Chapter 12: Nervous Tissue

Ganglia

Ganglia

clusters of neuron cell bodies in PNS

Divisons of the nervous system

central nervous system and peripheral nervous system

Neurons

the primary communicative cell in the nervous system

Axon

the extension of a neuron specialized for the rapid conduction of signals to distant points

axoplasm

cytoplasm of axon

axon collaterals

side branches of the axon

Myelination in the PNS

Schwann cells wrap around a single nerve fiber up to 100 times; leading to the neurolemma and endoneurium

Neurolemma

the thick outermost coil of the myelin sheath

-contains the nucleus of Schwann cells & most of its cytoplasm

Endoneurium

fibrous CT around a Schwann cell

The endoneurium and neurilemma are important factors in:

nerve regeneration in the PNS

Myelination in the CNS

-oligodendrocytes myelinate nerve fibers in its immediate vicinity

-never fibers in the CNS do not have a neurolemma or endoneurium

-cannot regenerate CNS fibers

Nodes of Ranvier

gaps in the myelin sheath

Internodes

myelin covered segments from one gap to the next

Trigger Zone

axon hillock and initial segment

-not covered in myelin

-important role in initiating a nerve signal

nervous system functions

sensory input, integration, motor output

What are nerves?

bundles of nerve fibers (axons)

Sensory Input

Information gathered by sensory receptors about internal and external environment changes

-ex. taste, hearing, pain, temperature

Integration

To process and interpret sensory input and decide if action is needed

motor output

Activation of effector organs (muscles and glands) produces a response

Central Nervous System (CNS)

consists of the brain and spinal cord

-integration and control center

Neuron type exclusively in the Central Nervous System

interneuron

Peripheral Nervous System (PNS)

consists of cranial and spinal nerves

-detects a stimulus and conducts a response

-consists of sensory (afferent) and motor (efferent) neurons

Divisions of Peripheral Nervous System

Sensory and Motor

Sensory (afferent) division

Carry information TO the CNS from sense organs (receptors)

Sensory Division is divided into 2 subdivisions:

Somatic Sensory- fibers come from skin, skeletal muscles, and joints

Visceral Sensory- fibers come from visceral organs (liver, stomach)

Motor (efferent) Division

Carry information AWAY from CNS to muscles or glands

Motor Division is subdivided into 2 subdivisions:

Somatic Motor (voluntary)-skeletal muscle innervation; nerve cells tell muscles to move.

Autonomic Visceral Motor (involuntary)- divided into 2 parts

Autonomic Motor Division subdivisions:

Sympathetic Division- puts your body's system on alert; think fight of flight

Parasympathetic Division- relaxes the body's systems; rest and digest*

Universal functions of a neuron

excitability, conductivity, and secretion

Functional Classes of Neurons

afferent, efferent, interneurons

sensory (afferent) neurons

Detect stimuli and transmit information about them toward the CNS

Interneurons (association neurons)

-Lie entirely within CNS, connecting motor and sensory neurons (about 90% of all neurons)

-Receive signals from many neurons and carry out integrative functions (make decisions on responses)

motor (efferent) neurons

send signals to muscles and glad cells

Soma

cell body of a neuron

Why can neurons not divide?

They do not contain centrioles for mitosis.

-once a neuron dies, it does not come back.

Dendrites

Branchlike extensions from the neuron's soma that are specialized to receive information.

Axolemma

plasma membrane of axon

myelin sheath

covers the axon of some neurons and helps speed neural impulses

synaptic knob of axon terminal

the rounded area at the end of axon terminals that contain synaptic vesicles of neurotransmitters

Multipolar Neuron

A neuron with a single axon and multiple dendrites; the most common type of neuron in the CNS.

Bipolar Neuron

a neuron with one axon and one dendrite; can be found in olfactory cells, the retina, and inner ear

unipolar neuron

a neuron with a single process of a dendrite and axon

anaxonic neuron

-many dendrites but no axon

-communicate locally (do not produce action potentials)

-found in the retina, brain, and adrenal gland

all cell products of a neuron are made in the

soma

An axon uses two ways of transporting signals through the nervous system:

Anterograde Transport

Retrograde Transport

Aterograde transport

Movement down the axon, away from the soma

Retrograde transportation

Movement up the axon, toward the soma

neuroglial cells

-protect neurons and help them function

-bind neurons and form the framework for nervous tissue

-guide migrating neurons during development

Mature neurons are covered by glial cells nearly everywhere

outnumber neurons 10:1

4 types of neuroglial cells in CNS

astrocytes, oligodendrocytes, microglia, ependymal cells

2 types of neuroglia in PNS

satellite cells and Schwann cells

Oligodendrocytes

Forms the myelin sheath in CNS

-arm-like processes wrap around nerve fibers

ependymal cells

-line cavities of the brain and spinal cord,

-circulate and secrete cerebrospinal fluid (CSF)

microglial cells

-serve as the immune system in the brain

-wander through the CNS looking for debris and damage

-multiply in areas of damage

Astrocytes

-most abundant CNS neuroglia

-have perivascular feet that connect capillaries and form a seal- the Blood Brain Barrier

-Convert glucose from the blood to lactate to supply energy to neurons.

