Chapter 11- Fundamentals Of The Nervous System And Nervous Tissue

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Nervous System

Back 1

The master controlling and communicating system of the body

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Functions of the Nervous System

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Sensory input
Integration
Motor output

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Sensory Input

Back 3

monitoring stimuli- internal (blood sugar) and external (temperature)

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Integration

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interpretation of sensory input and decide what to do with it

ex: hormones or skeletal muscle

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Motor Output

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response to stimuli; muscle contraction

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Central nervous system (CNS)

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Brain and spinal cord

Integration and command center

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Peripheral nervous system (PNS)

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Paired spinal and cranial nerves

Carries messages to and from the spinal cord and brain

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Peripheral Nervous System (PNS): Two Functional Divisions

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*Sensory (Afferent) division

*Motor (Efferent) division

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Sensory (afferent) division

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afferent= towards brain

Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain

Visceral afferent fibers – transmit impulses from visceral organs to the brain

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Motor (efferent) division

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efferent=away from brain

Transmits impulses from the CNS to effector organs

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Motor Division: Two Main Parts

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Somatic nervous system

Autonomic nervous system (ANS)

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Somatic nervous system

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Conscious control of skeletal muscles

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Autonomic nervous system (ANS)

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Regulates smooth muscle, cardiac muscle, and glands

Divisions – sympathetic and parasympathetic

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The two principal cell types of the nervous system are:

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Neurons – excitable cells that transmit electrical signals

Supporting cells – cells that surround and wrap neurons

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Supporting Cells: Neuroglia

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-Provide a supportive scaffolding for neurons
-Segregate and insulate neurons
-Guide young neurons to the proper connections
-Promote health and growth

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Astrocytes

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Most abundant, versatile, and highly branched glial cells

They cling to neurons and their synaptic endings, and cover capillaries

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Function of Astrocytes

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Support and brace neurons

Anchor neurons to their nutrient supplies

Guide migration of young neurons (control capillary permeability)

Control the chemical environment

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Microglia

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small, ovoid cells with spiny processes

Phagocytes that monitor the health of neurons

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Ependymal cells

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range in shape from squamous to columnar

They line the central cavities of the brain and spinal column

have cilia- help circulate cerebral spinal fluid around brain & cord

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Oligodendrocytes

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branched cells that wrap CNS nerve fibers

Produce insulating covering called myelin sheath

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Schwann cells (neurolemmocytes)

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surround fibers of the PNS

help form myelin sheath in PNS

vital for regeneration of damaged peripheral nerve tissue

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Satellite cells

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surround neuron cell bodies

part of PNS; serve same function as astrocytes

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Neuron Structure

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Composed of a body, axon, and dendrites

Long-lived, amitotic, and have a high metabolic rate

amitotic= don't reproduce

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Neuron Plasma Membrane Function

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Electrical signaling

Cell-to-cell signaling during development

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Nerve Cell Body (Perikaryon or Soma)

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-Contains the nucleus and a nucleolus
-Is the major biosynthetic center
-Is the focal point for the outgrowth of neuronal processes
-Has well-developed Nissl bodies (rough ER)
-Contains an axon hillock – cone-shaped area from which axons arise

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Processes of Neuron

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Armlike extensions from the soma

Called tracts in the CNS and nerves in the PNS

There are two types: axons and dendrites

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Dendrites of Motor Neurons

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-Short, tapering, and diffusely branched processes
-They are the receptive, or input, regions of the neuron
-Electrical signals are conveyed as graded potentials (not action potentials)

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Axons: Structure

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-Slender processes of uniform diameter arising from the hillock
-Long axons are called nerve fibers
-Usually there is only one unbranched axon per neuron
-Rare branches, if present, are called axon collaterals
-Axonal terminal – branched terminus of an axon

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Axons: Function

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-Generate and transmit action potentials
-Secrete neurotransmitters from the axonal terminals
-Movement along axons occurs in two ways
-Anterograde — toward axonal terminal
-Retrograde — away from axonal terminal

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Myelin Sheath

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Whitish, fatty (protein-lipoid), segmented sheath around most long axons

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Function of myelin sheath

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Protect the axon

Electrically insulate fibers from one another

Increase the speed of nerve impulse transmission

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Myelin Sheath and Neurilemma: Formation

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Formed by Schwann cells in the PNS

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Schwann cell

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Envelopes an axon in a trough

Encloses the axon with its plasma membrane

Has concentric layers of membrane that make up the myelin sheath

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Nodes of Ranvier (Neurofibral Nodes)

