Myelin Sheaths Myelin Sheaths in the PNS

• Segmented structures composed of the • Formed by Schwann cells lipoprotein myelin • Develop during fetal period and in the first • Surround thicker axons year of postnatal life • Form an insulating layer • Schwann cells wrap in concentric layers • Prevent leakage of electrical current around the axon • Increase the speed of impulse conduction • Cover the axon in a tightly packed coil of membranes • Neurilemma • Material external to myelin layers

Myelin Sheaths in the PNS Unmyelinated Axons in the PNS

(a) Myelinated axon in PNS (b) Unmyelinated axons in PNS An axon wrapped with a fatty insulating sheath Axons that are not covered with an insulating sheath formed from Schwann cells Myelin sheath Schwann cell plasma membrane 1 A Schwann cell Schwann cell Schwann cell envelops an axon. Schwann cell cytoplasm cytoplasm Axon Axon Schwann cell Axons Schwann cell nucleus Neurilemma 1 A Schwann Neurilemma Schwann cell cell surrounds Axons nucleus multiple axons. 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around Cross section of a myelinated axon (TEM 30,000×) it in successive layers.

Cross section of unmyelinated axons (TEM 11,000×) 3 The Schwann cell Neurilemma cytoplasm is forced from 2 Each axon is Myelin between the membranes. The encircled by the sheath tight membrane wrappings surrounding the axon form Schwann cell the myelin sheath. plasma membrane.

Myelin Sheaths in the PNS Myelin Sheaths in the CNS

• Nodes of Ranvier—gaps along axon • Oligodendrocytes form the myelin sheaths • Thick axons are myelinated in the CNS • Myelination speeds up nerve transmission • Have multiple processes • Thin axons are unmyelinated • Coil around several different axons • Conduct impulses more slowly

1 Nerves Nerves • Nerves—cablelike organs in the PNS • Consists of numerous axons wrapped in … are organized into groups of axons connective tissue • Endoneurium—layer of delicate connective • Most nerves contain myelinated and tissue surrounding the axon nonmyelinated sensory and motor axons • Perineurium—connective tissue wrapping • Axon is surrounded by Schwann cells surrounding a nerve fascicle • You see many nerves in lab • Nerve fascicles—groups of axons bound into • Nerves of brachial plexus bundles • Radial, axillary, median, musculocutaneous, ulnar • Epineurium—whole nerve is surrounded by • Nerves of lumbosacral plexus tough fibrous sheath

Structure of a Nerve Gray and White Matter in the CNS Axon Myelin sheath • Gray matter Endoneurium Perineurium • Is gray-colored and surrounds hollow central Blood vessels cavities of the CNS Endoneurium Fascicle Forms H-shaped region in the spinal cord Perineurium Blood vessels • Fascicle • Dorsal half contains cell bodies of Nerve fibers interneurons (b) • Ventral half contains cell bodies of motor Epineurium neurons

Schwann cell • Primarily composed of neuronal cell bodies, nucleus Axon dendrites, nonmyelinated axons Myelin • Surrounds white matter of CNS in cerebral (a) Node of Ranvier cortex and cerebellum

Gray and White Matter in the CNS Gray and White Matter in the CNS

• White matter • Lies external to the gray matter of the CNS in PNS CNS Gray matter Short unmyelinated the spinal cord, but internal in the brain Sensory (afferent) interneurons fiber Cell bodies of • Dominated by myelinated axons Spinal interneurons and nerve motor neurons • Consists of axons passing between specific Neuroglia White matter regions of the CNS Fiber tracts of Motor (efferent) myelinated and • Tracts are bundles of axons traveling to fiber unmyelinated axons Hollow central cavity similar destinations

2 Integration Between the PNS and CNS Integration Between the PNS and CNS

• The CNS and PNS are functionally • Tracts of the CNS interrelated • Composed of interneurons that • Nerves of the PNS • Process and receive sensory information • Provide information pathways to and from • Direct information to specific CNS regions body periphery • Initiate appropriate motor responses • Afferent PNS fibers respond to sensory • Transport information from one area of the stimuli CNS to another • Efferent PNS fibers transmit motor stimuli from CNS to muscles and glands

Reflex Arcs Five Essential Components to the Reflex Arc • Receptor—site where stimulus acts • Reflex arcs—simple chains of neurons • Sensory neuron—transmits afferent impulses • Basis for simplest (reflex) behaviors to the CNS • Determine structural plan of the nervous • Integration center—consists of one or more system synapses in the CNS • Responsible for reflexes • Motor neuron—conducts efferent impulses • Rapid, autonomic motor responses from integration center to an effector Can be visceral or somatic • • Effector—muscle or gland cell • Responds to efferent impulses • Contracting or secreting

Components of a Reflex Arc Types of Reflexes Stimulus • Monosynaptic reflex Skin • Simplest of all reflexes • Just one synapse Interneuron 1 Receptor • The fastest of all reflexes 2 Sensory neuron • Knee-jerk reflex 3 Integration center

