Fundamentals of the Nervous System and Nervous Tissue: Part A

Fundamentals of the Nervous System and Nervous Tissue: Part A

*Functions of the Nervous System
Sensory input Information gathered by sensory receptors about internal and external changes Integration Interpretation of sensory input Motor output Activation of effector organs (muscles and glands) produces a response

Sensory input Integration Motor output Figure 11.1

*Divisions of the Nervous System
Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves carry messages to and from the CNS

Peripheral Nervous System (PNS)
Two functional divisions Sensory (afferent) division Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints Visceral afferent fibers—convey impulses from visceral organs

* Motor (efferent) division
Transmits impulses from the CNS to effector organs

Somatic (voluntary) nervous system
*Motor Division of PNS Somatic (voluntary) nervous system Conscious control of skeletal muscles

Figure 11.2 Central nervous system (CNS)
Peripheral nervous system (PNS) Brain and spinal cord Cranial nerves and spinal nerves Integrative and control centers Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Conducts impulses from receptors to the CNS Somatic sensory fiber Somatic nervous system Autonomic nervous system (ANS) Skin Somatic motor (voluntary) Visceral motor (involuntary) Conducts impulses from the CNS to skeletal muscles Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic division Mobilizes body systems during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS Figure 11.2

Autonomic (involuntary) nervous system (ANS)
*Motor Division of PNS Autonomic (involuntary) nervous system (ANS) Visceral motor nerve fibers Regulates smooth muscle, cardiac muscle, and glands Two functional subdivisions Sympathetic Parasympathetic

*Histology of Nervous Tissue
Two principal cell types Neurons—excitable cells that transmit electrical signals

*Histology of Nervous Tissue
Neuroglia (glial cells)—supporting cells: Astrocytes (CNS) – most abundant, control chemical environment (blood/brain barrier) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)

*Satellite Cells and Schwann Cells
Surround neuron cell bodies in the PNS Schwann cells (neurolemmocytes) Surround peripheral nerve fibers and form myelin sheaths Vital to regeneration of damaged peripheral nerve fibers

(forming myelin sheath)
Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber (e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS. Figure 11.3e

*Neurons (Nerve Cells)
Special characteristics: Long-lived ( 100 years or more) Amitotic—with few exceptions High metabolic rate—depends on continuous supply of oxygen and glucose Plasma membrane functions in: Electrical signaling Cell-to-cell interactions during development

Cell Body (Perikaryon or Soma)
Biosynthetic center of a neuron Spherical nucleus with nucleolus Well-developed Golgi apparatus Rough ER called Nissl bodies (chromatophilic substance)

and conducting region)
Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleolus Axon (impulse generating and conducting region) Impulse direction Nucleus Node of Ranvier Nissl bodies Axon terminals (secretory region) Axon hillock Schwann cell (one inter- node) Neurilemma (b) Terminal branches Figure 11.4b

Bundles of processes are called
Dendrites and axons Bundles of processes are called Tracts in the CNS Nerves in the PNS

*Dendrites Short, tapering, and diffusely branched Receptive (input) region of a neuron Convey electrical signals toward the cell body as graded potentials

The Axon One axon per cell Long axons (nerve fibers) Occasional branches (axon collaterals)

Numerous terminal branches
The Axon Numerous terminal branches Knoblike axon terminals called synaptic knobs or boutons Release neurotransmitters to excite or inhibit other cells*

*Axons: Function Conducting region of a neuron Generates and transmits nerve impulses (action potentials) away from the cell body

*Myelin Sheath Segmented protein-lipoid sheath around most long or large-diameter axons It functions to: Protect and electrically insulate the axon Increase speed of nerve impulse transmission --conduction in myelinated axons is about 30 times faster

Myelin Sheaths in the PNS
Schwann cells wraps many times around the axon Myelin sheath—concentric layers of Schwann cell membrane Neurilemma—peripheral bulge of Schwann cell cytoplasm

rotates around the axon, wrapping its plasma membrane loosely around
Schwann cell plasma membrane Schwann cell cytoplasm A Schwann cell envelopes an axon. 1 Axon Schwann cell nucleus The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. 2 Neurilemma The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. 3 Myelin sheath (a) Myelination of a nerve fiber (axon) Figure 11.5a

