Chapter 13 Flashcards
Vertebral column
Provide a sturdy shelter for the enclosed spinal cord. The vertebral ligaments, meninges, and cerebrospinal fluid provide additional protection.
Meninges
Singular is meninx; are three protective, connective tissue coverings that encircle the spinal cord and brain.
From superficial to deep, what are the three meninges?
- Dura mater
- Arachnoid mater
- Pia mater
Dura mater
Most superficial meninx. Is a thick, strong layer composed of dense irregular connective tissue.
Arachnoid mater
The middle meninx. Is a thin, avascular covering made of cells and thin, loosely arranged collagen and elastic fibers.
Subdural space
A thin space between the dura mater and the arachnoid mater which contains interstitial fluid.
Pia mater
Innermost meninx. Is a thin, transparent connective tissue layer that adheres to the surface of the spinal cord and brain. Consists of thin squamous to cuboidal cells within interlacing bundles of collagen fibers and some fine elastic fibers. Contains many blood vessels that supply oxygen and nutrients to the spinal cord.
Denticulate ligaments
Triangular-shaped membranous extensions of the pia mater that suspend the spinal cord in the middle of its dural sheath. Extend along the entire length of the spinal cord and protect it against sudden displacement that could result in shock.
Subarachnoid space
Space between the arachnoid mater and the pia mater which contains shock-absorbing cerebrospinal fluid.
Spinal meninges
Surround the spinal cord. Are continuous with the cranial meninges. Cover the spinal nerves up to the point where they exit the spinal column through the intervertebral foramina.
Cranial meninges
Encircle the brain. Are continuous with the spinal meninges.
Epidural space
Space between the dura mater and the wall of the vertebral canal.
Spinal cord
Roughly oval in shape and is flattened slightly anteriorly and posteriorly.
In adults, the spinal cord extends from the ______ to ______. In newborn infants, it extends to ______. Length ranges from ______, with a maximum diameter of approximately ______ in the lower cervical region and is smaller in the thoracic region and at its inferior tip.
Medulla oblongata; L2; L3 or L4; 42 to 45 cm (16-18 in.); 1.5 cm (0.6 in.)
When viewed externally, what two conspicuous enlargements can be seen on the spinal cord?
- Cervical enlargement
- Lumbar enlargement
Cervical enlargement
The superior enlargement. Extends from C4-T1. Nerves to and from the upper limbs arise from here.
Lumbar enlargement
The inferior enlargement. Extends from T9-T12. Nerves to and from the lower limbs arise from here.
Conus medullaris
Tapering, conical structure where the spinal cord terminates.
Filum terminale
An extension of the pia mater that extends inferiorly, fuses with the arachnoid mater, and dura mater, and anchors the spinal cord to the coccyx. Arises from the conus medullaris.
Spinal nerves
Paths of communication between the spinal cord and specific regions of the body. There are 31 pairs of spinal nerves total.
Of the spinal nerves, there are __ pair(s) of cervical nerves, __ pair(s) of thoracic nerves, __ pair(s) of lumbar nerves, __ pair(s) of sacral nerves, and __ pair(s) of coccygeal nerves.
8; 12; 5; 5; 1
Roots
Two bundles of axons that connect each spinal nerve to a segment of the cord using rootlets.
Posterior (dorsal) root/rootlets
Contain only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the CNS.
Posterior (dorsal) root ganglion
A swelling which contains the cell bodies of sensory neurons.
Anterior (ventral) root/rootlets
Contain axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands).
Cauda equina
Roots of lower spinal nerves that do not leave the vertebral column at the same level they exit the cord. Angle inferiorly alongside the filum terminale in the vertebral column. Resemble wisps of hair.
What does the white matter of the spinal cord consist of?
Consists primarily of bundles of myelinated axons of neurons.
What are the two grooves that penetrate the white matter and divide it into left and right sides?
- Anterior median fissure
- Posterior median sulcus
Anterior median fissure
A wide groove on the anterior (ventral) side.
Posterior median sulcus
Narrow furrow on the posterior (dorsal) side.
Gray commissure
Forms the crossbar of the H.
