity which kills neurons, attack by the immune system, or both

Developmental Aspects of the CNS CNS is established during the first month of development Gender-specific areas appear in both brain and spinal cord, depending on presence or absence of fetal testosterone Maternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the developing CNS Smoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damage

Developmental Aspects of the CNS The hypothalamus is one of the last areas of the CNS to develop Visual cortex develops slowly over the first 11 weeks Neuromuscular coordination progresses in superior-to-inferior and proximal-to-distal directions along with myelination Growth and maturation of the nervous system continues throughout childhood and reflect progressive myelination The brain reaches its maximum weight in the young adult

Developmental Aspects of the CNS Age brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80s Shrinkage of brain accelerates in old age Excessive use of alcohol causes signs of senility unrelated to the aging process