Chapter 12 Spinal cord and spinal nerves

Cranial reflex

when the site of integration is the brain stem

IPSP

Inhibitory postsynaptic potential voltage change away from threshold making an AP less likely hyperpolarization making it more negative usually a result of opening Cl- or K+ gates

ventricles of the brain

4 cavities in the brain called ventricles There is one lateral ventricle (ventricles 1 and 2) in each hemisphere of the brain, separated by a thin membrane called the septum pellucidum The 3rd ventricle is a narrow slit like cavity along the midline of the brain superior to the hypothalamus between the halves of the thalamus The 4th ventricle is between the brain stem and the cerebellum

reflex

A reflex is a fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus

Compare and contrast monosynaptic and polysynaptic reflexes

Although the stretch reflex is monosynaptic, a polysynaptic reflex arc to the antagonistic muscle occurs at the same time

Describe the circulation of human cerebrospinal fluid/CSF (fig 14.4)

CSF is formed at a rate of ~20 mL/hr, and reabsorbed at about the same rate Thus the pressure and volume of CSF is fairly constant CSF produced in the choroid plexuses of the lateral ventricles flow into the third ventricle Passes through the interventricular foramina More CSF is added in the 3rd ventricle, then it flows into the 4th ventricle Passes through the aqueduct of the midbrain More CSF is added in the 4th ventricle, then it flows into the subarachnoid space Enters through three openings, the median aperture and paired lateral apertures It then circulates through the central canal of the spinal cord and in the subarachnoid space CSF is reabsorbed into the blood by arachnoid villi Fingerlike extensions of the arachnoid mater that project into the dural venous sinuses

Describe the formation of human cerebrospinal fluid/CSF (fig 14.4)

CSF is produced by the choroid plexuses, networks of blood capillaries in the walls of the ventricles Ependymal cells joined by tight junctions cover capillaries of the choroid plexuses The ependymal cells extract and filter substances (mostly water) from the blood plasma to create CSF This secretion is bidirectional, allowing for transport of metabolites from the nervous tissue back to the blood Because the ependymal cells are connected with tight junctions, all material entering the CSF must go through the ependymal cells This creates the blood-cerebrospinal fluid barrier Functions to protect the brain and spinal cord from potentially harmful chemicals and microorganisms

List and/or diagram the parts of a typical reflex arc

Nerve impulses that produce a reflex follow specific pathways between the sensory receptors, CNS and effectors, and are called reflex arcs or reflex circuit All reflex arcs have 5 functional components sensory receptor Sensory neuron Integrating center Motor neuron Effector

monosynaptic reflexes

Occurs in response to stretching of muscles, resulting in contraction of the muscle Can be elicited by tapping on tendons attached to muscles at the elbow, knee, wrist and ankle Slight stretching of the muscle stimulates receptors in the muscles called muscle spindles In response to the stretch, one or more nerve impulses are sent along a somatic sensory neuron to the spinal cord In the spinal cord (integrating center) the sensory neuron makes an excitatory synapse with (and thereby activating) a motor neuron If the excitation is strong enough, an action potential is sent along the motor neuron axon to the stretched skeletal muscle The nerve impulse causes acetylcholine release at the neuromuscular junction, and contraction of the muscle Although the stretch reflex is monosynaptic, a polysynaptic reflex arc to the antagonistic muscle occurs at the same time

PNS

The PNS has two important parts. They are the motor division and the sensory division. The sensory division collects the impulses from the sensory receptors in areas like skin, muscles, and organs, and also carries those impulses through the nerves to the CNS. The motor division collects the outgoing messages from the CNS and delivers them to the appropriate body organs, telling them what to do. The motor division does the opposite from the sensory division.

lumbar plexus

The anterior rami of spinal nerves L1-L4 form the lumbar plexus pass obliquely outward, between the superficial and deep heads of the psoas major muscle anterior to the quadratus lumborum muscle Supplies the anterolateral abdominal wall, external genitalia and parts of the lower limbs The largest nerve originating from the lumbar plexus is the femoral nerve Damage to the femoral nerve is indicated by an inability to extend the leg and loss of sensation in the skin over the anteromedial aspect of the thigh The obturator nerve supplies the adductor muscles of the hip and skin of the medial aspect of the thigh Damage to this nerve, can result from pressure of the fetal head during pregnancy

sacral plexus

The anterior rami of spinal nerves L4-L5 and S1-S4 form the sacral plexus Is situated largely anterior to the sacrum Supplies the buttocks, perineum and lower limbs The sciatic nerve is the largest in the body Injury to the sciatic nerve results in sciatica, pain that may extend from the buttock down the posterior and lateral aspect of the leg and lateral aspect of the foot The anterior rami of spinal nerves S4-S5 and the coccygeal nerves make up the small coccygeal plexus This plexus produces the anococcygeal nerves Supplies a small area of skin in the coccygeal region

