BIO 220 CH 12: Nervous Tissue

The Nervous System

The master control and communications system of the body

Sensory receptor--Sensory input (\_______)--\> Integration --Motor Output (\_______)--\> Effector

afferent, efferent

Effectors \=

muscles, glands, adipose tissue

Afferent \=

approaches central nervous system

Efferent \=

exiting central nervous system

The nervous system Includes various organs

- Brain and spinal cord
-Receptors of sense organs (eyes, ears, ...)
- Nerves that connect to other systems

Nervous tissue contains 2 kinds of cells:

- Neurons for intracellular
-Neuroglia (glial cells)
- Essential to survival and function of neurons
- Preserve structure of nervous tissue

Central nervous system (CNS)

- Brain and spinal cord
- Consist of nervous tissue
Peripheral nervous system (PNS) includes all nervous tissue outside CNS

Peripheral nervous system (PNS) delivers

sensory information to the CNS

Peripheral nervous system (PNS) Carries

motor commands to peripheral tissues

Peripheral nervous system (PNS) Nerves (peripheral nerves)

- Bundles of axons w/connective tissues and blood vessels
- Cranial nerves connect to brain
- Spinal nerves attach to spinal cord

Functional divisions of the PNS

afferent division, efferent divsion, receptors, effectors

Afferent division

- Carries sensory information
- From receptors in peripheral tissues and organs to CNS

Efferent division

- Carries motor commands
- From CNS to effector (muscles, glands, adipose tissue)

Receptors

- Detect changes or respond to stimuli
- May be neurons or specialized ells
- May be single cells or complex sensory organs (e.g., eyes, ears)

Effectors

Target organs that respond to motor commands (muscles/glands/adipose tissue)

Efferent division of PNS contains

Somatic Nervous System (SNS) and Autonomic Nervous System (ANS)

Somatic nervous system (SNS)
- Controls skeletal muscle contractions
- Both voluntary and involuntary (reflexes)
- Effector \= \________ \_______

skeletal muscle

Autonomic nervous tissue (ANS)
- Controls subconscious actions, contractions of smooth and cardiac muscle, and glandular secretions
- \__________ division
- Parasympathetic division

Sympathetic

Neurons

- Basic functional units of the nervous system
- Send and receive signals
- Function in communication, information processing, and control

Neuron Cell body (soma)
- \______ nucleus and nucleolus
- Perikaryon (cytoplasm)
- Mitochondria (produce energy)
- RER and \_______ (synthesize proteins)

Large, ribosomes

Neuron Dendrites
○ Short and highly branched processes extending from cell body
○ Dendritic spines
- Fine processes on dendrites
- \_______ information from other neurons
- \___-\___% of neuron surface area

Receive, 80, 90

Neuron Axon
○ Single, long \________ process
- \_________ process
○ Propagates electrical signals (action potentials)
○ \________ - cytoplasm of axon
- Contains neurofibrils, neurotubules, enzymes and organelles

cytoplasmic, Longest, Axoplasm

Structures of the axon

axolemma, axon hillock, collaterals, axon terminals

Axolemma

- Plasma membrane of the axon
- Covers the axoplasm

Axon hillock

Thick region that attaches initial segment to cell body

Collaterals

branches of the axon

Axon terminals

(synaptic terminals/knobs)

Structrural classifications of neurons

- Anaxonic Neurons
- Bipolar Neurons
- Unipolar Neurons
- Multipolar Neurons

Anaxonic neurons
- \_______
- All cell processes look similar
- Found in \_______ and special sense organs

Small, brain

Bipolar neurons
- Small and \_____
- \_____ dendrite and \____ axon
- Found in special sense \______ (sight, smell, hearing)

rare, one, one, organs

Unipolar neurons (pseudounipolar neurons)
- Axon and dendrites are \______
- Cell body to \___ side
- Most \________ neurons of PNS

fused, one, sensory

Multipolar neurons
- Have one \_____ axon and two or more dendrites
- Common in the \______
- All \________ neurons that control skeletal muscles

long, CNS, motor

Functional classifications of neurons

Sensory, Motor, or Interneurons

Sensory neurons (afferent neurons)
○ \_______
○ Cell bodies grouped in sensory \________
○ Processes (afferent fibers) extend from sensory receptors to \____
○ Somatic sensory neurons
- Monitor \________ environment
○ Visceral sensory neurons
- Monitor \________ environment