-Regulate the chemical composition of fluid by absorbing excess neurotransmitters and ions

Blood Brain Barrier (BBB)

A selective barrier that prevents unwanted materials from leaving the blood & entering fluid around the CNS

satellite cells

-surround neuron cell bodies in PNS

-provide electrical insulation around the soma

-regulate the chemical environment of neurons

Schwann cells

produce myelin sheath in PNS

-assist in regeneration of damaged fibers

Myelination

-insulates the axon and increases signal speed

-consists of the plasma membrane of glial cells

-is 80% lipid and the rest protein

unmyelinated nerve fibers

-Many CNS and PNS fibers are unmyelinated

-In PNS, Schwann cells hold 1 to 12 small nerve fibers in surface grooves

-The membrane folds once around each fiber

-called "unmyelinated" but still somewhat myelinated; essential to help control the environment around the axon

The speed of a nerve signal depends on:

1. the diameter of the fiber

2. presence or absence of myelin

Larger Fibers will:

create faster signals

Presence of a myelin sheath

increases signal speed

Regeneration can only happen if

-The soma is intact and some of the neurolemma

-if the neuron is located in the PNS

An unstimulated neuron's resting membrane potential is:

-70 mV

The Na+/K+ pump is important for

maintaining the RMP and gradient

2 Types of disturbances at the RMP

1. Local Potential (graded)

2. Action Potential

Local potentials are how the body:

senses stimuli in the environment

Properties of Local Potentials

-graded

-decremental

-reversible

-either excitatory or inhibitory

graded

-vary in magnitude with stimulus strength

-Stronger stimuli open more Na+ gates

Decremental

get weaker the farther they spread from the point of stimulation

reversible

if stimulation ceases, the cell quickly returns to its normal resting potential

excitatory or inhibitory

Some neurotransmitters make the membrane less likely to send a signal (inhibitory)

if the local potential is strong enough...

-they overcome their decremental nature and cause a neuron to send a signal; stimulating the trigger zone

-the trigger zone has a high concentration of ion-gated channels; when these open they stimulate an Action Potential

Local potentials regulate:

Neuron signaling (Action Potential)

Action Potential

1. At RMP, Na+ and K+ channels are closed. Na+/K+ pump is maintaining RMP= -70 mV

2. Once the threshold is met, -55mV, depolarization begins. Na+ channels open, and Na+ enters the cell. K+ channels open after.

3. At peak of an action potential, +35 mV, repolarization begins. Na+ channels close, K+ channels fully open and K+ rushes out.

4. Membrane temporarily hyperpolarized, -71 to -75 mV. This is caused by more K+ leaving the cell than Na+ entering

Action Potential Characteristics

Not Graded

Not Decremental

Not Reversible

Signal Conduction is Nerve Fibers

-Action potentials do not travel down the axon

-An action potential triggers another, then another (like dominos)

-all the action potentials together are called a nerve signal

Unmyelinated Signal Conduction

-unmyelinated fibers have voltage-gated channels along their entire length

-Action potential at the trigger zone causes Na+ to enter the axon & diffuse into adjacent regions

-Depolarization opens voltage-gated channels, creating a new action potential. Creating a chain reaction until nerve signal reaches the end of the axon or synapse.

Myelin affects the axon in two ways:

1. reduces the leakage of sodium out of the cell; creates a higher concentration on Na+ inside the cell

2. Insulates the Axon; results in sodium moving more freely through the cell

In myelinated fibers, where does action potential occur?

In the gaps between the myelin

Saltatory Conduction

Action potential "jumps" down the axon- only in the gaps between myelin.

-results in a much faster nerve signal (120 m/s vs. 2 m/s)

Chemical Synapse

the most common type of synapse

-have a synaptic cleft and neurotransmitters are used to communicate across the cleft

Electrical Synapse

Less common than chemical

-occur between neurons in cardiac and smooth muscle

-Gap junctions joint adjacent cells; no cleft or neurotransmitters

-Advantage of quick transmission

presynaptic neuron

-the first neuron in the signal path

-releases neurotransmitters

postsynaptic neuron

the second neuron; responds to the neurotransmitter

The presynaptic neuron can synapse with the postsynaptic neuron in multiple ways:

Axodendritic synapse

axosomatic synapse

axoaxonic synapse

axodendritic synapse

axon to dendrite

axosomatic synapse

axon to soma

axoaxonic synapse

axon to axon

4 categories of neurotransmitters

acetylcholine, amino acids, monoamines, neuropeptides

Acetylcholine

stimulates skeletal muscle

Norepinephrine and Epinephrine

regulates dreaming, waking, and mood; excites cardiac muscle

Dopamine

elevation of mood and control of skeletal muscles

Seratonin

sleep/wake cycles, thermoregulation, and mood

Endorphin

suppresses pain; makes you feel good

excitatory cholinergic synapse

-uses acetylcholine

-nerve signal arrives at synaptic cleft and opens Ca+2 gated channels.

-Calcium enters knob and triggers exocytosis of ACh. ACh diffuses across cleft and binds to postsynaptic receptors.

-The receptors open ion channels that allow Na+ and K+ diffuse.

-Entry of Na+ causes depolarization; if depolarization is strong enough, Action Potential will occur at trigger zone.

inhibitory GABA-ergic synapse

-employs y-aminobutyric acid as its neurotransmitter

-nerve signal triggers release of GABA into synaptic cleft

-their receptors are chloride channels

-Cl- enters cell and makes the inside more negative than the resting membrane potential

-postsynaptic neuron is inhibited, and less likely to fire

The postsynaptic neuron's response is dependent on:

whether EPSP or IPSP is more prominent

- more IPSP than EPSP- no response

-more EPSP than IPSP- action potential may be generated

( this is referred to as summation)

temporal summation

Presynaptic neuron fires very rapidly or very slowly

Spatial summation

multiple presynaptic neurons stimulate one postsynaptic neuron