4 Motor neuron

5 Effector

Spinal cord (in cross section)

3 Types of Reflexes Types of Reflexes

1 Sensory (stretch) receptor • Polysynaptic reflex 2 Sensory (afferent) neuron • More common type of reflex • One or more interneurons are part of the 3 pathway • Withdrawal reflexes • Interneurons signal the motor neuron to 4 Motor (efferent) neuron contract muscle involved

5 Effector organ

(a) Monosynaptic stretch reflex

Types of Reflexes Neuronal Circuits

• Diverging circuit—one presynaptic neuron 1 Sensory receptor 2 Sensory (afferent) neuron 3 Interneuron synapses with several other neurons (divergence) • Converging circuit—many neurons synapse on a single postsynaptic neuron (convergence)

4 Motor (efferent) neuron • Reverberating circuit—circuit that receives

5 Effector organ feedback via a collateral axon from a neuron (b) Polysynaptic withdrawal reflex in the circuit

Figure 12.12 Types of neuronal circuits.

Input Types of Input Processing

• Serial processing Input 1 • Neurons pass a signal to a specific

Input 2 Input 3 destination along a single pathway from one to another

Output Many outputs • Parallel processing

Diverging circuit to Converging circuit • Input is delivered along many pathways; a multiple pathways single sensory stimulus results in multiple perceptions Input Output

4 Simplified Design of the Nervous System Simplified Design of the Nervous System

• Three-neuron reflex arcs • Sensory neurons—located dorsally • Basis of the structural plan of the nervous • Cell bodies outside the CNS in sensory system ganglia • Similar reflexes are associated with the brain • Central processes enter dorsal aspect of the spinal cord • Motor neurons—located ventrally • Axons exit the ventral aspect of the spinal cord

Simplified Design of the Nervous System Simplified Design of the Nervous System

• Interneurons—located centrally Gray matter • Synapse with sensory neurons White matter Cerebrum Withdrawal reflex. A • Interneurons are neurons confined to CNS painful stimulus triggers nerve impulses in a Parallel processing. • Long chains of interneurons between sensory sensory neuron, which Simultaneously, the initiate the polysynaptic nerve impulses travel Brain stem and motor neurons withdrawal reflex. on an axon branch that Sensory extends into the white neuron matter. This ascending fiber carries the nerve impulses to the brain. Cervical spinal cord Motor Interneuron neuron

Simplified Design of the Nervous System Disorders of the Nervous System Integration in gray matter. Multiple interneurons process the nerve impulses to localize the stimulus, identify its source, and plan a response. This complex processing is illustrated here in a simplified manner. • Multiple sclerosis • Common cause of neural disability Voluntary motor response. A nonreflexive motor response is initiated in the gray matter • An autoimmune disease and transmitted down a descending fiber in the white matter to stimulate somatic • Immune system attacks the myelin around motor neurons. axons in the CNS • Varies widely in intensity among those affected • More women than men are affected • When men are affected, disease develops quicker and is more devastating • Cause is incompletely understood

5 Neuronal Regeneration Neuronal Regeneration

• Neural injuries may cause permanent • CNS—neuroglia never form bands to guide dysfunction re-growing axons and may hinder axon • If axons alone are destroyed, cells bodies growth with growth-inhibiting chemicals often survive and the axons may regenerate • No effective regeneration after injury to the spinal cord and brain • PNS—macrophages invade and destroy axon distal to the injury • Axon filaments grow peripherally from injured site • Partial recovery is sometimes possible

Regeneration of the Peripheral Nerve Fiber Nervous Tissue Throughout Life

• Nervous system develops from the dorsal

Axon sprouts, Endoneurium Schwann cells 1 The axon Aligning Schwann cells 3 ectoderm becomes form regeneration tube or filaments, Droplets fragmented at grow through a of myelin the injury site. regeneration tube formed by • Invaginates to form the neural tube and neural Schwann cells. crest

Fragmented • Neural tube walls begin as neuroepithelial Fine axon sprouts axon Site of nerve damage or filaments cells

2 Macrophages Schwann cell Site of new myelin 4 The axon • These cells divide and become Schwann cell Macrophage clean out the dead sheath formation regenerates, and axon distal to the a new myelin injury. sheath forms. neuroblasts

Single enlarging axon filament

Nervous Tissue Throughout Life

Ectoderm

(b) Week 5. Neuroepithelial cells of the neural tube divide and migrate externally to become (a) 28 days. neuroblasts and Neural tube and Neural Neural neuroglia. neural crest form tube crest from invaginating ectoderm. Neuroblasts Neuroepithelial cells Neuroepithelial cells Sensory neurons from neural crest (c) Week 6. Neural crest cells form the sensory neurons. Alar plate: Axons form interneurons Dorsal neuroblasts white matter form the alar plate Basal plate: (future interneurons). motor neurons Long axons extending from the interneurons form the white matter. Ventral neuroblasts Neuroepithelial cells Central form the basal plate cavity (future motor neurons).