Central canal
A small space in the center of the gray commissure. Extends the entire length of the spinal cord and is filled with cerebrospinal fluid.
Anterior (ventral) white commissure
Connects the white matter of the right and left sides of the spinal cord.
What does the gray matter of the spinal cord consist of?
Consists of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia. Is shaped like an H or a butterfly.
Nuclei
Functional groups made of clusters of neuronal cell bodies.
______ receive input from receptors via sensory neurons, and ______ provide output to effector tissues via motor neurons.
Sensory nuclei; motor nuclei
Horns
Regions that gray matter is subdivided into on each side of the spinal cord.
Posterior (dorsal) gray horns
Contain axons of incoming sensory neurons as well as cell bodies and axons of interneurons.
Anterior (ventral) gray horns
Contain somatic motor nuclei.
Somatic motor nuclei
Clusters of cell bodies and somatic motor neurons that provide nerve impulses for contraction of skeletal muscles.
Lateral gray horns
Horns between the posterior and anterior gray horns. Present only in thoracic and upper lumbar segments of the spinal cord. Contain autonomic motor nuclei.
Autonomic motor nuclei
Clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands.
Columns
Three broad areas that the anterior and posterior gray horns divide the white matter into.
What are the three columns?
- Anterior (ventral) white columns
- Posterior (dorsal) white columns
- Lateral white columns
Tracts
Bundles of axons in the CNS. Found in each column. May extend long distances up or down the spinal cord.
Sensory (ascending) tracts
Consist of axons that conduct nerve impulses toward the brain. Sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain.
Motor (descending) tracts
Consist of axons that carry nerve impulses from the brain. Sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain.
Describe the nine steps of how sensory input and motor output is processed in the spinal cord
- Sensory receptors detect a sensory stimulus.
- Sensory neurons convey this sensory input in the form of nerve impulses along their axons, which extend from sensory receptors into the spinal nerve and then into the posterior root. From the posterior root, axons of sensory neurons may proceed along three possible paths (steps 3, 4, and 5).
- Axons of sensory neurons may extend into the white matter of the spinal cord and ascend to the brain as part of a sensory tract.
- Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons whose axons extend into the white matter of the spinal cord and then ascend to the brain as part of a sensory tract.
- Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons that in turn synapse with somatic motor neurons that are involved in spinal reflex pathways.
- Motor output from the spinal cord to skeletal muscles involves somatic motor neurons of the anterior gray horn. Many somatic motor neurons are regulated by the brain. Axons from higher brain centers form motor tracts that descend from the brain into the white matter of the spinal cord. There they synapse with somatic motor neurons either directly or indirectly by first synapsing with interneurons that in turn synapse with somatic motor neurons.
- When activated, somatic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the anterior gray horn and anterior root to enter the spinal nerve. From the spinal nerve, axons of somatic motor neurons extend to skeletal muscles of the body.
- Motor output from the spinal cord to cardiac muscle, smooth muscle, and glands involves autonomic motor neurons of the lateral gray horn. When activated, autonomic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the lateral gray horn, anterior gray horn, and anterior root to enter the spinal nerve.
- From the spinal nerve, axons of autonomic motor neurons from the spinal cord synapse with another group of autonomic motor neurons located in the peripheral nervous system (PNS). The axons of this second group of autonomic motor neurons in turn synapse with cardiac muscle, smooth muscle, and glands.
Spinal nerve
Connect the CNS to sensory receptors, muscles, and glands in all parts of the body.
Mixed nerve
What a spinal nerve is classified as, since the posterior root contains sensory axons and the anterior root contains motor axons.
Endoneurium
Innermost layer of spinal nerves that contain individual axons. Axons can be myelinated or unmyelinated. Consists of a mesh of collagen fibers, fibroblasts, and macrophages.
Fascicles
Bundles of groups of axons with their endoneurium.
Perineurium
Thick middle layer of spinal nerves. Contain fascicles. Consists of up to 15 layers of fibroblasts within a network of collagen fibers.
Epineurium
Outermost layer of spinal nerves. Consists of fibroblasts and thick collagen fibers. Extensions of the epineurium fill the spaces between fascicles. Contain blood vessels.