*Unmyelinated Axons Thin nerve fibers are unmyelinated One Schwann cell may incompletely enclose 15 or more unmyelinated axons

Myelin sheath Process of oligodendrocyte Nerve fibers
(d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 11.3d

*Multiple Sclerosis (MS)
An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence Myelin sheaths in the CNS become nonfunctional scleroses Shunting and short-circuiting of nerve impulses occurs Impulse conduction slows and eventually ceases

*Multiple Sclerosis: Treatment
Some immune system–modifying drugs, including interferons and Copazone: Hold symptoms at bay Reduce complications Reduce disability

*White Matter and Gray Matter
Dense collections of myelinated fibers Gray matter Mostly neuron cell bodies and unmyelinated fibers

*Structural Classification of Neurons
Three types: Multipolar—1 axon and several dendrites Most abundant Motor neurons and interneurons Pyramidal neuron – high branching, found in cerebral cortex, hippocampus and amygdala Bipolar—1 axon and 1 dendrite Rare, e.g., retinal neurons

*Structural Classification of Neurons
Unipolar (pseudounipolar)—single, short process that has two branches: Peripheral process—more distal branch, often associated with a sensory receptor Central process—branch entering the CNS

Table 11.1 (1 of 3)

Table 11.1 (2 of 3)

*Functional Classification of Neurons
Three types: Sensory (afferent) Transmit impulses from sensory receptors toward the CNS Motor (efferent) Carry impulses from the CNS to effectors

*Functional Classification of Neurons
Interneurons (association neurons) Shuttle signals through CNS pathways; most are entirely within the CNS

Table 11.1 (3 of 3)

Reflex Arc* Components of a reflex arc (neural path)
Receptor—site of stimulus action Sensory neuron—transmits afferent impulses to the CNS Integration center—either monosynaptic or polysynaptic region within the CNS Motor neuron—conducts efferent impulses from the integration center to an effector organ Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

1 2 3 4 5 Stimulus Skin Interneuron Receptor Sensory neuron
Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) Figure 13.14

The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + – Excitatory synapse Inhibitory synapse The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b Figure (2 of 2)

*Role of Membrane Ion Channels
Proteins serve as membrane ion channels Two main types of ion channels Leakage (nongated) channels—always open

*Role of Membrane Ion Channels
Gated channels (three types): Chemically gated (ligand-gated) channels—open with binding of a specific neurotransmitter Voltage-gated channels—open and close in response to changes in membrane potential Mechanically gated channels—open and close in response to physical deformation of receptors

Figure 11.6 Receptor Neurotransmitter chemical attached to receptor
Na+ Na+ Na+ Na+ Chemical binds Membrane voltage changes K+ K+ Closed Open Closed Open (a) Chemically (ligand) gated ion channels open when the appropriate neurotransmitter binds to the receptor, allowing (in this case) simultaneous movement of Na+ and K+. (b) Voltage-gated ion channels open and close in response to changes in membrane voltage. Figure 11.6

A junction that mediates information transfer from one neuron:
*The Synapse A junction that mediates information transfer from one neuron: To another neuron, or To an effector cell

*The Synapse Presynaptic neuron—conducts impulses toward the synapse Postsynaptic neuron—transmits impulses away from the synapse

Axodendritic synapses Dendrites Axosomatic synapses Cell body
Axoaxonic synapses (a) Axon Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron (b) Figure 11.16

Less common than chemical synapses
*Electrical Synapses Less common than chemical synapses Neurons are electrically coupled (joined by gap junctions) Communication is very rapid, and may be unidirectional or bidirectional Are important in: Embryonic nervous tissue Some brain regions

Specialized for the release and reception of neurotransmitters
*Chemical Synapses Specialized for the release and reception of neurotransmitters Typically composed of two parts Axon terminal of the presynaptic neuron, which contains synaptic vesicles Receptor region on the postsynaptic neuron

*Termination of Neurotransmitter Effects
Within a few milliseconds, the neurotransmitter effect is terminated Degradation by enzymes Reuptake by astrocytes or axon terminal Diffusion away from the synaptic cleft

*Neurotransmitters Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies 50 or more neurotransmitters have been identified Classified by chemical structure and by function

