13.1: The Spinal Cord

Conduction

Nerve fibers conduct sensory and motor information up and down the spinal cord

Neural Integration

Spinal neurons receive input from multiple sources, integrate it, and execute appropriate output (ex: bladder control)

Locomotion

spinal cord contains central pattern generators: groups of neuronal circuits that coordinate repetitive sequences of contractions/outputs to the extensor/flexor muscles for walking

Reflexes

Involuntary responses to stimuli that are vital to posture, coordination and protection; involve brain, spinal cord, and peripheral nerve

Spinal Cord

Cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull and passes through the vertebral canal as far as the inferior margin of the first lumbar vertebra (L1); gives rise to 31 pairs of spinal nerve (passes through intervertebral foramina) and is segmented

Segment Areas of Spinal Cord

Cervical, thoracic, lumbar, and sacral regions; named for the vertebral column from which the spinal nerve emerge

Cervical Enlargement

Inferior cervical region of the spinal cord, that gives rise to nerves of the upper limbs

Lumbar Enlargement

Lumbosacral region of the spinal cord that gives rise to nerves in the pelvic region and lower limbs

Medullary Cone (conus medullaris)

Inferior to the lumbar enlargement; cord tapers to a point inferior to lumbosacral enlargement

Cauda Equina

From medullary cone and lumbar enlargement; bundle of nerve roots that occupy the vertebral canal from L2 to S5; innervates the pelvic organs and lower limbs

Meninges of Spinal Cord

Three fibrous membranes that enclose the brain and spinal cord; function is to separate the soft tissue of the CNS from the bones of the vertebrae and skull

Dura Mater

Most superficial layer of meninge; dural sheath; tough collagenous membrane; surrounds spinal cord and is separated from vertebrae by epidural space

Epidural Space

Space between the sheath and vertebral bone; contains blood vessels, adipose tissue, and loose connective tissue

Arachnoid Mater

Middle layer of meninge; adheres to dural sheath and is separated from pia by fibers spanning the subarachnoid space that is filled with cerebrospinal fluid (CSF); consists of simple squamous epithelial, arachnoid membrane, and loose collagenous and elastic fibers (Lumbar puncture (spinal tap) takes sample of CSF)

Subarachnoid Space

Between arachnoid and pia mater; filled with cerebrospinal fluid (CSF)

Pia Mater

Deepest layer of meninges; delicate, translucent membrane that follows contours of spinal cord and continues beyond medullary cones as a fibrous strand (terminal filum); forms part of coccygeal ligament

Gray Matter in Cross Section

In the brain and spinal cord; contains somas, dendrites, and proximal parts of the axons of neurons and little myelin; function is to synthesize integration (information processing) in CNS; Central core (looks like butterfly or H) 2 dorsal/posterior horns and 2 ventral/anterior horns on both sides; connected by gray commisure, both commisures are connected by a central canal (most are collapsed in adults but sometimes remain open and are lined with ependymal cells and filled with CSF)

Lateral Horns of Spinal Cord

Visible from T2 through L1; contains neurons of sympathetic nervous system; function is to send their axons out of the cord via the ventral root along with somatic efferent fibers

Spinal Nerves Relationship with the Gray Matter of the Spinal Cord

Spinal nerve branches into 2 main parts, dorsal root and ventral root; they enter/connect with the spinal cord at the dorsal and ventral horn respectively

Dorsal Root/Horn

Carry sensory information to spinal cord (CNS); either connect with interneurons at the level they enter or travel and connect with a different interneuron

Ventral Root/Horn

Thicker than dorsal horns; contain large somas of somatic motor neuron; motor information

White Matter in Cross Section

In the brain and spinal cord; surrounds the gray matter; contains abundance of myelinated axons; axons are generally in bundles (tracts) that course up and down the cord; function is to provide communication/carry signals between different levels of the CNS

Funiculi/Columns

3 Pairs of the white matter bundles on axons; dorsal/posterior, lateral, and ventral/anterior columns

Fasciculi/Tracts

Subdivisions of the funiculi columns

Spinal Tracts

All nerve fibers in a given tract have similar origin, destination, and function

Decussation

Tracts that cross the midline (right and left) as they go up or down the brainstem and spinal cord

