Chapter 11: Functional Organization of Nervous Tissue

5 functions of the nervous system (Smart Ions Hear Melodious Covers)

A. Sensory input - Monitor internal/external stimuli
B. Integration - Brain and spinal cord process sensory input and initiate responses.
C. Homeostasis - Regulate and coordinate physiology.
D. Mental activity - Consciousness, thinking, memory, emotion.
E. Control - Muscles and glands.

Components of the nervous system (4 things)

1. Brain
2. Spinal cord
3. Nerves
4. Sensory receptors

Central NS (CNS) components

brain and spinal cord

Peripheral NS (PNS) components

sensory receptors and nerves

Sensory receptors

endings of neurons/specialized cells that detect: temperature, pain, touch, pressure, light, sound, and odors

Where are sensory receptors found?

skin, muscles, organs, etc.

Nerve

a bundle of axons + sheaths that connects CNS to sensory receptors, muscles, and glands

Cranial nerves

originate from the brain; 12 pairs

Spinal nerves

originate from spinal cord; 31 pairs

Ganglion

neuron cell bodies outside CNS

Plexus

Axons (and sometimes cell bodies) outside CNS

How many pairs of cranial and spinal nerves are in the nervous system?
A. 13,12
B. 12, 31
C. 31,13
D. 12, 21

B. 12, 31

Nerves connect to all the following structures except?
A. Nerves
B. Muscles
C. Glands
D. Cartilage

D. Cartilage

2 divisions of the PNS

A. Sensory (afferent): transmits action potentials from receptors to CNS.
B. Motor (efferent): transmits action potentials from CNS to effectors (muscles, glands)

2 sections of the motor division of the PNS

1. Somatic NS
2. Autonomic NS

Somatic NS (SNS) characteristics (4 things)

- carries action potentials from CNS to skeletal muscles
- is voluntarily controlled
- single neuron system
- cell bodies located within the CNS

Autonomic NS (ANS) characteristics (3 things)

- carries action potentials from CNS to smooth and cardiac muscle & glands
- is involuntarily controlled
- two neuron system (CNS to ganglion, and ganglion to effector)
- 3 divisions: sympathetic, parasympathetic, enteric

Sympathetic division of ANS

most active during physical activity (fight or flight)

Parasympathetic division of ANS

regulates resting functions such as digesting food & emptying the bladder

Enteric division of ANS

plexuses in wall of digestive tract (reflex)

Order of the nervous system

Receptor -> Sensory NS -> CNS -> Motor NS -> Effector

2 types of cells in the nervous system

1. Neurons (nerve cells)
2. Neuroglia (glial cells)

Neurons

receive stimuli and transmit action potentials

Organization of neurons

- Dendrites: input
- Cell body or soma
- Axons: output

Parts of a neuron (3 things)

A. Cell body (includes the nucleus & Nissl bodies)
B. Dendrites (include dendritic spines)
C. Axons (include axon hillock, initial segment, trigger zone, axoplasm, axolemma, presynaptic terminals/terminal boutons, & synaptic vesicles)

Nissl bodies

chromatophilic substances like the rough ER that are the primary site of protein synthesis

Dendritic spine

little protuberances where axons synapse with the dendrite

Axon hillock

single axon arises

Initial segment

beginning of axon

Trigger zone

site where action potentials are generated (made up of the axon hillock and initial segment)

Axoplasm

the cytoplasm of the axon

Axolemma

the axon's plasma membrane

Neuroglia

support and protect neurons

2 axonic transport mechanisms

A. Away from cell body (anterograde): can move proteins, organelles, and vesicles (Supplies for growth, repair, renewal) away from the cell body to the presynaptic terminal
B. Into cell body (retrograde): Damaged organelles, recycled plasma membrane, and substances taken in by endocytosis can be transported up the axon to the cell body (Rabies and herpes virus into CNS)

Classification types of neurons

functional and structural

Functional classification of neurons

1. Sensory (afferent): action potentials toward CNS
2. Motor (efferent): action potentials away from CNS
3. Interneurons (association neurons): action potentials from neuron to neuron in CNS

Structural classification of neurons

1. Multipolar: many dendrites with a single axon, usually motor neurons (Ex: most neurons in CNS)
2. Bipolar: one dendrite one axon, usually sensory neurons (Ex: retina of the eye and nose)
3. Pseudo-unipolar: a single process that divides into two branches, usually sensory neurons

Which statement is totally true?
A. Sensory neurons are afferent neurons - conduct toward the CNS
B. Motor neurons are afferent neurons - conduct away from the CNS
C. Sensory neurons are efferent neurons - conduct toward the CNS
D. Motor neurons are efferent neurons - conduct toward the CNS

