The Nervous System: The Spinal Cord & Spinal Nerves

Chapter 9Anatomy & Physiology I

•ROLE OF THE NERVOUS SYSTEM Structural divisions—anatomic (CNS and PNS)Functional divisions—physiologic (somatic and autonomic)

•NEURONS AND THEIR FUNCTIONS Structure of a neuron (cell body, cell fibers, myelin sheath)Types of neurons (afferent, efferent, interneuron)Nerves and tracts—bundles of neuron fibers

•NEUROGLIA Nonconducting cellsProtect and support nervous tissue

•THE NERVOUS SYSTEM AT WORK Nerve impulse (action potential)Synapse—junction between neurons (neurotransmitter, presynaptic, postsynaptic, receptors)

•SPINAL CORD LocationStructure of the spinal cordReflex arc—pathway through the nervous systemMedical procedures involving the spinal cordDiseases and other disorders of the spinal cord

•SPINAL NERVES—31 PAIRS RootsMixed nerves—combine sensory and motor fibersBranches of the spinal nervesDisorders of the spinal nerves—peripheral neuritis, sciatica, herpes zoster, Guillain-Barré

•AUTONOMIC NERVOUS SYSTEM (VISCERAL NERVOUS SYSTEM) CharacteristicsDivisions of the autonomic nervous systemCellular receptorsFunctions of the autonomic nervous system

Outline

I. Role of the Nervous System

• Chief coordinating center for all body systems• Conditions internally & externally are

constantly changing. These changes stimuli.• Nervous system detects & responds to changes

(stimuli) so the body can adapt & maintain homeostasis >> nerves carry message to and from Centers (Brain and the spinal cords)

• Sensors- Nerves that detect stimuli• Effector – any tissue that carries out a

command from the NS; they are muscles or glands

Anatomic Divisions of the Nervous System

1. Structural Division• Central nervous system(CNS)– Brain & – Spinal cord

• Peripheral nervous system (PNS)– Cranial nerves– Spinal nerves– Nerves carrying messages to &

from the body

2. Functional Division• Somatic nervous system – Controlled voluntarily – Effectors are skeletal muscles– No further subdivision

• Autonomic nervous system – involuntarily controlled– Effectors: smooth muscle, cardiac muscle, glands– Subdivided into:

• Sympathetic nervous system and• Parasympathetic nervous system

Neurons and Their Functions

Neurons: Functional cell of the nervous system

Neuron Structure• Cell body:

– Contains nucleus & organelles– Ganglion – a collection of nerve cell bodies that are located

outside • Cell fibres:

– Dendrite – fibers that conduct nerve impulses to the nerve body• Highly branched, tree-like appearance• Receptors of the NS – receive stimuli to NS

– Axon - fibers that conduct nerve impulses away from the nerve body• Deliver impulses to another neuron, a muscle or a gland• Single long fiber

Myelin Sheath

• Covering of fatty material over some axons that acts as an insulator

• Schwann cell – a glial cell that produces myelin in the PNS in layers

• Nodes of Ranvier – spaces between the myelin sheath

• Nerve impulses jump from node to node, increasing their speed of conduction along an axon

Myelin Sheath - Neurilemma

• Neurilemma – outer membrane of a Schwann cell that allows some peripheral nerves to repair themselves if injured

• Neurilemma creates a sleeve into which a nerve can regenerate

• CNS is not myelinated by Schwann cells so have no mechanism by which to repair themselves

Myelination & Axon Regeneration

White Matter vs Gray Matter• White matter – myelinated part of the NS• Gray matter – unmyelinated part of the NS

Types of Neurons

• Sensory neurons (afferent neurons)

– relay information to the CNS

• Motor neurons (efferent neurons)– relay information from

the CNS; efferent neurons

• Interneurons – relay information within

the CNS

Nerves & Tracts• Nerve – a bundle of nerve

fibers within the PNS• Tract – a bundle of nerve

fibers within the CNS• Fascicles – a bundle of nerve

fibers within a nerve or tract• Endoneurium – connective

tissue surrounding an individual nerve fiber

• Perineurium – connective tissue surrounding a neural fascicle

• Epineurium – connective tissue surrounding an entire nerve

Neuroglia aka Glial Cells

• Support nervous tissue & bind it to other structures (i.e. Schwann cells)

