CH12: Neural Tissue

Nervous system

The master controlling and communicating system of the body

Sensory input

Monitoring stimuli- receptors

Integration

Interpretation of sensory input- CNS

Motor output

Response to stimuli- effectors (skeletal, smooth, cardiac muscles, and all glands)

Central nervous system

Brain and spinal cord
Integration and command center

Peripheral nervous system

All nervous structures outside of the brain and spinal cord
Carries messages to and from the spinal cord and brain
2 divisions: sensory (afferent) and motor (efferent) division

Sensory division

Afferent division
Transmits sensory input from receptors to CNS
Contains: somatic afferent fibers, visceral afferent fibers, and special sensory fibers

Somatic afferent fibers

Carry sensory input from skin, skeletal muscles, and joints to the CNS

Visceral afferent fibers

Transmit sensory input from visceral organs to the CNS

Special sensory fibers

transmit sensory input for vision, smell, tastes, balance and hearing to CNS

Motor division

Efferent division
Transmits motor output from the CNS to effector organs
2 parts: somatic nervous system and autonomic nervous system

Somatic nervous system

Conscious control of skeletal muscles

Autonomic nervous system

Subconscious control of smooth muscle, cardiac muscle and glands
Divisions- sympathetic and parasympathetic

Enteric nervous system

100 million neurons in walls of digestive tract
-as many or more than a spinal cord
-use the same neurotransmitters as the brain
Initiates and coordinates visceral reflexes locally
-without instructions from CNS
Can be influenced by ANS

Histology of nervous tissue

The 2 principal cell types of the nervous system are: neurons and neuroglia
Highly cellular; little extracellular space
-tightly packed

Neurons

Excitable cells that transmit electrical signals
Functional units of the nervous system
Composed of a body, axon, and dendrites
Long-lived and amitotic (no division occurs)
Plasma membrane function in ELECTRICAL SIGNALING

Neuroglia

(Supporting cells)
Cells that surround and wrap neurons
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth

Soma

Nerve cell body
contains the nucleus with a nucleolus and many organelles
Has no centrioles
Has well-developed Nissl bodies (rough ER)
Contains an axon hillock- cone shaped area from which axons arise
Nuclei and ganglia

Nuclei

clusters of neuron cell bodies in CNS

Ganglia

clusters of neuron cell bodies in PNS

Neuron processes

Armlike processes that extend from cell body
2 types: dendrites and axon
CNS contains both neuron cell bodies and their processes
PNS contains chiefly neuron axon
Tracts and nerves

Tracts

Bundles of neuron axons in CNS

Nerves

Bundles of neuron axons in PNS

Dendrites

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

Axons structure

AKA nerve fibers
Slender processes of uniform diameter arising from the axon hillock
Usually there is only one unbranched axon per neuron
Occasional branches are called AXON COLLATERALS

Telodendria

Small branches arising from axon extensions end

Axon terminals

AKA end synaptic bulbs
Arise from telodendria components that communicate with target cell

Axolemma

Plasma membrane of axon

Axons function

Generate and transmit action potentials
Secrete neurotransmitters from the axonal terminals
Sends signal away toward target cell

Multipolar

3 or more processes
Motor and interneurons
MOST COMMON

Bipolar

2 processes (axon and dendrite)
Special sensory

Unipolar

One T-like process (2 axons)
Sensory information from skin, muscles, etc

Axoaxonic

All cell processes look alike

Sensory

Afferent
Transmit impulses TOWARD the CNS

Motor

Efferent
Carry impulses AWAY from the CNS

Interneurons

Association neurons
shuttle signals through CNS pathways
99% of neurons

Astrocytes

ONLY in CNS
Most abundant, versatile, and highly branched neuroglial cells
They cling to neurons and cover capillaries

Astrocytes function

Support and brace neurons
Anchor neurons to their nutrient supplies
Guide migration of young neurons
Help control the chemical environment as part of blood brain barrier

Microglia

ONLY in CNS
Small, oval cells with spiny processes
-phagocytes (immune cells to protect)

Ependymal cells

ONLY in CNS
Range in shape from squamous to columnar
-they line the ventricles of the brain and the central canal of the spinal cord
-assist in producing, circulating, and monitoring cerebrospinal fluid

Oligodendrocytes

ONLY in CNS
Branched cells that wrap CNS nerve fibers and form myelin sheath

Schwann cells

ONLY in PNS
Surround fibers of the PNS and form myelin sheath
"Electrical wire is to electrical tape as peripheral neurons is to ____ ____"