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Gaps in the myelin sheath between adjacent Schwann cells

They are the sites where axon collaterals can emerge

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White matter

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dense collections of myelinated fibers

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Gray matter

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mostly soma and unmyelinated fibers

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3 structural classifications of a neuron

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*Multipolar- 3 or more processes (99% of neurons)

*Bipolar- 2 processes (axon & dendrite)

*Unipolar- single, short process

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3 functional classifications of a neuron

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Sensory (afferent) — transmit impulses toward the CNS

Motor (efferent) — carry impulses away from the CNS

Interneurons (association neurons) — shuttle signals through CNS pathways

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Neurons are highly irritable/excitable-

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True

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Action potentials, or nerve impulses, are:

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Electrical impulses carried along the length of axons

Always the same regardless of stimulus

The underlying functional feature of the nervous system

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Conduction Velocities of Axons

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Conduction velocities vary widely among neurons

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Rate of impulse propagation is determined by:

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Axon diameter – the larger the diameter, the faster the impulse

Presence of a myelin sheath – myelination dramatically increases impulse speed

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Saltatory Conduction

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Current passes through a myelinated axon only at the nodes of Ranvier

Action potentials are triggered only at the nodes and jump from one node to the next

Much faster than conduction along unmyelinated axons

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Synapses

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*A junction that mediates information transfer from one neuron:

-To another neuron
-To an effector cell

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Presynaptic neuron

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conducts impulses toward the synapse

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Postsynaptic neuron

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transmits impulses away from the synapse

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Axodendritic

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synapses between the axon of one neuron and the dendrite of another

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Axosomatic

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synapses between the axon of one neuron and the soma of another

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Other types of synapses include:

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-Axoaxonic (axon to axon)
-Dendrodendritic (dendrite to dendrite)
-Dendrosomatic (dendrites to soma)

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Electrical synapses:

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Are less common than chemical synapses

Correspond to gap junctions found in other cell types

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Why are electrical synapses important to CNS?

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-Arousal from sleep
-Mental attention
-Emotions and memory
-Ion and water homeostasis

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Chemical Synapses

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Specialized for the release and reception of neurotransmitters

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Chemical Synapses Typically composed of two parts:

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Axonal terminal of the presynaptic neuron, which contains synaptic vesicles

Receptor region on the dendrite(s) or soma of the postsynaptic neuron

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Synaptic Cleft

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Fluid-filled space separating the presynaptic and postsynaptic neurons

Prevents nerve impulses from directly passing from one neuron to the next

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Transmission across the synaptic cleft:

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Is a chemical event (as opposed to an electrical one)

Ensures unidirectional communication between neurons

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Step 1 of Synaptic Cleft: Information Transfer

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Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca2+ channels

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Step 2 of Synaptic Cleft: Information Transfer

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Neurotransmitter is released into the synaptic cleft via exocytosis in response to synaptotagmin

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Step 3 of Synaptic Cleft: Information Transfer

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Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron

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Step 4 of Synaptic Cleft: Information Transfer

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Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect

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Termination of Neurotransmitter Effects

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Neurotransmitter bound to a postsynaptic neuron:
-Produces a continuous postsynaptic effect
-Blocks reception of additional “messages”
-Must be removed from its receptor

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Removal of neurotransmitters occurs when they:

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Are degraded by enzymes

Are reabsorbed by astrocytes or the presynaptic terminals

Diffuse from the synaptic cleft

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Synaptic Delay

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*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

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Postsynaptic Potentials

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Neurotransmitter receptors mediate changes in membrane potential according to:
-The amount of neurotransmitter released
-The amount of time the neurotransmitter is bound to receptors

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The two types of postsynaptic potentials are:

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EPSP – excitatory postsynaptic potentials

IPSP – inhibitory postsynaptic potentials

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EPSPs are graded potentials that can initiate an action potential in an axon

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Use only chemically gated channels

Na+ and K+ flow in opposite directions at the same time

Postsynaptic membranes do not generate action potentials

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Neurotransmitter binding to a receptor at inhibitory synapses:

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Causes the membrane to become more permeable to potassium and chloride ions

Leaves the charge on the inner surface negative

Reduces the postsynaptic neuron’s ability to produce an action potential

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Summation

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A single EPSP cannot induce an action potential

EPSPs must summate temporally or spatially to induce an action potential

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Temporal summation

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presynaptic neurons transmit impulses in rapid-fire order

