chapter 13: spinal cord anatomy

antagonists to the stretching reflex

- brain uses muscle fibers inside of muscle spindles to regulate muscle spindle sensitivity. This adjusts how the muscle spindle responds to stretching and sets overall muscle done (small degree of contraction present while the muscle is at rest). It helps avert injury by preventing overstretching of muscles. - there is a polysynaptic reflex for antagonist muscles that act at the same time as the stretching reflex. -The axon from the muscle spindle sensory neuron synapses with inhibitory interneurons. These interneurons synapse with a motor neuron and inhibits it to prevent excitation of the antagonist muscles. -as a result, while stretched muscles contract during a reflex, the antagonistic muscles relax to allow the contraction to happen. -reciprocal innervation: components of neural circuit simultaneously cause contraction of muscle and relaxation of antagonist

tendon reflex

- feedback mechanism to control muscle tension by causing muscle relaxation before muscle force causes tendons to be torn -can override the stretch reflex when the tension is too great -sensory receptors are called tendon organs which are located in a tendon near junction with the muscle. They detect and respond to changes in muscle tension 1. as tension increases in a muscle, the tendon organ is stimulated (depolarized to threshold) and creates nerve impulses that are sent to the spinal cord through a sensory neuron 2. in the spinal cord, the sensory neuron synapses with an inhibitory interneuron that inhibits a motor neuron. This causes hyperpolarization of the motor neuron and causes less nerve impulses. 3. less nerve impulses from the motor neuron causes the muscle to relax and relieve excess tension.

antagonists to tendon reflex arc

- sensory neuron from tendon organ also synapses with excitatory interneuron that synapses with a motor neuron that controls antagonist muscles to the tendon reflex arc ==> triggers the contraction of antagonists and creates reciprocal innervation - sensory neuron also relays nerve impulses to the brain through sensory tracts and informs brain about state of muscle tension throughout body

reflex arc (steps)

- sensory receptor responds to specific stimulus by producing a generator potential (graded potetial). If the generator potential reaches threshold, it triggers a nerve impulse in the sensory neuron. - nerve impulses reach the axon terminals of the sensory neuron that are located in the gray matter of spinal cord or brain stem. -Relay neurons send nerve impulses to the area of brain that allows conscious awareness that reflex has occurred. -Information is integrated through monosynaptic reflex arc (if there is synapse between sensory and motor neurons) or polysynaptic reflex arc (if sensory neuron synapses with interneurons and the interneurons synapse with the motor neuron) -impulses triggered by the integrating center exit the CNS along motor neuron -motor neuron sends nerve impulse to the effector. If the effector is a skeletal muscle, it is a somatic reflex. If the effector is cardiac muscle, smooth muscle, or gland, it is an autonomic reflex. -can determine if there is damage or disease along the reflex arc if the reflex arc is absent or abnormal.

meninges (general)

- three protective, connective tissue coverings that encircle the spinal cord or brain (dura mater, arachnoid mater, and pia mater) -spinal meninges cover the spinal nerves up to the point where they exit the spinal column through the intervertebral foramen -spinal cord is also protected by epidural space or cushion of fat and connective tissue that is located between the dura mater and vertebral column (bone)

dermatomes

- trigeminal nerve (V) serves the skin of face and scalp -dermatome: area of skin that provides sensory input to the central nervous system via spinal nerves or trigeminal nerve -can locate damage to the spinal cord based on knowing which spinal cord segments supply each dermatome. If skin in a region is stimulated but not perceived, the nerves supplying the dermatome are damaged.

