CH12: Neural Tissue

Nervous system

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