Chapter 11 Fundamentals of the Nervous System and Nervous Tissue Anatomy and Physiology I(2) Mr. Scott

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 Motor output is the response generated from the information

Functions of the Nervous System Each muscle in the body is supplied by a particular level or segment of the spinal cord and by its corresponding spinal nerve. The muscle, and its nerve make up a myotome. This is approximately the same for every person and are as follows: C3,4 and 5 supply the diaphragm (the large muscle between the chest and the belly that we use to breath). C5 also supplies the shoulder muscles and the muscle that we use to bend our elbow . C6 is for bending the wrist back. C7 is for straightening the elbow. C8 bends the fingers. T1 spreads the fingers. T1 –T12 supplies the chest wall & abdominal muscles. L2 bends the hip. L3 straightens the knee. L4 pulls the foot up. L5 wiggles the toes. S1 pulls the foot down. S3,4 and 5 supply the bladder. bowel and sex organs and the anal and other pelvic muscles.

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 Afferent (Sensory) = arriving; Efferent (Motor) = exiting Location of afferent versus efferent branches from spinal cord demonstrates evolution

Motor Division of PNS Somatic (voluntary) nervous system Conscious control of skeletal muscles Autonomic (involuntary) nervous system (ANS) Visceral motor nerve fibers Regulates smooth muscle, cardiac muscle, and glands Two functional subdivisions Sympathetic Parasympathetic Sympathetic Nervous System (Pushing) Heart rate increased and force increased, bronchial muscles relaxed, Eye pupil dialation, intestinal motility reduced, bladder sphincter closed, decrease kidney output Parasympathetic Nervous System (PNS)  Increases: digestion intestinal motility, fuel storage (increases insulin activity), resistance to infection, rest and recuperation, circulation to nonvital organs, (skin, extremities...), endorphins, the "feel good" hormones  Decreases: heart rate blood pressure temperature, kidney output, urine secretion, digestion, pupil constriction,

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) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)

Astrocytes Capillary Neuron Astrocyte Most abundant, versatile, and highly branched glial cells Cling to neurons, synaptic endings, and capillaries Support and brace neurons Neuron Help determine capillary permeability Guide migration of young neurons Control the chemical environment Participate in information processing in the brain Help with exchanges between capillaries and nuerons Help with cleanup of potassium and neurotransmitters from neurons Astrocyte

Small, ovoid cells with thorny processes Microglia Small, ovoid cells with thorny processes Migrate toward injured neurons Phagocytize microorganisms and neuronal debris Help with monitoring health of neurons Help remove diseased or damage neurons by becoming phagocytes Important because cells of the immune system are not allowed in the central nervous system

Range in shape from squamous to columnar May be ciliated Ependymal Cells Range in shape from squamous to columnar May be ciliated Line the central cavities of the brain and spinal column Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities Cilia helps to circulate cerebrospinal fluid

(c) Ependymal cells line cerebrospinal fluid-filled cavities. Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal fluid-filled cavities. Figure 11.3c

Oligodendrocytes Branched cells Processes wrap CNS nerve fibers, forming insulating myelin sheaths

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 Both of these are neuroglia of the peripheral nervous system Satellite cells – similar in function to the astrocytes Schwann cells – similar to oligodendrocytes

Special characteristics: 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 Amitotic = no divide Important in signaling cells during development to form

Cell Body (Perikaryon or Soma) Spherical nucleus with nucleolus Well-developed Golgi apparatus Rough ER called Nissl bodies (chromatophilic substance) Cell body is also called the Perikaryon or soma Endoplasmic reticulum of the neurons is probably the most well developed and active of any cells in the body Lots of Golgi bodies

Cell Body (Perikaryon or Soma) Network of neurofibrils (neurofilaments) Axon hillock—cone-shaped area from which axon arises Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS 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 arising from the axon hillock Long axons (nerve fibers) Occasional branches (axon collaterals) Numerous terminal branches (telodendria) Knoblike axon terminals (synaptic knobs or boutons) Secretory region of neuron Release neurotransmitters to excite or inhibit other cells Telodendria Axon terminals

Axons: Function Conducting region of a neuron Generates and transmits nerve impulses (action potentials) away from the cell body Molecules and organelles are moved along axons by motor molecules in two directions: Anterograde—toward axonal terminal Examples: mitochondria, membrane components, enzymes Retrograde—toward the cell body Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins

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

Segmented sheath around most long or large-diameter axons Myelin Sheath Segmented sheath around most long or large-diameter axons It functions to: Protect and electrically insulate the axon Increase speed of nerve impulse transmission Myelin in PNS Schwann cells wraps many times around the axon Myelin sheath—concentric layers of Schwann cell membrane Neurilemma—peripheral bulge of Schwann cell cytoplasm Nodes of Ranvier Myelin sheath gaps between adjacent Schwann cells Sites where axon collaterals can emerge

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 Sheaths in the CNS Formed by processes of oligodendrocytes, not the whole cells Nodes of Ranvier are present No neurilemma Thinnest fibers are unmyelinated

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

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 Interneurons (association neurons) Shuttle signals through CNS pathways; most are entirely within the CNS Except for a few all sensory neurons are unipolar Impulses from great toe travel a meter before they reach a cell body in a ganglion close to the spinal cord. Efferent = exiting Most motor neurons are multipolar Multipolar—1 axon and several dendrites Most abundant Motor neurons and interneurons Bipolar—1 axon and 1 dendrite Rare, e.g., retinal 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

Resting Membrane Potential Differences in ionic makeup Inside the neuron has lower concentration of Na+ and Cl– than outside Inside has higher concentration of K+ and negatively charged proteins (A–) than outside

Resting Membrane Potential Negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell Sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for Na+ and K+ Because K+ is always leaking out and Na+ is always leaking in – you would think the concentrations would eventually even out BUT the Na-K pump transports Na out and K in to maintain concentration differences.