reflex

-a fast, involuntary, unplanned sequence of actions from a particular stimulus -spinal reflex: when integration takes place in the spinal cord gray matter (ex. patellar reflex) -cranial reflex: when integration occurs at the brain stem instead of spinal cord -somatic reflexes: contraction of skeletal muscles -visceral reflexes: contraction of smooth muscle, cardiac muscle, and glands but not generally consciously perceived

distribution of spinal nerves

-after passing through the intervertebral foramen, spinal nerves divide into several branches called rami -posterior ramus serves the deep muscles and skin of the posterior surface of the trunk while the anterior ramus serves muscles and structures of the supper and lower limbs and skin on lateral and anterior surfaces of the trunk -meningeal branch: reeenters the vertebral cavity through the intervertebral foramen and supplies vertebrae, blood vessels, and meninges of the spinal cord -rami communicates: components of the autonomic nervous system

intercostal nerves

-anterior rami of spinal nerves T2 - T12 that do not form plexuses. They directly connect to the structures they supply in intercostal spaces. -T2 supplies with the axilla and posteromedial aspect of the arm -T3-T6 extend along costal grooves of the ribs and into the intercostal muscles and skin of the anterior and lateral chest wall -T7-T12 supply intercostal muscles and abdominal muscles -posterior rami supply deep back muscles and skin of the posterior thorax

arachnoid mater

-arachnoid mater: thin, avascular covering comprised of collagen and elastic fiers. It is deep to the dura mater and contiuous through the foramen magnum with the arachnoid mater of the brain. -subdural space: space between the dura mater and arachnoid mater that contains interstitial fluid

plexuses

-axons from anterior rami of spinal nerves that do not go directly to the body structures that they supply and form plexuses (does not include T2-T12) -plexus: networks created by joining axons from anterior rami with one another ==> principal ones are the cervical plexus, brachial plexus, lumbar plexus, and sacral plexus

stretch reflex

-causes contraction of skeletal muscle in response to stretching of the muscle -occurs in a mono synaptic reflex arc where a single sensory neuron forms a synapse with a single motor neuron in the central nervous system 1. muscle spindles detect slight stretching of the muscle 2. muscle spindle sends nerve impulses to the spinal cord through the posterior root of the spinal nerve 3. in the spinal cord, the sensory neuron makes excitatory synapse with a motor neuron the anterior gray horn 4. motor neuron creates a nerve impulse which propagates from anterior root of spinal cord and peripheral nerves to the stimulated muscle. The axon terminals on the motor neuron form neuromuscular junctions with the stretched muscle fibers. 5. Ach is released and causes muscle contractions in the skeletal muscle fiber that relieves stretching. -ipsilateral reflex: sensory nerve impulses enter spinal cord on the same side that the motor nerve impulses leave it

end of the spinal cord (external anatomy)

-conus medullaris is the end of the spinal cord and at the level of the intervertebral disc between the first and second lumbar vertebrae. -filum terminale is an extension of the pia mater underneath the conus medullaris that fuses with the arachnoid and dura mater to anchor the spinal cord to the coccyx

dura mater

-dura mater: thick strong layer of connective tissue, most superficial of the three meninges. It forms a sac from foramen magnum to the second sacral vertebra. It is also continuous with the epineurium or outer covering of the spinal and cranial nerves.

connective tissue coverings of spinal nerves

-each spinal nerve and cranial nerve has layers of protective connective tissue -endoneurium is layer of tissue that wrapped around individual axons within a nerve (myelinated and unmyelinated) -perineurium is connective tissue that wraps around a bundle of axons called fasicles -epineurium is connective tissue that wraps around the entire nerve and its extensions fills in spaces between fasicles -the dura mater and epineurium fuse when the nerve passes through the intervertebral foramen

internal anatomy of spinal cord (gray matter)