Unipolar, ganglia, CNS, external, internal

Types of sensory receptors

interoceptors, exteroceptors, proprioceptors

Interoceptors
- Monitor \_______ systems (e.g., digestive, urinary)
- Internal \______ (stretch, deep pressure, pain)

internal, senses

Exteroceptors
- Monitor \_______ environment (e.g., temperature)
- \_______ senses (e.g., sight, smell, hearing)

external, complex

Proprioceptors (type of mechanoreceptor)
- Monitor position and movement of \_______ muscles and joints

skeletal

Types of motor neurons

somatic and visceral

Motor neurons (efferent neurons)
○ Carry instructions from \_____ to peripheral effectors
- Via \_______ fibers (axons)

CNS, efferent

Somatic motor neurons of SNS

Innervate skeletal muscles

Visceral motor neurons of ANS

Innervate all other peripheral effectors
□ Smooth and cardiac muscle, glands, adipose tissue

Motor neurons signals from CNS to visceral effectors cross \_______ \_____ that divide axons into
- Preganglionic fibers
- Postganglionic fibers

autonomic ganglia

Interneurons
- Found in \_____
- Located \________ sensory and motor neurons

CNS, between

Interneurons are responsible for
- Distribution of \_______ information
- Coordination of \_______ activity

sensory, motor

Interneurons are involved in

higher functions
- memory, planning, learning

Neuroglia
○ Support and protect \________
○ Make up \_____ the volume of the nervous system
○ Different types in CNS and PNS

neurons, half

Types of neuroglia in the CNS

- Astrocytes
- Ependymal cells
- Oligodendrocytes
- Microglia

Astrocytes have

large cell bodies with many processes

Astrocytes function to
- \_______ neurons
- Control \________ environment - maintain ion concentration
- Maintain blood brain barrier (BBB)
- Create three-dimensional framework for \_____
- \______ damaged nervous tissue
- \______ neuron development & synapse formation

Nourish, interstitial, CNS, Repair, Guide

Ependymal cells
- Form epithelium that lines \_______ \______ of spinal cord and ventricles of brain
- \_______ and monitor cerebrospinal fluid (CSF)
- Have \_____ that help circulate CSF

central canal, Produce, cilia

Oligodendrocytes
○ Have \______ cell bodies with few processes
○ \______ cooperate to form a myelin sheath
- Myelin insulates myelinated axons
- Increases speed of action potentials
- Makes nerves (tracts) appear white
○ \_________—myelinated segments of axon
○ Nodes (nodes of Ranvier) lie between \__________

small, Many, Internodes, internodes

White matter

Regions of CNS with many myelinated axons

Gray matter of CNS

Contains unmyelinated axons, neuron cell bodies, and dendrites

Microglia
○ \_______ and \_____ numerous neuroglia
○ Have \______ fine-branched processes
○ \_______ through nervous tissue
○ \________ that engulf & remove invading organisms
○ \_______ up cellular debris, wastes, and pathogens
○ Originates in \_____ \_________

Smallest, least, many, Migrate, Phagocytes, Clean, bone marrow

Neuroglia of the PNS

Insulate neuronal cell bodies and most axons

Neuroglia of the PNS Two types:

- Satellite cells
- Schwann cells

Satellite cells
○ Surround \_______ (clusters of neuronal cell bodies)
○ \_______ interstitial fluid around neurons

ganglia, Regulate

Schwann cells (neurolemmocytes)
○ \_____ myelin sheath around axons
○ \________—outer surface of Schwann cell
○ A myelinating Schwann cell sheaths only one \_____
○ \_______ Schwann cells sheath entire axon

Form, Neurolemma, axon, many

Organization in the PNS & CNS

gray matter and white matter

Gray matter
- \________ are collections of neuron cell bodies in the PNS
- \________ are collections of neuron cell bodies in the CNS

Ganglia, Nuclei

White matter
- \______ - Bundles of axons in the PNS
- \_______ - Bundles of axons in the PNS

Nerves, Tracts

Neural responses to injuries
○ Wallerian degeneration
- \______ distal to injury degenerates
○ Schwann cells (PNS)
- Form \_____ for new growth
- Wrap around new \_____ Axon, path, axon

Nervous tissue regeneration in CNS
○ Limited by \_______, which
- Produce \______ tissue
- Release chemicals that \_______ regrowth

astrocytes, scar, block

Passive processes acting across cell membrane

chemical, electrical, electrochemical gradients

Chemical gradients

Concentration gradients of ions (Na+, K+)

Electrical gradients

Charges are separated by cell membrane

Electrochemical gradient

- Sum of chemical and electrical forces acting on an ion across the membrane
- A form of potential energy

More sodium \______ cell

outside

More potassium \______ the cell

inside

Active processes across the membrane

Sodium-potassium exchange pump

Sodium-potassium exchange pump
- Powered by __ATP
- \______ 3 Na+ for every 2 K+ brought in
- Balances passive forces of diffusion
- Stabilizes resting membrane potential (\____ mV)