Contralateral

When the origin and destination of a tract are on opposite sides of the body

Ipsilateral

When the origin and destination of the tract are on the same side of the body

Ascending Tracts

Carry sensory signals up the spinal cord; travel across 3 neurons from origin (receptors) to destination in the sensory area in the brain; 1st-order neuron detects stimulus and transmit signal to spinal cord/brainstem, 2nd-order neuron continues to the thalamus at the upper end of brainstem, and 3rd-order neuron carries signal the rest of the way to sensory region of cerebral cortex

Gracile Fasciculus

Ascending tract; carries signals for vibration, visceral pain, deep and discriminative touch, and proprioception from lower limbs and lower trunk; carries signals from midthoracic and lower parts of body; terminates at gracile nucleus of medulla oblongata; first order nerve fiber travels up ipsilateral of spinal cord; starts below vertebra T6 and composes the entire posterior funiculus

Cuneate Fasciculus

Ascending tract; first order neuron carries same sensory signals as gracile fasciculus (signals for vibration, visceral pain, deep and discriminative touch, and proprioception); its signals are from upper limb and chest; occupies lateral portion of posterior funiculus; second order neuron of gracile and cuneate nuclei decussate and form the medial lemniscus (tract leading to thalamus); third order neuron goes from thalamus to cerebral cortex, carrying signals to cerebral hemisphere

Spinothalamic Tract

Ascending tract; passes up the anterior and lateral funiculi of spinal cord; carries signals for pain (anywhere), pressure, temperature, light touch, tickle, and itch; first-order neurons end in posterior horn of spinal cord, second-order neurons start in posterior horn, then decussate and form the spinothalamic tract, third-order neurons continue from there to cerebral cortex

Spinoreticular Tract

Ascending tract; travels up anterolateral system; carries pain signals resulting from tissue injury; first-order neurons enter posterior horn and immediately synapse with second-order neurons, second-order neurons decussate to opposite anterolateral system ascend the cord and end in reticular formation: loosely organized core of gray matter in the medulla and pons, third-order neurons continue from the pons to the thalamus, and fourth-order neurons complete the path to the cerebral cortex

Spinocerebellar Tract

Ascending tract; anterior and posterior spinocerebellar tracts travel through lateral funiculus; carry proprioceptive signals from limbs and trunk up to the cerebellum; first-order neurons originate in the muscles and tendons and end in posterior horn of the spinal cord, second-order nerves ascend spinocerebellar tracts and end in cerebellum providing it with feedback needed to coordinate movements; posterior spinocerebellar tract stays ipsilateral while anterior spinocerebellar tracts cross over and travel up contralateral side, but cross back to end in ipsilateral cerebellum

Descending Tracts

Carry motor signals down brainstem and spinal cord; upper motor neuron originates in cerebral cortex or brainstem and terminates on a lower motor neuron, lower motor neuron neurosoma is in brainstem or spinal cord that terminates at target organ

Corticospinal Tracts

Descending tract; carries signals from cerebral cortex for precise, finely coordinated movements; starts at cerebral cortex and travels the pyramids on medulla oblongata, most decussate in lower medulla forming the lateral corticospinal tract on contralateral side of spinal cord; some fibers form the anterior (ventral) corticospinal tract that descends in the ipsilateral side of spinal cord and decussates inferiorly

Tectospinal Tract

Descending tract; begins in midbrain region (tectum); Crosses to contralateral side of midbrain; reflex turning of head in response to sights and sounds

Lateral and Medial Reticulospinal Tracts

Descending tract; adrenaline rush; originate in the reticular formation of brainstem; control muscles of upper and lower limbs, especially those for posture and balance; contain descending analgesic pathways; reduce the transmission of pain signals to brain

Lateral and Medial Vestibulospinal Tracts

Descending tract; begin in brainstem vestibular nuclei; receive impulses for balance from inner ear; control extensor muscles of limbs for balance control

Poliomyelitis

Caused by poliovirus; destroys motor neurons in brainstem and anterior horn of spinal cord; signs of polio include muscle pain, weakness, and loss of some reflexes, followed by paralysis, muscular atrophy, and respiratory arrest; virus spreads by fecal contamination of water

Amyotrophic Lateral Sclerosis (ALS)

Destruction of motor neurons and muscular atrophy; also sclerosis (scarring) of lateral regions of the spinal cord; astrocytes fail to reabsorb the neurotransmitter glutamate from the tissue fluid which accumulates to toxic levels; early signs are muscular weakness, difficulty speaking, swallowing, and using hands; sensory and intellectual functions remain unaffected