Chapter 11 - Nervous Tissue/System

Nerve

a bundle of axons outside the brain and spinal cord

The two main cell types in nervous tissue

neurons and glial cells

Neurons

electrically excitable cells of the nervous system

Glial cells

supportive cells with many functions

Nerve

a bundle of axons outside the brain and spinal cord

Cranial nerves

originate from the brain; 12 pairs

Spinal nerves

originate from spinal cord; 31 pairs

Ganglion

collection of neuron cell bodies outside the brain and spinal cord

Plexus

extensive network of axons, and sometimes neuron cell bodies, located outside CNS

Functions of glial cells

• Maintaining homeostasis - Regulate and coordinate activities to maintain balance

• Receiving sensory input - Monitor internal and external stimuli

• Integrating information - Brain and spinal cord process sensory input and initiate responses

• Controlling muscles and glands

• Establishing and maintaining mental activity - Consciousness, thinking, memory,and emotion

Central nervous system (CNS)

brain and spinal cord

Peripheral nervous system (PNS)

sensory receptors and nerves

Afferent (Sensory)

transmits action potentials from receptors TOWARD the CNS

Efferent (Motor)

transmits action potentials from CNS to effectors (muscles, glands). Action potentials AWAY from CNS

Sympathetic

Fight or flight. Prepares body for physical activity

Parasympathetic

Rest and digest. Regulates resting functions such as digesting food or emptying of the urinary bladder.

Somatic nervous system

from CNS to skeletal muscles. Voluntary; single neuron system

Autonomic nervous system

from CNS to smooth muscle, cardiac muscle and certain glands. Subconscious or involuntary control.

• Two neuron system: first from CNS to ganglion; second from ganglion to effector.

Enteric

plexuses within the wall of the digestive tract

Three major parts of the neuron

Neuron cell body (soma)

Dendrites

Axons

Neuron cell body (soma)

Typical cell functions such as protein synthesis and housekeeping; contains Nissl bodies (rough ER)

Dendrites

cell extensions of the cell body that receive information from other neurons or sensory receptors.

Short and often highly branched with extensions called dendritic spines.

Conducts currents toward the cell body.

Axons

arises from axon hillock of the cell body then becomes the initial segment.

Part of the trigger zone where action potentials are generated; contains axoplasm and axolemma.

Ends at the presynaptic terminal containing synaptic vesicles full of neurotransmitters

Interneurons

within CNS from one neuron to another.

Multipolar

most neurons in CNS; motor neurons.

Bipolar

sensory in retina of the eye and nasal cavity.

Pseudo- unipolar

single process that divides into two

branches. Part that extends to the periphery has dendrite-like sensory receptors.

Anaxonic

no axons, only dendrites; found in brain and retina where they only communicate using graded potentials.

Astrocytes

Star-shaped with processes that form feet

that cover the surfaces of neurons, blood

vessels, and the pia mater. Lots of microfilaments for support

Reactive astrocytosis

CNS injury sites are walled-off to limit spread of

inflammation

blood-brain barrier

regulate what substances reach the CNS from the blood

Ependymal Cells

Line brain ventricles and spinal cord central canal. Specialized versions of ependymal form choroid plexuses

Choroid plexus

within certain regions of ventricles. Secrete cerebrospinal fluid. Cilia help move fluid thru the cavities of the brain. Have long processes on basal surface that extend within the brain tissue, may have astrocyte-like functions

Microglia

specialized CNS macrophages. Respond to inflammation, phagocytize necrotic tissue, microorganisms, and foreign substances that invade the CNS

Oligodendrocytes

form insulating myelin sheaths by wrapping cytoplasmic extensions around axons. A single oligodendrocyte can form myelin sheaths around portions of several axons.

Schwann cells

wrap around portion of only one axon to form myelin sheath. Outer layer of the wrap is the neurilemma that contains most of the cytoplasm, nucleus and organelles. Cell membrane primarily phospholipid.

Satellite cells

surround neuron cell bodies in sensory and autonomic ganglia; provide support, nutrients, and protection from heavy-metal poisons.

Myelinated axons

Myelin protects and insulates axons from one another, speeds transmission, functions in repair of axons.

Made of white matter. Degeneration of myelin sheaths occurs in multiple sclerosis and some cases of diabetes mellitus

nodes of Ranvier

gaps within the myelin sheaths, since they aren’t continuous

Unmyelinated axons

rest in invaginations of Schwann cells or oligodendrocytes. Not wrapped around the axon; gray matter.

Nervous tissue’s response to injury

A cut nerve either heals or becomes permanently damaged.

Within 3 to 5 days, axon distal from the cut breaks into segments due to a lack of nutrients; distal portion dies.

• Schwann cells of the degenerating axon begin to degenerate.

• Macrophages invade area to phagocytize the myelin.

• Schwann cells enlarge, divide, and form a column where the axon had been.

• If the ends of a regenerating axon come into contact with the Schwann cell column, the axon grows more rapidly and is more likely to re-establish contact with the nerve target.

• Regeneration in the CNS is limited because oligodendrocytes do not produce the columns of cells to guide growing axons.

Gray matter

unmyelinated axons, cell bodies, and dendrites. Integrative functions.

(The cortex of the brain is gray matter)

White matter

myelinated axons. Propagate action potentials

Nuclei

clusters of neuron cell bodies

Nerve tracts

bundles of myelinated axons

Ganglia

clusters of cell bodies

Nerves

bundles of axons with their connective tissue sheaths

Action potential

electrical signals. Action potentials allow for perception of the environment, performance of complex mental activities, and responses to stimuli via the transfer of information from one part of the body to another

Membrane potential

the result from ionic concentration differences across plasma membrane and permeability of membrane.

What are ion concentrations across a membrane a result of?