Satellite cells

ONLY in PNS
Surround neuron cell bodies
Regulate O2, CO2, nutrient, and neurotransmitter levels around neurons

Myelin sheath

Whitish, fatty (protein lipid), segmented sheath around most long axons
Protection of axon
Electrically insulates fibers from one another
Increase the speed of the nerve impulse transmission
NOT ALL AXONS HAVE A ____ ____

Formation

_____ of myelin sheath and neurolemma
formed by Schwann cell:
-envelopes an axon in a trough
-encloses the axon with its plasma membrnae
-lays concentric layers of membrane that make up the myelin sheath

Neurolemma

Remaining nucleus and cytoplasm of a Schwann cell

Nodes of Ranvier

Gaps in the myelin sheath between adjacent Schwann cells
Areas of bare axon

Unmyelinated axons

A Schwann cell surrounds nerve fibers, but coiling does not take place
Schwann cells partially enclose 15 or more axons

Regeneration

_____ of nerve fibers
Damage to nerve tissue is serious because mature neurons are amitotic
In the PNS, if the cell body of a damaged nerve remains intact, damage can be repaired
CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration
Slow process, depends on the length of damage and there is guarantee that it will be partially/fully repaired

PNS regeneration step 1

Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration); degeneration spreads down axon

PNS regeneration step 2

Macrophages clean dead axon debris; Schwann cells are stimulated to divide and form regeneration tube

PNS regeneration step 3

Axon filaments grow through regeneration tube

PNS regeneration step 4

Axon regenerates, and new myelin sheath forms

Axons of the CNS

Both myelinated and unmyelinated fibers are present
Myelin sheaths are formed by oligodendrocytes
Can wrap up to 60 axons at once
Nodes of Ranvier are widely spaced
There is no neurolemma

White matter

Dense collections of myelinated fibers

Gray matter

Mostly cell bodies and unmyelinated fibers

Neurophysiology

Neurons are highly excitable
Action potentials, or nerve impulses, are:
-Electrical impulses carried along the length of axons
-Always the same regardless of stimulus
-The underlying functional feature of the nervous system

Principles of Electricity

Opposite charges attract each other
Energy is required to separate opposite charges across a membrane
Energy is liberated when the charges move toward one another
If opposite charges are separated, the system has potential energy

Voltage

Measure of potential energy generated by separated charge (V)

Potential difference

Voltage measured between two points

Current

The flow of electrical charge between two points (I)

Resistance

Hindrance to charge flow (R)

Insulator

Substance with high electrical resistance (myelin sheath)

Conductor

Substance with low electrical resistance

Electrical Current and the Body

Reflects the flow of ions rather than electrons
There is a potential on either side of membranes when:
-The number of ions is different across the membrane
-The membrane provides a resistance to ion flow

Leak channels

Always open

Chemically gated channels

Open with binding of a specific neurotransmitter
ex: Na+
Closed when a neurotransmitter is not bound to the extracellular receptor
-Na+ cannot enter the cell
-Open when a neurotransmitter is attached to the receptor
-Na+ enters the cell

Voltage gated channels

Open and close in response to membrane potential
ex: Na+
1. Resting, closed but capable of opening
-Activation gate is closed, inactivation gate is open
2. Open (activated)
3. Closed, not capable of opening (inactivated)
-Inactivation gate is closed, activation gate is open
Closed when the intracellular environment is negative (Na+ cannot enter the cell)
Open when the intracellular environment is less negative (Na+ can enter the cell)

Mechanically gated channels

Open and close in response to physical deformation of receptors

Gated channels

When ____ ____ are open:
Ions moved quickly across the membrane
Movement is along their electrochemical gradients
An electrical current is created
Voltage changes across the membrane

Resting state

Voltage gated channels
Channels are closed; no potassium ions are able to cross plasma membrane

Activated state

Voltage gated channels
Channels are open; potassium ions are able to flow out of the cell along its concentration gradient

Increases

Sodium ion centration in the cytoplasm of a neuron ___ when its voltage-gated sodium ion channels open

Electrochemical gradient

THE COMBINATION OF...
Ions flow along their ELECTRICAL GRADIENT when they move toward an area of opposite charge
Ions flow along their CHEMICAL GRADIENT when they move from an area of high concentration to an area of low concentration

Resting membrane potential

The potential difference (-70mV) across the membrane of a resting neuron
Thin layer of negatively charged ions exists in cytosol on inside of cell; thin layer of positively charged ions exists on outside of cell
Lose more K+ and gain less Na+ equals charge will drop
Generated by:
Differences in ionic makeup of ICF and ECF
It is generated by different concentrations of Na+, K+, Cl-, and protein anions (A-)
Differential permeability of the plasma membrane