• Aid in repair of cells• Act as phagocytes to remove pathogens &

impurities (i.e. astrocytes)• Regulate the composition of fluids around

cells• Can reproduce

Glial Cells

The Nervous System at Work

• The NS works by means of electrical impulses sent along neuron fibers and transmitted between cells at junctions

Nerve Impulse

• Resting state a neuron at rest; polarized• Depolarization a neuron being stimulated,

causing an electrical charge• Repolarization the electrical charge leaves and the neuron returns to its resting state

Nerve Impulse – Resting State

• Resting state – a neuron at rest– Neurons at rest are polarized with a separation of + & - ions on

either side of its plasma membrane (like a battery)– - ions are on the inside of the plasma membrane– Resting neuron has a negative charge– The separation of ions creates the potential for creating energy

Nerve Impulse - Depolarization

• Depolarization – a neuron being stimulated, causing an electrical charge– An electrical, chemical or mechanical force stimulates the neuron– + ions (Na+ ions) rush into the neuron, causing the neuron to

suddenly change from negative to positive– The nerve impulse starts

Nerve Impulse - Repolarization

• Repolarization – the electrical charge leaves and the neuron returns to its resting state– The neuron cannot be stimulated until it returns to a negative state– The neuron must pump all of the + ions out; pumps out K+– The Na+/K+ pump describes the action of the neuron pumping + ions

Na+ & K+ back out of the cell– Requires energy (ATP)

Nerve Impulse

• Action potential – the traveling of an electric charge along the plasma membrane of a neuron - < 1/1000 of a second

Synapse

• Point of junction for transmitting a nerve impulse from one nerve axon to– Another nerve dendrite– A muscle at the neuromuscular junction– A gland

• Synaptic cleft – the space between one cell & another – Information must cross this cleft for the nerve impulse to

continue or create action– Can do this chemically, through neurotransmitters– Can also do this electrically, as in cardiac muscle, CNS,

smooth muscle

Neurotransmitters

• Chemical that crosses the synaptic cleft from the axon of one neuron to signal the receiving cell

• Receptors – on the receiving dendrite, motor end plate or gland that respond to specific neurotransmitters

Neurotransmitters

• Epinephrine aka adrenaline – ANS

• Norepinephrine aka noradrenaline– ANS

• Acetylcholine – – ANS– Acts between neurons & muscles at the

neuromuscular junction

Spinal Cord• The link between the brain & the peripheral nervous system• Spinal cord ends around T12 and then a tail of spinal nerves

called the cauda equina travels the rest of the way down the spinal canal

Spinal Cord Structure• Composed of both white

(myelinated) & gray (unmyelinated) matter

• White matter – outer layer• Gray matter – H shaped

middle section– Dorsal horn – back part of H– Ventral horn – front part of

H– Gray commissure – the

middle bar of the H, connecting the dorsal & ventral horn

Central canal: Contains cerebrospinal fluidPosterior median sulcus: divides the right and left portions of the posterior white matterAnterior median fissure : separates the right and left portions of the anterior white matter.

Spinal Cord Structure

Spinal Cord Structure• Ascending tracts – sensory (afferent) impulses entering

the spinal cord carrying information to the brain • Descending tracts – motor (efferent) impulses traveling

from the brain to the body

Reflex Arc• The complete pathway through the nervous system from the stimulus to the body’s response• Five parts to the reflex arc

1. Receptor End of dendrite,receives stimulus2. Sensory neuron afferent; carries messages to CNS from body3. CNS processes the information & coordinates response4. Motor neuronefferent; message sent from CNS to body5. Effector a muscle or gland outside the CNS that carries out a response

Reflex Activities

• Simple reflex – rapid automatic response– Involves very few neurons– a given stimulus always produces same response

• Spinal reflex – reflex arc passes through the spinal cord & does not travel to the brain

• Stretch reflex – muscle is stretched & responds by contracting– A simple spinal reflex

Reflex Arc

Spinal Nerves

• 31 pair of nerves that attach directly to the spinal cord; all spinal nerves are mixed nerves