Na+/K+ pump and membrane permeability

(Sodium ion/Potassium)

Sodium-potassium pump

Neurons expend ATP to maintain the uneven distribution of ions across the membrane.

The sodium-potassium pump pumps 3 Na+ out of the cell while also pumping 2 K+ into the cell for each ATP molecule used

Leak ion channels (non-gated ion channels)

Always open and responsible for permeability when membrane is at rest.

There are more leak ion channels for K+ and Cl-

Gated ion channels

Gated ion channels open and close because of some sort of stimulus. When they open, they change the permeability of the cell membrane

Ligand-gated

open or close in response to ligand such as neurotransmitter or hormone binding to receptor protein. Receptor proteins are usually glycoproteins

Voltage-gated

open or close in response to specific, small voltage changes across the cell membrane.

• At rest, membrane is negative on the inside relative to the outside.

• When cell is stimulated, that relative charge changes and voltage-gated ion channels either open or close. Most common voltage-gated channels are Na+ and K +

• In cardiac and smooth muscle, 2

Ca (2+) (calcium ion) are important.

Types of gated ion channels

Ligand-gated

Voltage-gated

Touch receptors

Temperature receptors

Touch receptors

respond to mechanical stimulation of the skin

Temperature receptors

respond to temperature changes in the skin

There are opposite charges across the membrane, so the membrane is ____

polarized

Potential difference

unequal distribution of charge exists between the immediate inside and immediate outside of the plasma membrane: −70 to −90 mV.

The resting membrane potential exists in an unstimulated (resting) cell, due to:

• Permeability characteristics of membrane.

• Differences in ion concentrations on each side of membrane.

Why is the membrane more permeable to K+?

due to many leak channels, K+ (potassium also diffuses from inside to outside the cell)

Positive charges also accumulate outside the membrane.

Negatively charged proteins can’t diffuse with K+

in a resting cell, there is a higher concentration of ___ inside the plasma membrane

K+

in a resting cell, there is a higher concentration of ___ outside the plasma membrane

Na+

Where are negatively charged proteins isolated?

the cytoplasm, since the negatively charged proteins can’t move easily across the plasma membrane

Sodium-potassium pump

maintains resting levels of ions across the plasma membrane

Depolarization

inside of cell becomes more positive; for example, from −70mV to −55mV.

Sodium ions are the most common way neurons become depolarized

Calcium ion entry also causes depolarization (cardiac muscle, for example)

Potassium ions diffuse out of the cell

Hyperpolarization

inside of cell becomes more negative; for example, from −70mV to −90mV

Potassium ions most common way neurons become hyperpolarized

Chloride ions also diffuse into the cell

Graded potential

Can summate or add onto each other to reach threshold

Four phases of action potentials

1. Depolarization

2. Repolarization

3. Afterpotential

4. Return to resting potential

All-or-none principle

No matter how strong the stimulus, as long as it is greater than threshold, then an action potential will occur.

Repolarization

the change in membrane potential that returns it to a negative value

Afterpotential

A period of hyperpolarization occurs because the voltage-gated K+ channels remain open for a slightly longer time than it takes to bring the membrane potential back to its original resting level.

Refractory period

Sensitivity of area to further stimulation decreases for a time. Two types of refractory periods: absolute and relative

Absolute refractory period

Complete insensitivity exists to another stimulus.

• From beginning of action potential until near end of repolarization.

• No matter how large the stimulus, a second action potential cannot be produced

Relative refractory period

A stronger-than-threshold stimulus can initiate another action potential.

Continuous conduction

Action potential in one site causes action potential at the next location. Cannot go backwards because initial action potential site is depolarized yielding one-way conduction of impulse

Saltatory Conduction

the propagation of action potentials along myelinated axons from one node of Ranvier to the next. Skips/hops over the myelinated regions of membrane

Synapse

junction between two cells. The site where action potentials in one cell cause action potentials in another cell.

There are two types of cells in synapse: presynaptic and postsynaptic

Presynaptic

cell that transmits signal toward the synapse

Postsynaptic

target cell receiving the signal.

Two types of synapses: electrical and chemical

Electrical synapses

Cells connected by gap junctions that allow graded current to flow between adjacent cells. Found in cardiac and smooth muscle. Contractile activity amongst a group of cells.

Connexons

protein tubes in cell membrane

Chemical synapses

Composed of the presynaptic terminal, synaptic cleft, and postsynaptic membrane.

Presynaptic terminal

end of the presynaptic cell axon

Synaptic cleft

space between the presynaptic terminal and

the postsynaptic membrane

Postsynaptic membrane

membrane of other neurons, muscle cells, or gland cells

synaptotagmin

a Ca2+ (calcium ion) sensor within synaptic vesicle membranes

Are neurotransmitters excitatory or inhibitory?

Neurotransmitters are excitatory in some cells and

inhibitory in others

How many receptors can neurotransmitters fit in?

Neurotransmitters only fit in one receptor

Neuron communication steps

1. Graded potential (if stimulation is adequate, graded potential reaches threshold)

2. Depolarization at axon hillock

3. Repolarization

4. Action potential propagation

5. Synaptic communication

How many types of neurotransmitters are there?

at least 100 different types

Criteria for being a neurotransmitter

Must be synthesized by a neuron and stored within

synaptic vesicles in presynaptic terminals.

• An action potential must stimulate its exocytosis in the synaptic cleft.

• It must bind to a specific receptor on the postsynaptic membrane.

• Must evoke a response in the postsynaptic cell.

What is the classification of neurotransmitters based on?

Chemical structure

Effect on the postsynaptic membrane

The mechanism of action at their target

Ionotropic effect

binding to ion channels

Metabotropic effect

binding to G-protein-linked receptors