Ionic makeup

ECF has higher concentration of Na+ than ICF
-balanced chiefly by chloride ions (Cl-)
ICF has higher concentration of K+ than ECF
-balanced by negatively charged proteins (A-)
K+ plays most important role in membrane potential

Permeability

Impermeable to A-
Slightly permeable to Na+ (through leakage channels)
25 times more permeable to K+ (more leakage channels)
Always sodium leaking into channels

Sodium potassium pump

Stabilizes the resting membrane potential by maintaining the concentration gradients for Na+ and K+
Repolarization
-restores the resting electrical conditions of the neuron
-Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is restored by the ___ ___ ___

Membrane potential changes

Concentrations of ions across membrane change
Membrane permeability to ions changes
Produces graded and action potentials
Used as signals to receive, integrate, and send information
Caused by depolarization, repolarization, and hyperpolarization

Graded potentials

Incoming signals operating over short distances
Short-lived, local changes in membrane potential
Occur on dendrites and cell bodies
Magnitude varies directly with the strength of the stimulus
Stronger stimulus -> more voltage changes; farther current flows
Sufficiently strong graded potentials can initiate action potentials

Action potentials

Long-distance signals of axons
A brief reversal of membrane potential
Action potentials are only generated by muscle cells and on axons of neurons
They do not decrease in strength over distance
They are the principal means of neural communication
The nerve impulse
ALL OR NONE

Depolarization

The inside of the membrane becomes less negative

Repolarization

The membrane returns to its resting membrane potential
-Same for graded

Hyperpolarization

The inside of the membrane becomes more negative than the resting potential

Graded depolarization

Can excite the neuron into generating and action potential
A shift in membrane potential toward 0mV
-movement of Na+ through channel
-produces local current
-Depolarizes nearby plasma membrane (graded potential)
-change in potential is proportional to stimulus

1 GD

Step ? of graded depolarization
Stimulation
Membrane exposed to chemical that opens the sodium ion channels

2 GD

Step ? of graded depolarization
Graded potential
Spread of sodium ions along inner surface produces a local current that depolarizes adjacent portions of the plasma membrane

Graded hyperpolarization

Increasing negativity of the resting potential
Result of opening a potassium channel
Opposite effect of opening a sodium channel
Positive ions move out, not into cell

Resting state

Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Each Na+ channel has two voltage-regulated gates- activation and inactivation

Activation gates

Closed in the resting state

Inactivation gates

Open in the resting state

Ap depolarization

Na+ permeability increases; membrane potential reverses
Na+ gates are opened, K+ gates are closed
Graded potential must be able to depolarize axon strongly enough to reach level called threshold (-55mV)
At threshold, depolarization becomes self-generating
Membrane potential rises to +30mv

Threshold

Not all depolarization events produce APs
For axon to fire, depolarization must reach ___
-voltage at which the AP is triggered
Membrane has been depolarized by ~15mV
Na+ permeability increases
Na+ influx exceeds K+ efflux
The positive feedback cycle begins

AP repolarization

Sodium inactivation gates close
Membrane permeability to Na+ declines to resting levels
As sodium gates close, voltage-sensitive K+ gates open
K+ exits the cell and internal negativity of the resting neuron is restored
Drops back down towards -70mV

AP hyperpolarization

Potassium gates remain open, causing an excessive efflux of K+
This efflux causes ____ of the membrane
Drops below -70mV
Flow of ions through leak channels restores resting membrane potential

Absolute refractory period

Time from beginning of depolarization into mid-repolarization
Coincides with voltage gated sodium channels being activated and inactivated
Prevents the neuron from generating another action potential
Ensures that each ___ ___ ____ is separate

Relative refractory period

Mid-repolarization through hyperpolarization
-Voltage gated Na+ channels returned to resting state; able to open again
-K+ channels are activated
Second action potential possible with larger than normal stimulus

Propagation

Allows AP to be transmitted from initial segment down entire axon length toward axon terminals
Na+ influx through voltage gates in one membrane area cause local currents that cause opening of Na+ voltage gates in adjacent membrane areas
-Leads to depolarization of that area, which in turn causes depolarization in next area.

Velocities of axons

Propagation
Velocities vary
Rate of impulse is determined by: axon diameter (the larger the faster) and presence of myelin sheath (presence increases speed)

Continuous propagation

Unmyelinated axon
Impulse continues smoothly down the axon