Anatomy ch 13

• three CT coverings that encircle the spinal cord and brain
• The spinal meninges surrounds the spinal cord and are continuous with
• The cranial meninges surrounds the brain
• 1. Dura mater
• 2. Arachnoid mater
• 3. Pia Mater

• 1. Epidural space - a space that covers the dura mater - contains a cushion of fat and CT
• 2. Dura mater (first meninx) - composed of dense, irregular CT - forms a sac from the foramen magnum to the second sacral vertebra
• 3. Subdural space - between the dura mater and arachnoid mater - contains interstitial fluid
• 4. Arachnoid mater (second Meninx) - avascular covering - looks like web - collagen fibers and elastic fibers
• 5. Subarachnoid space - contains cerebrospinal fluid ( shock absorber and suspension system for spinal cord and brain)
• 6. Pia Mater - innermost meninx (meningial layer) - adheres to the surface of the spinal cord and brain - HIGHLY VASCULAR - supplies blood and nutrients to spinal cord - squamous to cuboidal cells with collagen and elastic fibers

needle is inserted into the Subarachnoid space between L4 and L5 (below spinal cord so less chance of injury to spinal cord) and cerebrospinal fluid is removed for diagnostic purposes

Supracristal line - line drawn across the highest points of the iliac crest passes through the spinous process of the L4 vertebra

triangular shaped membranous extensions of the pia mater that suspend the spinal cord in the middle of its dural sheath

• project laterally and fuse with the arachnoid mater and and innersurface of the dura mater between the anterior and posterior nerve roots of the spinal nerve on either side
• 

spinal cord - roughly cylindrical - extends from medulla oblingata (inferior part of brain) to the superior border of L2

cervial enlargement- extend from C4 to T1 - contains nerves to and from the upper limbs

lumbar enlargement - from T9 to T12 - nerves to and from the lower limbs

conus medullaris - tapering where spinal cord terminates between L1 and L2

filum terminale - arises from the conus medullaris - extension of pia mater that extends inferiorly and blends with the arachnoid mater and dura mater - ANCHORS THE SPINAL CORD TO THE COCCYX

spinal nerves - paths of communication between the spinal cord and specific regions of the body. 31 pairs of spinal nerves emerge at regular intervals from the intervertebral foramina.

cauda equina - the roots of spinal nerves for the lumbar, sacral and coccygeal regions. Fall away from the conus medullaris like a horses tail

posterior (dorsal) root - contains only sensory axons - conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the CNS

Dorsal root ganglion - swelling of the posterior root that contains the cell bodies of sensory neurons

anterior (ventral) root - contains axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands)

Anterior median fissure - wide grove on the anterior (ventral) side - divided white matter into left and right

Posterior median sulcus - narrow furrow on the posterior (dorsal) side

Grey commisure - forms crossbar of grey H

Central canal - in the grey commisure - runs entire length of spinal cord and is filled with cerebrospinal fluid - at its superior end it is continuous with the fourth ventrical in the medulla oblongata

anterior white commisure - anterior to the grey commisure - connects the white matter of the right and left side of the spinal cord

posterior (dorsal) grey horns - contain cell bodies and axons of interneurons as well as axons of incoming sensory neurons ( cells bodies of the sensory neurons are in the dorsal root ganglion)

anterior (ventral) grey horns - contain somatic motor nuclei which are clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles

lateral grey horns - present only in thoracic and upper lumbar segments of the spinal cord - contain autonomic motor nuclei which are clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands

• white columns 1) anterior white columns 2) posterior white columns 3) lateral white columns
• each column contains distince bundles of anons having a common origin or destination

• tracts - bundles of axons in the CNS-may extend long distances up and down the spinal cord - white matter in the white columns
• sensory ascending tracts - axons that conduct nerve impulses toward the brain
• motor descending tracts - axons that carry nerve impulses from the brain

spinal nerves and the nerves that branch from them are part of the PNS - a typical spinal nerve has two connections to the cord - a posterior root and an anterior root

Mixed nerves - the posterior and anterior root joint to form a spinal or mixed nerve at the intervertebral foramin - called mixed because the posterior root contains sensory axons and the anterior root contains motor anons

intervertebral foramina - spaces in the adjoining vertebrae that the nerves pass through. spinal nerve C1-C7 exit above the corresponding vertebrae, C8 exits between C7- T1. nerves T1-L5 exit below their corresponding vertebrae.