CNS

The central nervous system or the CNS contains the brain and the spinal cord. All together, the brain and the spinal cord serve the nervous system's command station. When the sensory input reaches the CNS, the spinal cord and the brain figure outs what it exactly means. After, they quickly orders out the body parts that needs to move faster.

Cervical plexus

The cervical plexus is formed by the anterior roots of cervical nerves C1-C4, with a little contributions of C5 Supply the skin and muscles of the head, neck, superior part of the shoulders and chest The phrenic nerves of the cervical plexus supply the diaphragm Originates from C3, C4 and C5 Severing of the spinal cord above the origin of the phrenic nerve results in respiratory arrest, as nerve impulses are no longer reaching the diaphragm

Contrast the organs of the CNS and PNS

The nervous system has two different major parts. The two parts are the central nervous system and the peripheral nervous. Everything else but the CNS it is known as the peripheral nervous system. The peripheral nervous system or PNS contains the nerves, which leave the brain and the spinal cord and travel to certain areas of the body. The peripheral nervous system's main job is to send information gathered by the body's sensory receptors to the CNS as quickly as possible. Once the CNS has understood the information, the PNS will relay the specific orders back out the body. These nerves which carry information in a way of nerve impulses to and from the brain are called cranial nerves. The nerves that carry impulse to and are carrying information from the spine are called spinal nerves.

Explain the functional difference between the dorsal root and the ventral root of a human spinal nerve.

The posterior or dorsal root and rootlets contain only sensory axons These conduct nerve impulses from sensory receptors to the central nervous system Each posterior root has a swelling called the posterior or dorsal root ganglion This swelling contains the cell bodies of sensory neurons The anterior or ventral root and rootlets contain axons of motor neurons These conduct nerve impulses from the CNS to effectors

polysynaptic reflexes

The tendon reflex operates as a safety mechanism to ensure tendons are not damaged by lifting loads that are too heavy Increased tension applied to the tendon causes the tendon organ (the sensory receptor) to depolarize to threshold The nerve impulse is propagated along a sensory neuron to the CNS Within the spinal cord, an the sensory neuron synapses with two interneurons One inhibitory and one excitatory The inhibitory interneuron inhibits the motor neuron serving the muscle being contracted This results in fewer impulses being sent, and relaxation of the muscle, lowering the tension on the tendon The excitatory interneuron synapses with motor neurons of the antagonist muscle, resulting in contraction of that muscle

white matter

The white matter of the spinal cord is also organized into regions The grey matter separates the white matter into three columns on each side of the spinal cord; the anterior, posterior and lateral white columns Each column contains bundles of axons having a common origin or destination known as tracts Sensory tracts (also called ascending tracts) consist of axons that conduct impulses towards the brain Motor tracts (also called descending tracts) consist of axons that conduct impulses away from the brain Both sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain

List each of the major spinal nerve plexuses and the spinal nerves that make up each.

There are 5 major plexuses in the human body Cervical plexus, brachial plexus, lumbar plexus, sacral plexus and coccygeal plexus

coccygeal plexus

This plexus produces the anococcygeal nerves Supplies a small area of skin in the coccygeal region

Compare and contrast spinal cord gray matter and white matter.