1, Ejects, -70

Na+ and K+ are the primary determinants of \________ potential
- Na+ and K+ channels are either passive or active

membrane

Passive ion channels (\_____channels)
○ Are always \______
○ \__________ changes with conditions

leak, open, Permeability

Active ion channels (gated ion channels)
○ Open and close in response to \_______
○ At resting membrane potential, most are \______
- Chemically gated
- Voltage gated
- Mechanically gated

stimuli, closed

Chemically gated ion channels
- Also called ligand-gated ion channels
- Open when they \____ specific chemicals (e.g., ACh)
- Found where \______ communicate with other cells

bind, neurons

Voltage-gated ion channels
○ Respond to changes in \________ potential
○ Found in \______ of neurons and sarcolemma of skeletal and cardiac
○ Activation gate opens when stimulated (\____ mV stimulus)
○ Inactivation gate closes to stop ion movement
○ Three possible states
- \______ but capable of opening
- Open (activated)
- Closed and incapable of opening (\__________)

membrane, axons, -55, Closed, inactivated

All plasma (cell) membranes produce electrical signals by \____ movements
○ Membrane potential is particularly important to \_______

ion, neurons

Resting membrane potential
○ The membrane potential of a resting cell
○ The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in \_______ composition
- \_________ fluid contains high concentrations of Na+ and Cl-
- \________ contains high concentrations of K+ and negatively charged proteins
○ Cells have selectively permeable membranes
○ Membrane permeability varies by ion

ionic, Extracellular, Cytosol

Passive processes acting across cell membrane

current and resistance

Current

Movement of charges to eliminate a potential difference

Resistance

How much the membrane restricts ion movement

What influences the resistance across the membrane?

If resistance is high, current is small

Subthreshold stimulus

A stimulus too small to create an action potential in a motor neuron

Graded potentials (local potentials \= doesn't reach threshold!)
○ \_______ in membrane potential
- That cannot spread far from site of \_________
○ Produced by any stimulus that opens \______ channels
○ Effect spreads passively, due to local currents
○ May involve \__________ or hyperpolarization
○ \_________ stimuli produce greater changes in membrane potential and affect a larger area

Changes, stimulation, gated, depolarization, Stronger

Hyperpolarizing brings membrane potential \_______ from threshold (inhibitory)

further

Depolarizing brings membrane potential \________ to the threshold (stimulatory)

closer

• Example:
○ Sodium ions move \_______ to plasma membrane
- Producing \______ current
- Which \__________ nearby regions of plasma membrane (graded potential)
- Change in potential is proportional to stimulus

parallel, local, depolarizes

• Graded potentials
○ Repolarization

When the stimulus is removed, membrane potential returns to normal

• Graded potentials
○ Hyperpolarization

Results from opening chemically gated potassium (K+) ion channels

Which direction is K+ going to move?

Out the cell
- Opposite effect of opening sodium ion channels

Graded potential
○ Temporary, \_________ change in resting potential
○ Caused by a \________

localized, stimulus

Action potential
○ Is an \________ impulse
○ Produced by graded potential that reaches threshold (\____mV)
○ Propagates along surface of \_____ to synapse

electrical, -55, axon

Action potentials (nerve impulses)
○ Propagated changes in membrane potential
○ Affect an \______ excitable membrane
○ Begin at initial segment of axon
○ Do not \_______ as they move away from source
○ Stimulated by a \________ potential that depolarizes the axolemma to threshold
- Threshold for an axon is -60 to -55 mV

entire, diminish, graded

All-or-none principle
○ Any stimulus that changes the membrane potential to \________
- Will cause an action potential
○ All action potentials are the \______
- No matter how large the stimulus
○ An action potential is either triggered (100%) or not (0%)

threshold, same

Generation of action potentials:
○ Step 1 - Chemical Stimulus:
- \_________ to threshold

Depolarization

Generation of an Action Potential
• Step 2:
○ Activation of voltage-gated \____ channels
○ Na+ rushes into \_______
○ Inner membrane surface changes from negative to \_______
○ Results in rapid depolarization

Na+, cytosol, positive

Generation of an Action Potential
• Step 3:
○ Inactivation of Na+ channels and \________ of K+ channels
○ At +30 mV, inactivation gates of voltage-gated Na+ channels close
○ Voltage-gated K+ channels open
○ K+ moves out of \________
○ Repolarization begins

activation, cytosol

Generation of an Action Potential
• Step 4:
○ Voltage-gated K+ channels begin to \______
○ As membrane reaches normal resting potential
○ \__ continues to leave cell
○ Membrane is briefly \__________ to -90 mV

close, K+, hyperpolarized