-gray matter inside of the spinal cord consists of dendrites and cell bodies of neurons, unmyeliated axons, and neuroglia. It is shaped like a butterfly or letter H -gray commissure: crossbar of the H and contains the central canal in the center that contains cerebrospinal fluid. There is anterior white commissure in front of the gray comissure which connects white matter on the right and left sides of the spinal cord -gray matter contains neuronal cell bodies that form functional groups called nuclei. Sensory nuclei receive input from sensory neurons while motor nuclei provide output to effector tissues using motor neurons. -gray matter is divided into regions called horns. The posterior gray horns contain cell bodies and axons of interneurons and sensory neurons while the anterior gray horns contain somatic motor nuclei for contraction of skeletal muscles. -lateral gray horns are only present in thoracic and upper lumbar segments of the spinal cord and regulate activity of cardiac muscle, smooth muscle, and glands

spinal cord and vertebra length in adults and children

-in adults, it extends from the medulla oblongata to the superior border of the second lumbar vertebra -in new born infants, it extends to the third or fourth lumbar vertebra -the spinal cord does not extend the entire length of the adult vertebral column because elongation of the spinal cord stops earlier than growth of the vertebral column

spinal nerves

-parallel bundles of axons and associated neuroglial cells warapped in connective tissue -first cervical pair of spinal nerves emerges from spinal cord between occipital bone and atlas. C1-C7 spinal nerves exit the vertebral canal above their corresponding vertebrae. C8 exits the vertebral canal between vertebrae C7 and T1. -spinal nerves T1-L5 exit the vertebral canal below their corresponding vertebrae. -roots from sacral spinal nerves and coccygeal spinal nerves enter the sacral canal. S1-S4 exit the sacral canal via the anterior and posterior sacral foramina while spinal nerves S5 and Co1 exit via the sacral hiatus. -posterior and anterior roots unite to form a spinal nerve at the intervertebral foramen. Because posterior root contains sensory axons and anterior root contains motor axons, spinal nerves are considered mixed nerves.

spinal nerves (external anatomy)

-paths of communication between the spinal cord and specific regions of the body -there are 31 pairs of spinal nerves at emerge at regular intervals from intervertebral foramen -there are 8 pairs of cervical spinal nerves, 12 pairs of thoracic, 5 pairs of lumbar, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves -spinal nerves exit from the vertebral canal through the intervertebral formane between adjacent vertebrae. -However, the lumbar, sacral, and coccygeal spinal nerves leave the column at the angle inferiorly and toward the filum terminale and create the cauda equina

pia mater

-pia mater: thin tissue that adheres to the surface of spinal cord and brain and made of collagen and elastic fiber bundles. There are many blood vessels that supply oxygen and nutrients to the spinal cord. -pia mater also contains denticulate ligaments or thickenings of the pia mater that fuse with the arachnoid mater. They suspend the spinal cord in the middle of the dural sheath and protects the spina cord from sudden displacement. -subarachnoid space is space between the arachnoid mater and pia mater that contains cerebrospinal fluid.

roots and rootlets (external anatomy)

-roots: 2 bundles of axons that connect spinal nerves to the segment of the cord, made of rootlets (smaller bundles of axons) -the posterior root contains sensory axons from sensory receptors into the central nervous system -the posterior root has a swelling called the posterior root ganglion that contains cell bodies of sensory neurons. -anterior root contain axons of motor neurons

spinal cord physiology

-spinal cord maintains homeostasis by nerve impulse propagation and integration of information -white matter tracts are highways for nerve impulse propagation while gray matter receives and integrates incoming and outgoing information

spinal cord segment variation

-spinal cord segments vary in amount of gray matter, which is largest in cervical and lumbar segments because they are responsible for sensory and motor innervation of the limbs. -more sensory and motor tracts in supper segments of spinal cord than lower segments -amount of white matter decreases from cervical to sacral segments because there are more ascending axons added to spinal cord whit matter to form sensory tracts near cervical segments and motor tracts decrease in thickness as more descending axons leave motor tracts to synapse with neurons in gray matter of spinal cord

propagate up the spinal cord

-spinothalamic tract: conveys nerve impulses for pain, temperature, itching, pressure, and crude touch -posterior column contains the gracile fasciulus and cuneate fasciculus that convey nerve impulses for light pressure, vibration, proprioception, and discriminative touch -these tracts keep the CNS informed of changes in external and internal environments. It is processed by interneurons in the spinal cord and brain -responses to the nerve impulses from these tracts are brought by motor activities (cerebral cortex controls precise voluntary movements while other brain regions regulate automatic movements)

internal anatomy of the spinal cord (white matter)