*Chemical Classes of Neurotransmitters
Acetylcholine (Ach)* Released at neuromuscular junctions and some ANS neurons Synthesized by enzyme choline acetyltransferase Degraded by the enzyme acetylcholinesterase (AChE)*

Chemical Classes of Neurotransmitters
Biogenic amines include: Catecholamines Dopamine, norepinephrine (NE), and epinephrine* Indolamines Serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and the biological clock

Amino acids include: GABA—Gamma ()-aminobutyric acid Glycine Aspartate Glutamate

Peptides (neuropeptides) include: Substance P Mediator of pain signals Endorphins Act as natural opiates; reduce pain perception Gut-brain peptides Somatostatin and cholecystokinin

Purines such as ATP: Act in both the CNS and PNS Produce fast or slow responses Induce Ca2+ influx in astrocytes Provoke pain sensation

Gases and lipids Nitric oxide (NO) Synthesized on demand Activates the intracellular receptor guanylyl cyclase to cyclic GMP Involved in learning and memory Carbon monoxide (CO) is a regulator of cGMP in the brain

Gases and lipids Endocannabinoids Lipid soluble; synthesized on demand from membrane lipids Bind with G protein–coupled receptors in the brain Involved in learning and memory

Functional Classification of Neurotransmitters
Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) Determined by the receptor type of the postsynaptic neuron GABA and glycine are usually inhibitory Glutamate is usually excitatory Acetylcholine Excitatory at neuromuscular junctions in skeletal muscle Inhibitory in cardiac muscle

Neurotransmitter overview

Peripheral Nervous System (PNS)
All neural structures outside the brain Sensory receptors Peripheral nerves and associated ganglia Motor endings

Central nervous system (CNS)
Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division Figure 13.1

Sensory Receptors Specialized to respond to changes in their environment (stimuli) Activation results in graded potentials that trigger nerve impulses Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain

*Classification of Receptors
Based on: Stimulus type Location Structural complexity

*Classification by Stimulus Type
Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina) Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

Unencapsulated Dendritic Endings
Thermoreceptors Cold receptors (10–40ºC); in superficial dermis Heat receptors (32–48ºC); in deeper dermis

Nociceptors Respond to: Pinching Chemicals from damaged tissue Temperatures outside the range of thermoreceptors Capsaicin

Light touch receptors Tactile (Merkel) discs Hair follicle receptors

Table 13.1

*Encapsulated Dendritic Endings
All are mechanoreceptors Meissner’s (tactile) corpuscles—discriminative touch Pacinian (lamellated) corpuscles—deep pressure and vibration Ruffini endings—deep continuous pressure Muscle spindles—muscle stretch Golgi tendon organs—stretch in tendons Joint kinesthetic receptors—stretch in articular capsules

Table 13.1

*Classification of Nerves
Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Pure sensory (afferent) or motor (efferent) nerves are rare Types of fibers in mixed nerves: Somatic afferent and somatic efferent Visceral afferent and visceral efferent Peripheral nerves classified as cranial or spinal nerves

Contain neuron cell bodies associated with nerves
Ganglia Contain neuron cell bodies associated with nerves Dorsal root ganglia (sensory, somatic) (Chapter 12) Autonomic ganglia (motor, visceral) (Chapter 14)

“On occasion, our trusty truck acts funny—very good vehicle anyhow”
Cranial Nerves Twelve pairs of nerves associated with the brain Most are mixed in function; two pairs are purely sensory Each nerve is identified by a number (I through XII) and a name “On occasion, our trusty truck acts funny—very good vehicle anyhow”

The cranial nerves are:
I - Olfactory nerve Old II - Optic nerve Opie III - Occulomotor nerve Occassionally IV - Trochlear nerve Tries V - Trigeminal nerve/dentist nerve Trigonometry VI - Abducens nerve And VII - Facial nerve Feels VIII - Vestibulocochlear nerve/Auditory nerve Very IX - Glossopharyngeal nerve Gloomy X - Vagus nerve Vague XI - Accessory nerve/Spinal accessory nerve And XII - Hypoglossal nerve hypoactive

Odor Of Orangutan Terrified Tarzan After Forty Voracious Gorillas Viciously Attacked Him
Old Opie Occasionally Tries Trigonometry And Feels Very Gloomy, Vague And Hypoactive