When a transverse section of spinal column is viewed, a region of white matter surrounds an inner core of grey matter The white matter is split by two groves into left and right halves Anterior median fissure and posterior median sulcus These divisions separate the white matter into two primary bundles of myelinated neurons The grey matter of the spinal cord is arranged into an overall H shape Is composed of the cell bodies of neurons, unmyelinated axons, dendrites and neuroglia The grey commissure forms the crossbar of the H In its center is the central canal, which is continuous with the 4th ventricle of the medulla oblongata

dermatome map

a diagram of the cutaneous regions innervated by each spinal nerve

EPSP

excitatory postsynaptic potential voltage change approaches threshold making an AP more likely depolarization usually a result of opening a Na+ channel

Spinal reflex

occur when the site of integration is the spinal cord grey matter

2. Describe the anatomy of the human spinal cord( fig 13.1, 13.2 and 13.3)

• Consisting of ~100 million nerves, the spinal cord is the part of the CNS that extends from the brain • Contain neural circuits that allow for some of the most rapid responses in the body • When you pick up something hot, your spinal cord reflex will respond by relaxing the muscles responsible for grasping the item • Often times this will occur before you become consciously aware that the item was hot • The grey matter of the spinal cord is a site of integration, processing incoming stimuli • The white matter of the spinal cord functions as a "highway" for sensory information to travel to the brain and motor responses travel to effectors • Nervous tissue of the CNS is very delicate and does not respond well to injury or damage • Thus, it must be protected well • The first layer of protection is the bones of the vertebral column and skull • These structures surround the brain and spinal column, protecting it from physical trauma • The second layer of protection is the meninges • These are three membrane layers that surround the brain and spinal cord • The third layer of protection is the cerebrospinal fluid • Surrounds the brain and spinal column providing a shock absorbing cushion

3. Describe the anatomy and physiology of human meninges( fig 13.1, 14.2, 14.4)

• Dura mater is the outermost of the meninges • Is a thick, strong layer of dense connective tissue • Is continuous with the dura mater of the brain; the spinal portion begins at the foramen magnum and extends to the second sacral vertebra • The dura mater is also continuous with the epineurium, the outer covering of spinal and cranial nerves • Arachnoid mater is the middle layer of the meningeal membranes • Is a thin, avascular covering consisting of cells and loosely arranged collagen and elastic fibers • Is named because it resembles a spider web • Is continuous with the arachnoid mater of the brain • Between the dura mater and the arachnoid mater is a small space called the subdural space • Pia mater is the innermost of the meninges • Is a thin, transparent layer of connective tissue that adheres to the surface of the spinal cord and brain • Consists of squamous to cuboidal cells with interlacing bundles of collagen fibers and a few fine elastic fibers • The pia mater contains most of the blood vessels that supply oxygen to the spinal cord • Triangular lateral extensions of the pia mater form denticulate ligaments • These ligaments fuse with arachnoid and dura mater along the entire length of the spinal cord • They function to protect the spinal cord against sudden displacement by "tethering" it • Between the pia mater and the arachnoid mater is the subarachnoid space, which contains cerebrospinal fluid

Effector

• Effector: The part of the body that responds to the nerve impulse, such as a gland or muscle, is the effector. The action the effector produces is called a reflex

grey matter

• In the grey matter of the spinal cord and brain, clusters of neuronal cell bodies form function groups called nuclei • Sensory nuclei receive input from sensory neurons • Motor nuclei provide output to effectors via motor neurons • The grey matter of each side of the spinal cord is divided into regions called horns (or grey horns) • These contain cell bodies and axons of interneurons as well as axons of incoming sensory neurons • The anterior grey horns contain somatic motor nuclei and provide nerve impulses for contraction of skeletal muscles • The posterior grey horns contain cell bodies and interneurons as well as axons of incoming sensory neurons • The lateral grey horns are only present in the thoracic and upper lumbar segments, contain autonomic motor nuclei and regulate the activity of cardiac and smooth muscle and glands

Integrating center

• Integrating center: one or more regions of grey matter within the CNS act as an integrating center In the simplest type of reflex, the integrating center is a single synapse between a sensory and motor neuron • This is known as a monosynaptic reflex arc More commonly, an integration center involves one or more interneurons, which relay information to other interneurons as well as to a motor neuron • These are known as polysynaptic reflex arcs

Motor neuron

• Motor neuron: impulses triggered by the integrating center will propagate out of the CNS along a motor neuron to the body part that will respond

Sensory neuron

• Sensory neuron: the nerve impulse will propagate along the axon of the sensory neuron to the axon terminals in the grey matter of the brain or spinal cord • Relay neurons in the grey matter will send the impulses to the area of the brain that allow conscious awareness that the reflex has occurred

sensory receptor

• Sensory receptor: The distal end of a sensory neuron or an associated sensory structure serves as the sensory receptor • Will respond to a change in the environment, either internal or external, by producing a graded potential called a generator potential • If the generator potential reaches threshold, one or more nerve impulses will be produced in the sensory neuron