-white matter surrounds an inner core of gray matter. -the white matter consists of bundles of myelinated axons of neurons. It is divided into left and right sides by an anterior median fissure and posterior median sulcus -divided into three broad regions: anterior white columns, posterior white columns, and lateral white columns -each region is made of tracts or bundles of axons that carry similar information and run up and down the spinal cord (sensory tracts consist of axons that conduct nerve impulses to the brain while motor tracts consist of axons that carry nerve impulses from the brain)

processing of sensory input and motor output by the spinal cord

1. sensory receptors detect the sensory stimulus 2. sensory neurons convey the sensory input along their axons, into the spinal nerve, and into posterior root 3. axons of the sensory neurons can extend into the white matter of the spinal cord and ascend to the brain -enter the posterior gray horn and synapse with interneurons (their axons extend into the white matter of the spinal cord and ascend to the brain) -enter the posterior gray horn and synapse with interneurons and somatic motor neurons involved in spinal reflex pathways 4. axons from higher brain centers form motor tracts that descent from brain into white matter of the spinal cord. They synapse with somatic motor neurons in the white matter 5. somatic motor neurons convey motor output using nerve impulses along their axons and enter the spinal nerve through anterior gray horn and root. From the spinal nerve, they extend to the skeletal muscles of the body. -if motor output goes from spinal cord to smooth muscle, cardiac muscle, glands, then need to activate autonomic motor neurons of the lateral gray horn. In this case, the autonomic motor neurons form nerve impulses along their axon and enter the spinal nerve. 6. if going from the spinal cord to smooth muscle, cardiac muscle, or glands, axons of autonomic motor neurons from spinal cord synapse with another group of autonomic motor neurons in the PNS. The axons of the second group of autonomic motor neurons synapse with cardiac muscle, smooth muscle, or glands.

flexor reflex

1. stepping on sharp object stimulates dendrites of pain sensitive neuron. this causes nerve impulse to generate and propagate into the spinal cord 2. sensory neuron activates interneurons that extend to many spinal cord segments 3. interneurons activate motor neurons which generate nerve impulses. Ach is released by the motor neurons and causes the flexor muscles in thigh to contract and withdraw and leg or limb. - moves limb away from source of damaging stimuli -ipsilateral because the incoming and outgoing impulses enter and exit the same side of the spinal cord -example of intersegmental reflex because a single sensory neuron activates several motor neurons and stimulates multiple effectors

crossed extensor reflex

1. stepping on something sharp stimulates sensory receptor of pain sensitive neuron in right foot 2. sensory neuron generates nerve impulse that reaches the spinal cord and activates interneurons that synapse with motor neurons on the left side of the spinal cord (incoming pain signals cross the opposite side through interneurons) 3. interneurons excite motor neurons which send nerve impulses to the extensor muscles. Ach is released by the motor neurons and causes extensor muscles of the left limb to contract to support weight of the body and maintain balance. -example of a contralateral reflex arc because the sensory nerve impulses enter one side of the spinal cord while the motor nerve exits the spinal cord from the other side -crossed extensor reflex synchronizes the extension of the contralateral limb with the flexion and withdrawal of the stimulated limb

enlargements in the spinal cord (external anatomy)

cervical enlargement: superior enlargement in the spinal cord that extends from the fourth cervical vertebra to the first thoracic vertebra. It is responsible for nerves that go to and from the upper limbs. lumbar enlargement: inferior enlargement of the spinal cord that extends from the ninth to twelfth thoracic vertebra and responsible for neurons that go to and from the lower limbs.