Rami
Branches that a spinal nerve divide into.
Posterior (dorsal) ramus
Serves the deep muscles and skin of the posterior surface of the trunk.
Anterior (ventral) 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
Branch that reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges.
Rami communicantes
Branches of a spinal nerve that are components of the autonomic nervous system.
Plexus
Network of axons from the anterior rami of spinal nerves (except for T2-T12) that do not go directly to the body structures they supply.
The principal plexuses are the ______ plexus, ______ plexus, ______ plexus, ______ plexus, and ______ plexus (which is smaller than the others).
Cervical; brachial; lumbar; sacral; coccygeal
Intercostal nerves
AKA thoracic nerves; the anterior rami of spinal nerves T2-T12 that do not form plexuses. Directly connect to the structures they supply in intercostal spaces.
Dermatome
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.
Cervical plexus
Formed by the roots (anterior rami) of spinal nerves C1-C5. Supplies the skin and muscles of the head, neck, and superior part of the shoulders and chest.
Brachial plexus
Formed by the roots (anterior rami) of spinal nerves C5-C8 and T1. Provides almost the entire nerve supply of the shoulders and upper limbs.
Lumbar plexus
Formed by the roots (anterior rami) of spinal nerves L1-L4. Supplies the anterolateral abdominal wall, external genitals, and part of the lower limbs.
Sacral plexus
Formed by the roots (anterior rami) of spinal nerves L4-L5 and S1-S4. Supplies the buttocks, perineum, and lower limbs. The largest nerve in the body, called the sciatic nerve, arises from here.
What are the two principal functions of the spinal cord in maintaining homeostasis?
- Nerve impulse propagation
- Integration of information
What are the two main routes that the nerve impulses from sensory receptors propagate up the spinal cord to the brain along?
- Spinothalamic tract
- Posterior column
Spinothalamic tract
Conveys nerve impulses for sensing pain, temperature, itch, and tickle.
Posterior column
Conveys nerve impulses for touch, pressure, vibration, and conscious proprioception (the awareness of the positions and movements of muscles, tendons, and joints). Consists of two tracts: gracile fasciculus and cuneate fasciculus.
The sensory information is integrated (processed) by ______ in the spinal cord and brain.
Interneurons
Motor output
Travels down the spinal cord to skeletal muscles.
What are the two types of descending pathways of motor output?
- Direct motor pathways
- Indirect motor pathways
Direct motor pathways
AKA pyramidal pathways; convey nerve impulses that originate in the cerebral cortex and are destined to cause voluntary movements of skeletal muscles.
Direct motor pathways include the ______, ______, and ______ tracts.
Lateral corticospinal; anterior corticospinal; corticobulbar
Indirect motor pathways
AKA extrapyramidal pathways; convey nerve impulses from the brainstem to cause automatic movements and help coordinate body movements with visual stimuli. Indirect pathways also maintain skeletal muscle tone, sustain contraction of postural muscles, and play a major role in equilibrium by regulating muscle tone in response to movements of the head.
Indirect motor pathways include ______, ______, ______, ______, and ______ tracts.
Rubrospinal; tectospinal; vestibulospinal; lateral reticulospinal; medial reticulospinal
Reflex
A fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus. Some reflexes are inborn, and others are learned or acquired.
What is the difference between a spinal reflex and a cranial reflex?
Spinal reflex: when integration takes place in the spinal cord gray matter (Eg. Patellar reflex (knee jerk)).
Cranial reflex: when integration occurs in the brainstem (Eg. The tracking movements of your eyes as you read this sentence).
What is the difference between somatic reflexes and autonomic (visceral) reflexes?
Somatic reflexes: involve contraction of skeletal muscles.
Autonomic (visceral) reflexes: generally, are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands (Eg. Heart rate, digestion, urination, and defecation).
Reflex arc
AKA reflex circuit; pathway followed by nerve impulses that produce a reflex.
What are the five functional components of a reflex arc?
- Sensory receptor
- Sensory neuron
- Integrating center
- Motor neuron
- Effector
A reflex pathway having only one synapse in the CNS is termed a ______. More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron. A ______ involves more than two types of neurons and more than one CNS synapse.