Hypoglossal nerve (XII)
Filaments of olfactory nerve (I) Frontal lobe Olfactory bulb Olfactory tract Optic nerve (II) Temporal lobe Optic chiasma Infundibulum Optic tract Facial nerve (VII) Oculomotor nerve (III) Vestibulo- cochlear nerve (VIII) Trochlear nerve (IV) Trigeminal nerve (V) Glossopharyngeal nerve (IX) Abducens nerve (VI) Vagus nerve (X) Cerebellum Accessory nerve (XI) Medulla oblongata Hypoglossal nerve (XII) (a) Figure 13.5 (a)

A person attempting to show his teeth and raise his eyebrows with Bell's palsy on his right side Bell's palsy is the most common acute mononeuropathy Cranial Nerve VII Caused by a herpes virus

Yes (hearing and balance)
Cranial nerves I – VI Sensory function Motor function PS* fibers I Olfactory Yes (smell) No No II Optic Yes (vision) No No III Oculomotor No Yes Yes IV Trochlear No Yes No V Trigeminal Yes (general sensation) Yes No VI Abducens No Yes No Cranial nerves VII – XII Sensory function Motor function PS* fibers VII Facial Yes (taste) Yes Yes VIII Vestibulocochlear Yes (hearing and balance) Some No IX Glossopharyngeal Yes (taste) Yes Yes X Vagus Yes (taste) Yes Yes XI Accessory No Yes No XII Hypoglossal No Yes No *PS = parasympathetic (b) Figure 13.5 (b)

*Spinal Nerves 31 pairs of mixed nerves named according to their point of issue from the spinal cord 8 cervical (C1–C8) 12 thoracic (T1–T12) 5 Lumbar (L1–L5) 5 Sacral (S1–S5) 1 Coccygeal (C0)

Cervical plexus Cervical nerves C1 – C8 Brachial plexus Cervical
enlargement Thoracic nerves T1 – T12 Intercostal nerves Lumbar enlargement Lumbar nerves L1 – L5 Lumbar plexus Sacral plexus Sacral nerves S1 – S5 Cauda equina Coccygeal nerve Co1 Figure 13.6

Each spinal nerve connects to the spinal cord via two roots
*Spinal Nerves: Roots Each spinal nerve connects to the spinal cord via two roots Ventral roots Contain motor (efferent) fibers from the ventral horn motor neurons Fibers innervate skeletal muscles)

*Spinal Nerves: Roots Dorsal roots
Contain sensory (afferent) fibers from sensory neurons in the dorsal root ganglia Conduct impulses from peripheral receptors Dorsal and ventral roots unite to form spinal nerves, which then emerge from the vertebral column via the intervertebral foramina

Gray matter White matter Dorsal and Ventral root ventral rootlets
of spinal nerve Ventral root Dorsal root Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Rami communicantes Sympathetic trunk ganglion Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve. Figure 13.7 (a)

* Sciatica -- A common condition arising from compression of, or damage to, a nerve or nerve root. Restless legs syndrome (RLS) is a disorder of the part of the nervous system that affects the legs and causes an urge to move them. Because it usually interferes with sleep, it also is considered a sleep disorder

* Guillain-Barré syndrome -- causes muscle weakness, loss of reflexes, and numbness or tingling in your arms, legs, face, and other parts of your body. In GBS, the immune system attacks the myelin sheath of certain nerves. This causes nerve damage. May be triggered by a viral or bacterial infection

*Innervation of Skin Dermatome: the area of skin innervated by the cutaneous branches of a single spinal nerve All spinal nerves except C1 participate in dermatomes Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness

Anterior view (b) Posterior view Figure 13.12 C2 C3 C4 C5 T1 T2 T3 T2
L2 S1 L1 L3 C8 L4 S2 T12 S3 L5 C6 L1 L1 C6 S4 C7 S2 C7 S5 C8 S3 C8 L2 L2 S1 S2 S2 S1 L3 L3 L1 L5 L2 L5 L4 L4 L3 L5 L5 L4 S1 S1 Anterior view (b) Posterior view L4 L4 L5 L5 S1 Figure 13.12

Fundamentals of the Nervous System and Nervous Tissue: Part A

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Fundamentals of the Nervous System and Nervous Tissue: Part A

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