• endoneurium - protective CT layer around individual axons in a nerve - innermost layer
• perineurium - CT layer around fascicles - middle layer
• epineurium - outermost covering over the entire nerve

fascicles - axons arranged in bundles within the nerve

after passing through it intervertibral foramin, a spinal nerve divides into several branches

posterior ramus - serves the deep muscles and the skin of the posterior surface of the trunk

anterior ramus - serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk

meningeal branch - 1st branch - goes back into the vertebral cavity and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord and meninges

rami communicantes - components of the ANS - go to the sympathetic trunk ganglion - grey is closer to the spinal cord and white is more lateral

axons from the anterior rami of spinal nerves that from networks on both the left and right sides of the body by joining with axons from anterior rami of adjacent nerves

cervical plexus - supplies the skin and muscles of the head, neck, superior portion of the shoulders and chest, and diaphram

brachial plexus - supplies the shoulders and upper limbs

lumbar plexus - supplies the anterolateral abdominal wall, external genitals, and part of the lower limbs

sacral and coccygeal plexuses - supplies the buttocks, perineum, and lower limbs

the anterior rami of spinal nerves T2-T12 do not join plexuses but directly connect to the structures they supply in the intercostal spaces

an area of the skin that provides sensory input to the CNS via the posterior roots of one pair of spinal nerves or via the trigeminal (V) nerve

propagate nerve impulses to and from the CNS

white matter in the spinal cord are highways for nerve impulse propagation - sensory impulses toward the brain and motor nerve impulses from the brain

grey matter of the spinal cord receives and integrates incoming and outgoing information - summing of excitaory and inhibiotry postsynaptic signals

• the name of a tract indicates its position in the white matter and where it begins and ends
• eg corticospinal tract - starts in the cerebral cortex, ends in the spinal cord, going down - would be in the anterior white column

a fast, inborn, involuntary response to a change in the internal or external environment

spinal reflex - integration takes place in the spinal cord grey matter (patellar reflex)

cranial reflex - if integration occurs in the brain stem rather than the spinal cord (tracking movements of eyes as you read)

Somatic reflex - involve contraction of skeletal muscles

autonomic reflex - involve responses of smooth muscle, cardiac muscle, and glands

the pathway followed by nerve impulses that produce a reflex

• 1. Sensory receptor - responds to a stimulus by producing a generator or receptor potential - if generator potential reaches threshold level of depolarization it will trigger nerve impulse in the sensory neuron
• -stimulus - a change in the internal or external environment
• 2. Sensory neuron - axon conducts impulses from receptor to integrating center
• 3. Integrating center - one or more regions in the CNS (grey matter) that relay mpulses from sensory to motor neurons
• - monosynaptic reflex arc - reflex pathway with only one synapse in the CNS
• - polysynaptic reflex arc - involces two or more types of neurons and more than one CNS synapse
• 4. Motor neuron - axon conducts impulses from integrating center to effector
• 5. Effector - muscle or gland that responds to motor nerve impulse (if effector is skeletal muscle it is a somatic reflex - if effector is smooth muscle, cardiac muscle, or glands it is autonomic reflex)

• causes contraction of a skeletal muscle in response to stretching of the muscle - patellar reflex
• MONOSYNAPTIC REFLEX ARC
• IPSILATERAL REFELX - all monosynaptic reflexes are ipsilateral (sensory nerve impulses enter the spinal cord on the same side as the motor nerve impulses leave it
• MUSCLE TONE - small degree of contraction present when the muscle is at rest - set by how vigorously a muscle spindle responds to stretching
• RECIPROCAL INNERVATION - the components of a neural circuit simulaneously cause contraction of one muscle and relaxation of it antagonists

• 1. stretching of muscle stimulates sensory receptors in the muscle spindles - spindles monitor changes in the length of the muscle
• 2. muscle spindle generate nerve impulses - travel along a somatic sensory neuron through the posterior root into the cord
• 3. in the spinal cord (integration center) the sensory neuron make excitatory synapse - activates a motor neuron in anterior grey horn
• 4. nerve impulses from motor neuron travel from spinal cord into the anterior root to stimulated muscle
• 5. ACh is released by nerve impulses at NMJ - triggers action potentials and the muscle contracts

• operates as a feedback machanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that the muscle might be torn
• Tendon reflex is IPSILATERAL AND POLYSYNAPTIC

tendon (golgi tendon) organs - the sensory receptors for this reflex that lie in a tendon near its junction with a muscle

• 1. tendon organ senses tension in the tendon
• 2. nerve impulses along a sensory neuron to the spinal cord
• 3. in integration center (spinal cord) inhibitory signal to motor neuron
• 4. inhibitory neurotransmitters inhibit the motor neuron which generate fewer motor impulses
• 5. muscle relaxes

• step on a tack and immediately withdraw your leg
• POLYSYNAPTIC AND IPSILATERAL

INTERSEGMENTAL REFLEX ARC - a single sensory neuron can activate several motor neurons, thereby stimulating more than one effector

• 1. Sensory receptor - toe - pain-sensitive neuron
• 2. Sensory neuron - sends signal to spinal cord
• 3. Integrating center - spinal cord - activates interneurons that extend to several spinal cord segements
• 4. motor neurons- interneurons activate motor neurons in several spinal cord segments
• 5. effector- ACh released by motor neurons causes flexor muscles in thigh to contract moving leg back

when the flexor reflex causes you to withdraw a limb if you step on something it initiates a crossed extensor reflex to help you keep your balance

POLYSYNAPTIC AND CONTRALATERAL

Contralateral reflex arc - sensory impulses enter one side of the spinal cord and motor signals exit on the opposite side