Monosynaptic reflex arc; polysynaptic reflex arc
If the effector is skeletal muscle, the reflex is a ______. If the effector is smooth muscle, cardiac muscle, or a gland, the reflex is an ______.
Somatic reflex; autonomic (visceral) reflex
What are the four important somatic spinal reflexes?
- Stretch reflex
- Tendon reflex
- Flexor reflex
- Crossed extensor reflex
Stretch reflex
Operates as a feedback mechanism to control muscle length by causing muscle contraction. Occurs via a monosynaptic reflex arc. Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints. An example of a stretch reflex is the patellar reflex (knee jerk).
What are the five steps involved in a stretch reflex?
- Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles. The spindles monitor changes in the length of the muscle.
- In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord.
- In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with, and thereby activates, a motor neuron in the anterior gray horn.
- If the excitation is strong enough, one or more nerve impulses arises in the motor neuron and propagates, along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle.
- Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.
Ipsilateral reflex
Arrangement in which sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it.
Muscle tone
Small degree of contraction present while the muscle is at rest.
Although the stretch reflex pathway itself is monosynaptic (just two neurons and one synapse), a polysynaptic reflex arc to the ______ muscles operates at ______.
Antagonistic; the same time
Reciprocal innervation
Arrangement in which the components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonist. Prevents conflict between opposing muscles and is vital in coordinating body movements.
Tendon reflex
Operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn. Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. Like the stretch reflex, the tendon reflex is ipsilateral.
Tendon (Golgi tendon) organs
Sensory receptors for the tendon reflex. Lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by passive stretch or muscular contraction.
What are the five steps involved in a tendon reflex?
- As the tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold).
- Nerve impulses arise and propagate into the spinal cord along a sensory neuron.
- Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron.
- The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses.
- The muscle relaxes and relieves excess tension.
Flexor reflex
AKA withdrawal reflex; involves a polysynaptic reflex arc, specifically, a intersegmental reflex arc. Is ipsilateral. An example of this kind of reflex is stepping on a tack and withdrawing the limb. In response to such a painful stimulus, you immediately withdraw your leg.
What are the five steps involved in a flexor reflex?
- Stepping on a tack stimulates the dendrites (sensory receptor) of a pain-sensitive neuron.
- This sensory neuron then generates nerve impulses, which propagate into the spinal cord.
- Within the spinal cord (integrating center), the sensory neuron activates interneurons that extend to several spinal cord segments.
- The interneurons activate motor neurons in several spinal cord segments. As a result, the motor neurons generate nerve impulses, which propagate toward the axon terminals.
- Acetylcholine released by the motor neurons causes the flexor muscles in the thigh (effectors) to contract, producing withdrawal of the leg. This reflex is protective because contraction of flexor muscles moves a limb away from the source of a possibly damaging stimulus.
Intersegmental reflex arc
A type of reflex where nerve impulses from one sensory neuron ascends and descends in the spinal cord and activates interneurons in several segments of the spinal cord.
Crossed extensor reflex
Involves a contralateral reflex arc, unlike the flexor reflex which is an ipsilateral reflex. An example of this kind of reflex is stepping on a tack and maintaining balance as you shift your body weight from one foot to the other.
What are the five steps involved in a crossed extensor reflex?
- Stepping on a tack stimulates the sensory receptor of a pain sensitive neuron in the right foot.
- This sensory neuron then generates nerve impulses, which propagate into the spinal cord.
- Within the spinal cord (integrating center), the sensory neuron activates several interneurons that synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Thus, incoming pain signals cross to the opposite side through interneurons at that level, and at several levels above and below the point of entry into the spinal cord.
- The interneurons excite motor neurons in several spinal cord segments that innervate extensor muscles. The motor neurons in turn generate more nerve impulses, which propagate toward the axon terminals.
- Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the unstimulated left limb to contract, producing extension of the left leg. In this way, weight can be placed on the foot that must now support the entire body. A comparable reflex occurs with painful stimulation of the left lower limb or either upper limb.
