CH. 14 Spinal Cord and Spinal Nerves

Spinal Nerves (cervical, thoracic, lumbar, sacral, coccygeal)

- 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve - Spinal nerves are associated with adjacent vertebra - In cervical region, first pair of spinal nerves (C1) exits between skull and first cervical vertebra. Therefore, cervical nerves take their names from vertebra inferior to them. Spinal nerve between the last cervical vertebra and the first thoracic vertebra is C8. There are 7 cervical vertebra but 8 cervical nerves - Spinal nerves caudal to first thoracic vertebra take their names from the vertebra immediately superior to them - Each peripheral nerve is surrounded by 3 layers of CT: outer epineurium, central perineurium, and inner endoneurium - Epineurium is a tough fibrous sheath forming outermost layer of peripheral nerve, consisting of dense irregular CT - Perineurium is composed of collagenous fibers, elastic fibers, and fibrocytes, dividing the nerve into a series of compartments of axons called fascicles - Endoneurium surrounds individual axons and is composed of irregular CT

grey commissures

- A joining area of spinal cord containing axons crossing from one side of spinal cord to the other side - Anterior grey commissure and posterior grey commissure on anterior and posterior areas of central canal

stretch reflex

- A monosynaptic reflex providing autonomic regulation of skeletal muscle length - Tonic muscle contraction in response to stimulation of muscle proprioreceptors

Ascending vs Descending Tracts

- Ascending tracts carry sensory info from spinal cord toward the brain and descending tracts carry motor commands to spinal cord - Tracts are separated according to destination of motor neuron or source of sensory info - Axons within tract are uniform in diameter, myelination, and conduction speed - Columns within white matter of spinal cord contain tracts composed of axons sharing functional and structural characteristics

Examples of Nerves in Cervical, Brachial, and Lumbar Plexuses

- Cervical plexus innervates muscles of head, neck, chest, and diaphragm - Ex. phrenic nerve, lesser occipital nerve, greater auricular nerve - Brachial plexus innervates pectoral girdle and upper limbs - Ex. axillary nerve, musculocutaneous nerve, radial nerve, median nerve, ulnar nerve - Lumbar plexus and Sacral plexus innervate muscles of pelvis and lower limbs - Ex. Lumbar nerves = obturator nerve, femoral nerve - Ex. Sacral nerves = gluteal nerve, sciatic nerve, tibial nerve, common fibular nerve

posterior/anterior white columns

- Divisions of white matter of the spinal cord - Anterior white columns are located between anterior horns and anterior median fissure - Posterior white columns located between posterior horns and posterior median sulcus - Columns are organized into tracts composed of axons sharing functional and structural characteristics

dorsal/ventral ramus

- Dorsal ramus provides sensory and motor innervation to the skin and muscles of the back - Ventral ramus supplies ventrolateral body surface, structures in the body wall, and limbs

dorsal and ventral roots

- Dorsal roots contain afferent axons of sensory neurons in the dorsal root ganglion - Ventral root leaves spinal cord - Contains efferent axons of somatic motor neurons, and at some levels, efferent visceral motor neurons

cerebrospinal fluid

- Fluid that bathes exposed surfaces of CNS - Prevents contact between neural structures and surrounding bones, supports and cushions brain, and transports nutrients, wastes, and chemicals

Grey vs White Matter

- Grey matter - Areas in CNS dominated by nerve cell bodies, glial cells, and unmyelinated axons - White matter - Regions of CNS dominated by myelinated axons - Spinal cord contains central mass of grey matter containing cell bodies of neurons and neuroglia and a peripheral region of white matter containing myelinated and unmyelinated axons - Cell bodies of neurons within grey matter of spinal cord are organized into groups called nuclei that have specific functions - Axons of white matter are organized into tracts and columns

white/grey ramus

- Grey ramus = bundle of postganglionic sympathetic nerve fibers distributed to effectors in body wall, skin, and limbs by way of spinal nerve - White ramus = bundle of containing myelinated preganglionic axons of sympathetic motor neurons - White and grey ramus branch from spinal nerves T1 to L2

monosynaptic/polysynaptic reflex

- In a monosynaptic reflex, a sensory neuron synapses directly on a motor neuron (sensory afferent synapses directly on motor efferent) - In a polysnaptic reflex, interneurons interposed between the sensory fiber and the motor neurons - More complex reflex and has a longer delay between stimulus and response

pia mater

- Innermost layer of meninges that is bound to the underlying nervous tissue - Anchored to brain and spinal cord surface by astrocytes - Highly vascular membrane that supplies the spinal cord and brain

arachnoid mater

- Middle layer of cranial and spinal meninges - Encloses cerebrospinal fluid and protects the CNS

cervical plexus

- Network of nerves arising from the anterior rami of the first 4 cervical spinal nerves, and some fibers from C5 - Innervate areas of the head, neck, chest, and diaphragm

Reflex Arc

- Neural wiring of single reflex - Begins at receptor and ends at peripheral effector such as a muscle or gland cell 1. Receptor is stimulated and activated 2. Sensory neuron is activated -- Info is carried in the form of an action potential along an afferent fiber and through dorsal roots 3. Information processing in CNS begins when axon terminals of sensory neuron release neurotransmitters that reach postsynaptic membrane of motor neuron or interneuron controlling peripheral effectors 4. Motor neuron is activated when it is stimulated to threshold and conducts an action potential through ventral root of spinal nerve to the peripheral effector organ 5. Activation of motor neuron causes response by peripheral effector, such as skeletal muscle or gland

posterior/anterior/lateral grey horns

- Posterior horns = division of grey matter in spinal cord that contains somatic and visceral sensory nuclei - Anterior horns = contain somatic motor nuclei - Lateral horns = only found between segments T1 and L2 and contain visceral motor nuclei

epidural space

- Space between spinal dura mater and walls of vertebral foramen, containing blood vessels, areolar CT, and adipose tissue - Frequent site of injection for regional anesthesia

meninges

- Specialized membranes that cover and protect central nervous system and spinal nerve roots - Consist of 3 layers: duramater, arachnoid mater, and pia mater - Line the vertebral canal, providing protection, stabilization, nutrition, and shock absorption to spinal cord

dura mater

- Tough and fibrous outermost layer of cranial and spinal meninges - Cranial dura mater has 2 layers: outer periosteal cranial dura is fused to periosteum lining the cranial bones and an inner meningeal cranial dura.

reflex

An immediate involuntary motor response to a specific stimulus - Helps preserve homeostasis by making rapid adjustments in function of organs and organ systems

Innate vs Acquired Reflex

Innate and acquired reflexes are classified as development reflexes - Innate reflexes are genetically determined and acquired reflexes are learned following repeated exposure to stimulus - Development reflexes can change over time

brachial plexus

Network formed by branches of spinal nerves C5-T1 that innervate pectoral girdle and upper limbs

lumbar plexus

Network of nerves originating from anterior rami of T12-L4, and innervate the pelvic girdle and lower limbs

spinal/cranial reflex

Spinal and cranial reflexes are reflexes classified by processing site - Reflexes processed in the brain are called cranial reflexes, and spinal reflexes process in the spinal cord