Describe the cause of the condition known as Shingles

• Shingles is caused by herpes zoster, the virus that causes chicken pox • When an individual is infected with the virus initially, they develop chicken pox • The immune system responds, to clear the acute infection, but some virus enters the posterior root ganglion, where it remains latent • Normally our immune system prevents reactivation (the virus leaving the nerves) • As we age, our immune systems weaken • The virus can then leave the ganglion, travel down the sensory neurons to the skin • In the skin, it will cause pain, discoloration and blisters along the dermatome of the specific posterior root ganglion infected

Describe the general anatomy and physiology of a spinal nerve

• Spinal nerves are parallel bundles of axons and their associated neuroglial cells found in the PNS that connect the CNS to the sensors and effectors of the body • There are 31 pairs of spinal nerves • They are named and numbered according to the region and level of the vertebral column from which they emerge • The first spinal nerve emerges between the occipital bone and the atlas • Most of the other spinal nerves emerge through the intervertebral foramen between the vertebrae • Spinal nerves C1-C7 exit the vertebral canal above their corresponding vertebrae • Spinal Nerve C8 exits between vertebrae C7 and T1 • Spinal nerves T1 through L5 exit the vertebral canal below their corresponding vertebrae

Give the number of human spinal nerves, how they are named

• Spinal nerves are the 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 • 8 pairs of cervical nerves (C1-C8) • 12 pairs of thoracic nerves (T1-T12) • 5 pairs of lumbar nerves (L1-L5) • 5 pairs of sacral nerves (S1-S5) • 1 pair of coccygeal nerves (Co1) • Two bundles of axons, called roots, connect each spinal nerve to a segment of the spinal cord • As the root approaches the spinal cord, it divides into smaller bundles called rootlets

human spinal nerves, where they exit the vertebral column.

• Spinal nerves exit the spinal cord laterally through the intervertebral foramina • However, because the spinal cord is shorter than the vertebral column, nerves arising from the lumbar, sacral and coccygeal regions do not leave laterally • These nerves angle inferiorly along the filum terminale • The roots of these nerves are collectively named the cauda equina (means horse tail)

Brachial plexus

• The anterior rami of spinal nerves C5-C8 and T1 form the brachial plexus • The brachial plexus provides almost the entire nervous supply to the shoulders and upper limbs • 5 large terminal branches arise from the brachial plexus • The axillary nerve supplies the deltoid and teres minor muscles • The musculocutaneous nerve supplies the anterior muscles of the arm • The radial nerve supplies muscles on the posterior aspect of the arm and forearm • The median nerve supplies most of the muscles of the anterior forearm and some muscles of the hand • The ulnar nerve supplies the anteromedial muscles of the forearm and most muscles of the hand

List and identify the ventricles of the brain as well as the cranial meninges

• The brain is protected by the same three layers of meninges as the spinal cord • However, the cranial dura mater is composed of two layers, whereas the spinal dura mater has only one • Periosteal layer and the meningeal layer • The two layers are fused together except where they enclose the dural venous sinuses • These venous sinuses drain blood from the brain, delivering it to the internal jugular veins • Three extensions of the dura mater separate parts of the brain • Falx cerebri separates the two hemispheres of the cerebrum • Falx cerebelli separates the two hemispheres of the cerebellum • Tentorium cerebelli separates the cerebrum from the cerebellum • Additionally, there is no epidural space around the brain • Blood vessels that enter the brain pass along the surface of the brain • When the penetrate inward, they are sheathed by the pia mater

Explain what a dermatome map is and how it is used( fig 13.11)

• The skin of the body is supplied by somatic sensory neurons • Each spinal nerve somatic sensory neurons that supply predictable regions of the body • The area that a particular spinal nerve supplies is called a dermatome • Knowing what area a particular spinal nerve supplies makes it possible to locate damaged regions of the spinal column • If the skin supplied by a particular spinal nerve is stimulated, but the sensation is not perceived, the supplying spinal nerve is likely damaged • Knowing the innervation patterns can also be used therapeutically • Cutting posterior roots or infusing local anesthetics can block pain either permanently or transiently