Membrane Potentials That Act as Signals Membrane potential changes when: Ion concentrations on two sides change Permeability of membrane to ions changes Changes in membrane potential are signals used to receive, integrate and send information

Changes in Membrane Potential Depolarization A reduction in membrane potential (toward zero) Inside of the membrane becomes less negative than the resting potential Increases the probability of producing a nerve impulse Hyperpolarization An increase in membrane potential (away from zero) Inside of the membrane becomes more negative than the resting potential Reduces the probability of producing a nerve impulse Remember we start at -70 mV

Membrane Potentials That Act as Signals Two types of signals Graded potentials Incoming short-distance signals Action potentials Long-distance signals of axons

Occur when a stimulus causes gated ion channels to open Graded Potentials Occur when a stimulus causes gated ion channels to open E.g., receptor potentials, generator potentials, postsynaptic potentials Magnitude varies directly (graded) with stimulus strength Decrease in magnitude with distance as ions flow and diffuse through leakage channels Short-distance signals Current dies within a few millimeters of its origin

Action Potential (AP) Brief reversal of membrane potential with a total amplitude of ~100 mV Occurs in muscle cells and axons of neurons Does NOT decrease in magnitude over distance Principal means of long-distance neural communication

Graded Potentials vs. Action Potentials Chemically gated ion channels Stimulus is related to the strength Die out with increasing distance Due to leakage of the charge Short distance travel Voltage gated ion channels Stimulus is consistent Do not decrease with distance Long distance travel Graded Potentials can cause Action Potentials

The big picture 1 2 3 3 4 2 1 1 4 Resting state Depolarization Repolarization 3 4 Hyperpolarization Membrane potential (mV) 2 Action potential Depolorization is caused by Na+ flowing into cell Repolorization is caused by K+ flowing out Hyperpolorization (threshold) caused by continual K+ flowing out Threshold 1 1 4 Time (ms) Figure 11.11 (1 of 5)

Threshold At threshold: Membrane is depolarized by 15 to 20 mV Na+ permeability increases Na influx exceeds K+ efflux The positive feedback cycle begins Subthreshold stimulus—weak local depolarization that does not reach threshold Threshold stimulus—strong enough to push the membrane potential toward and beyond threshold AP is an all-or-none phenomenon—action potentials either happen completely, or not at all

Coding for Stimulus Intensity Action potentials do not vary and are independent of stimulus intensity How does the CNS tell the difference between a weak stimulus and a strong one? Strong stimuli can generate action potentials more often than weaker stimuli CNS determines stimulus intensity by the frequency of impulses Action potentials Stimulus The more impulses, the greater the intensity of the stimulus

Conduction velocities of neurons vary widely Effect of axon diameter Conduction Velocity Conduction velocities of neurons vary widely Effect of axon diameter Larger diameter fibers = less resistance to local current flow = faster impulse conduction Effect of myelination Continuous conduction in unmyelinated axons is slower than saltatory (jumping) conduction in myelinated axons

Effects of myelination Conduction Velocity Effects of myelination Myelin sheaths insulate and prevent leakage of charge Saltatory conduction in myelinated axons is about 30 times faster Voltage-gated Na+ channels are located at the nodes APs appear to jump rapidly from node to node 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 Some immune system–modifying drugs, including interferons and Copazone: Hold symptoms at bay Reduce complications Reduce disability

Nerve Fiber Classification Group A fibers Large diameter, myelinated somatic sensory and motor fibers Group B fibers Intermediate diameter, lightly myelinated ANS fibers Group C fibers Smallest diameter, unmyelinated ANS fibers Nerve fibers are classified according to: Diameter Degree of myelination Speed of conduction

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

Types of Synapses Axodendritic Axosomatic Between the axon of one neuron and the dendrite of another Axosomatic Between the axon of one neuron and the soma of another

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 In embryos they guide early neuronal development In brain responsible for normal jerky movements of the eyes and also the hippocampus which is associated with learning

Chemical Synapses Specialized for the release and reception of neurotransmitters Typically composed of two parts Axon terminal of the presynaptic neuron Receptor region on the postsynaptic neuron

Synaptic Cleft Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next

Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Involves release, diffusion, and binding of neurotransmitters Ensures unidirectional communication between neurons

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

Synaptic Delay Neurotransmitter must be released, diffuse across the synapse, and bind to receptors Synaptic delay—time needed to do this (0.3–5.0 ms) Synaptic delay is the rate-limiting step of neural transmission

Postsynaptic Potentials Graded potentials Strength determined by: Amount of neurotransmitter released Time the neurotransmitter is in the area Types of postsynaptic potentials EPSP—excitatory postsynaptic potentials IPSP—inhibitory postsynaptic potentials

Excitatory Synapses and EPSPs Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions Short distance signaling Moves the polarity towards AP

Inhibitory Synapses and IPSPs Neurotransmitter binds to and opens channels for K+ or Cl– Causes a hyperpolarization (the inner surface of membrane becomes more negative) Reduces the postsynaptic neuron’s ability to produce an AP

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 (sleep) and histamine (wakeful, appetite, inflammation) Broadly distributed in the brain Play roles in emotional behaviors and the biological clock Dopamine – Happy neurotransmitter Drugs like crystal meth flood the brain with pleasure so it does not produce its own Cocaine on the other hand blocks the reabsorption of dopamine so dopamine continues to stimulate over and over-after repeated stimulation the body makes less dopamine Another effect of the drug abuse is the release of glutamate (reponsible for learning)-this NT can be responsible for “jonesing”

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