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Spatial summation

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postsynaptic neuron is stimulated by a large number of terminals at the same time

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What happens when IPSPs summate with EPSPs

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They cancel each other out

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Neurotransmitters

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*Chemicals used for neuronal communication with the body and the brain
*50 different neurotransmitters have been identified
*Classified chemically and functionally

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Chemical Neurotransmitters

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Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Novel messengers: ATP and dissolved gases NO and CO

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Neurotransmitters: Acetylcholine

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-First neurotransmitter identified, and best understood
-Released at the neuromuscular junction
-Synthesized and enclosed in synaptic vesicles
-Degraded by the enzyme acetylcholinesterase (AChE)

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Acetylcholine is released by:

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All neurons that stimulate skeletal muscle

Some neurons in the autonomic nervous system

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Neurotransmitters: Biogenic Amines

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Broadly distributed in the brain

Play roles in emotional behaviors and our biological clock

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Biogenic Amines include:

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Catecholamines – dopamine, norepinephrine (NE), and epinephrine

Indolamines – serotonin and histamine

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Neurotransmitters: Amino Acids

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*Include:
-GABA – Gamma (γ)-aminobutyric acid
-Glycine
-Aspartate
-Glutamate

*Found only in the CNS

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Neurotransmitters: Peptides

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Act as natural opiates; reduce pain perception

Bind to the same receptors as opiates and morphine

Gut-brain peptides – somatostatin, and cholecystokinin

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Neurotransmitters: Peptides include:

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Substance P – mediator of pain signals

Beta endorphin, dynorphin, and enkephalins

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ATP (novel messenger of neurotransmitters)

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-Is found in both the CNS and PNS
-Produces excitatory or inhibitory responses depending on receptor type
-Induces Ca2+ wave propagation in astrocytes
-Provokes pain sensation

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Nitric oxide (NO) (novel messenger of neurotransmitters)

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Is involved in learning and memory

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Two functional classes of Neurotransmitters

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excitatory and inhibitory

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Excitatory neurotransmitters

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cause depolarizations 
(e.g., glutamate)

more positive

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Inhibitory neurotransmitters

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cause hyperpolarizations (e.g., GABA and glycine)

more negative

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Some neurotransmitters have both excitatory and inhibitory effects

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*Determined by the receptor type of the postsynaptic neuron

*Example: acetylcholine
-Excitatory at neuromuscular junctions with skeletal muscle
-Inhibitory in cardiac muscle

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Neurotransmitter Receptor Mechanisms- DIRECT

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*neurotransmitters that open ion channels

Promote rapid responses
Examples: ACh and amino acids

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Neurotransmitter Receptor Mechanisms- INDIRECT

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*neurotransmitters that act through second messengers

-Promote long-lasting effects
-Examples: biogenic amines, peptides, and dissolved gases

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Channel-Linked Receptors

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-Composed of integral membrane protein
-Mediate direct neurotransmitter action
-Action is immediate, brief, simple, and highly localized
-Ligand binds the receptor, and ions enter the cells
-Excitatory receptors depolarize membranes
-Inhibitory receptors hyperpolarize membranes

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G Protein-Linked Receptors

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Responses are indirect, slow, complex, prolonged, and often diffuse

These receptors are transmembrane protein complexes

Examples: muscarinic ACh receptors, neuropeptides, and those that bind biogenic amin

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Neural Integration: Neuronal Pools

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*Functional groups of neurons that:

-Integrate incoming information

-Forward the processed information to its appropriate destination

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Input fiber (simple neuronal pool)

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presynaptic fiber

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Discharge zone (simple neuronal pool)

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neurons most closely associated with the incoming fiber

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Facilitated zone (simple neuronal pool)

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neurons farther away from incoming fiber

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Divergent (circuit in neuronal pools)

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one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits

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Convergent (circuit in neuronal pools)

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opposite of divergent circuits, resulting in either strong stimulation or inhibition

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Reverberating (circuit in neuronal pools)

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chain of neurons containing collateral synapses with previous neurons in the chain

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Parallel after-discharge (circuit in neuronal pools)

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incoming neurons stimulate several neurons in parallel arrays

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Serial Processing (Patterns of Neural Processing)

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Input travels along one pathway to a specific destination

Works in an all-or-none manner

Example: spinal reflexes

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Parallel Processing (Patterns of Neural Processing)

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Input travels along several pathways

Pathways are integrated in different CNS systems

One stimulus promotes numerous responses

Example: a smell may remind one of the odor and associated experiences