• First part of the peripheral nervous system

• Attach to spinal cord by 2 branches– Dorsal root – receives sensory

information• Dorsal root ganglion – contains all

sensory (afferent) neuron cell bodies

– Ventral root – send motor information (efferent) to muscles and glands

Branches of the Spinal Nerves

• Plexus – branches off the spinal nerves that intertwine into a network– Cervical plexus – supplies

head & neck• phrenic nerve to diaphragm

– Brachial plexus – shoulder and upper extremities

– Lumbosacral plexus – pelvis & legs• Sciatic nerve – largest nerve in

body; to legs

Dermatome

• Dermatome the area of skin supplied by a single spinal nerve

Autonomic Nervous System

• Regulates glands, smooth muscle & cardiac muscle• Automatic and without conscious awareness• Motor Pathway (efferent) different in ANS than in

somatic NS (voluntary) because has 2 motor nerves– Spinal cord to ganglion via preganglionic neuron– Ganglion to effector via postganglionic neuron– Somatic NS has a single motor neuron that travels from

spinal cord directly to effector

Autonomic vs. Somatic Motor Neurons

Divisions of the Autonomic NS

• Sympathetic – “fight or flight” responses of organs– Accelerates organs that are needed for flight or fight – Brakes organs that are not needed for flight or fight

• Parasympathetic– Brings organs back into balance after stress or danger

has been alleviated – Reverses fight or flight effects

• Most organs are innervated by both sympathetic & parasympathetic fibers

Parasympathetic vs Sympathetic ANS

Sympathetic NS Structure

• Thoracolumbar – preganglionic efferent motor fibers begin in spinal cord at around T1 and continue to about L2

• Synapse with sympathetic ganglia near the spinal cord– Sympathetic chain – close to spinal cord from lower neck to upper

abdomen– Collateral ganglia – farther from spinal cord in lower abdomen & pelvis

• Celiac ganglion – digestive organs• Superior mesenteric ganglion – small intestine• Inferior mesenteric ganglion – large intestine, urinary & reproductive

organs

• Postganglionic fibers continue from ganglia to effector organs & glands– Act on effector organs by releasing epinephrine & norepinephrine

Parasympathetic NS Structure

• Craniosacral – preganglionic efferent motor fibers begin at the brainstem & then begin again at the sacrum

• Synapse with ganglia near the effector organs– Terminal ganglia

• Postganglionic efferent motor fibers from ganglia to effector organs– Act on effector organs by releasing acetylcholine

Diseases and Other Disorders of theSpinal Cord

•Multiple sclerosis (MS)–a disease in which the myelin sheath around axons is damaged and the neuron fibers themselves degenerate.–slows the speed of nerve impulse conduction and disrupts nervous system communication–evidence that it involves an attack on the myelin sheath by a person's own immune system, a situation described as autoimmunity. Genetic makeup, in combination with environmental factors, may trigger MS.–At this point, no cure has been found for MS, but drugs that stop the autoimmune response and drugs that relieve MS symptoms are currently under study.

•Amyotrophic lateral sclerosis–motor neurons are destroyed–The progressive destruction causes muscle atrophy and loss of motor control until finally the affected person is unable to swallow or talk.

•Poliomyelitis–is a viral disease–Infection of the gastrointestinal tract leads to passage of the virus into the blood, from which it spreads to the CNS. Poliovirus tends to multiply in the spinal cord's motor neurons, leading to paralysis, including paralysis of the breathing muscles.

•Tumors•Injuries: root -plegia, meaning “paralysis,”

– Monoplegia: paralysis of one limb– Diplegia: paralysis of both upper or both lower limbs– Paraplegia: paralysis of both lower limbs– Hemiplegia: paralysis of one side of the body– Tetraplegia: paralysis of all four limbs

Disorders of the Spinal Nerves

•Peripheral neuritis•Sciatica•Herpes zoster•Guillain-Barré syndrome

Disorders of the Spinal Nerves

• Peripheral neuritis or peripheral neuropathy: is the degeneration of nerves supplying the distal areas of the extremities. Causing symptoms of pain and paralysis. Causes include chronic intoxication (alcohol, lead, drugs), infectious diseases (meningitis), metabolic diseases (diabetes, gout), or nutritional diseases (vitamin deficiency, starvation).

• Sciatica: form of ^. pain along the sciatic nerve and its branches. The most common causes of this disorder are rupture of a disk between the lower lumbar vertebrae and arthritis of the lower spinal column.

• Herpes zoster (shingles): characterized by numerous blisters along the course of certain nerves (eg. Intercostal). Shingles is caused by a reactivation of a prior chickenpox virus infection and involves an attack on the sensory cell bodies inside the spinal ganglia.

• Guillain-Barré syndrome: polyneuropathy (disorder involving many nerves) muscle weakness caused by loss of myelin, with numbness and paralysis