A. Sensory neurons are afferent neurons - conduct toward the CNS

Neuroglia of CNS

A. Astrocytes
B. Ependymal cells
C. Microglia
D. Oligodendrocytes

Astrocytes characteristics (3 things)

1. Processes form feet that cover surface of neurons & blood vessels
2. Promote formation of the blood-brain barrier
3. Have an extensive cytoskeleton of microfilaments for support of blood vessels and nerves

5 functions of the astrocyte blood-brain barrier

1. Determines what substances can pass from the blood into the nervous tissue of the brain and spinal cord
2. Protects neurons from toxic substances
3. Allows exchange of nutrients and waste products between neurons and blood
4. Prevents fluctuations in blood composition from affecting the brain
5. Regulates extracellular brain fluid composition

Ependymal cells characteristics (3 things)

1. Line brain ventricles & spinal cord central canal
2. With blood vessels & support tissues they form Choroid plexuses in ventricles that secrete cerebrospinal fluid (CSF)
3. Cilia help move cerebral fluid through cavities of brain

Microglia

become mobile & phagocytic in response to inflammation (they seek out and phagocytize necrotic tissue, microorganisms, and foreign substances that invade the CNS)

What evidence of damage in an autopsy can microglia show?

In the event of infection, trauma, or stroke numerous microglia migrate to the damaged area

Oligodendrocytes characteristics (2 things)

1. Form myelin sheaths if they surround an axon many times
2. Single oligodendrocytes can form myelin sheaths around portions of several axons.

Neuroglia of PNS

A. Schwann cells/neurolemmocytes
B. Satellite cells

Schwann cells/neurolemmocytes

wrap many times around a portion of only one axon - forms myelin sheath

Satellite cells

1. Surround neuron cell bodies in sensory ganglia, providing support and nutrients to the neuron cell bodies (similar to astrocytes in the CNS)
2. Protect neurons from heavy metal poisons such as lead and mercury by absorbing them and reducing their access to the neuron cell bodies

Remember:

Oligodendrocytes in the CNS also wrap around axons to form myelinated or unmyelinated axons.

Myelinated vs unmyelinated axons

4 characteristics of myelinated axons

- protect & insulate axons from one another
- speed up transmission of action potentials
- make up "white" matter (lipids)
- have Nodes of Ranvier

Myelin sheaths up to 1 year old form ________ (rapidly or slowly?)

rapidly

Degeneration of myelin sheaths leads to:

multiple sclerosis

2 characteristics of unmyelinated axons

- Schwann Cell is not wrapped around the axon
- make up "gray" matter

White vs Gray Matter

In both the CNS & PNS nervous tissue is organized so that axons form bundles (white) while neuron cell bodies and their dendrites (gray) are grouped together.

T or F: Both the CNS & PNS contain areas of gray matter and white matter.

True

Gray matter

groups of neuron cell bodies and their dendrites with very little myelin

White matter

bundles of parallel myelinated axons

In the CNS, gray matter on the surface of the brain is called _________ & deeper clusters in the brain are called _________.

the cortex; nuclei

In the PNS, a cluster of neuron cell bodies is called _________.

a ganglion

White matter of the CNS forms nerve tracts that

propagate action potentials from one area of the CNS to another

In the PNS bundles of axons and their connective tissue sheaths are called _________.

a nerve

Choose the pair of terms that does NOT go together
A. Autonomic, voluntary
B. Somatic, voluntary
C. Autonomic, involuntary
D. Somatic, skeletal muscles

A. Autonomic, voluntary

Electrical properties result from: (2 things)

1. Ion concentration differences across plasma membrane (inside and outside)
2. Permeability of membrane to ions

Ion concentration of cell at resting potential

High Na+ & Cl- outside cell, high K+ & negative proteins inside cell (like a potato chip)

What does the sodium-potassium pump do?

Maintains the difference of Na+ and K+ by breaking down 1 ATP (into ADP) to move 3 Na+ out and 2 K+ in

Explanation of membrane permeability

A. Proteins (which have a negative charge) are made inside cell and can't cross the membrane
B. Cl- is repelled by proteins so they exit through non-gated Cl- channels.

Non-gated (Leakage) channels

channels that are always open, so they are responsible for membrane permeability to ions when the p.m. is unstimulated or at rest

There are many more leakage channels for ______ than for ______.

K+ & Cl-; Na+

At rest, more ______ are moving than ______.

K+ & Cl-; Na+

K+ is higher in concentration on the _______ than the _______, so it moves ________.

inside; outside, out

Because of the ion concentration differences across the membrane, K+ diffuses _______, and Na+ diffuses _______.

out of the cell, down its concentration gradient; into the cell, down its concentration gradient

Leak channels (non-gated):
A. Open in response to small voltage changes.
B. Open when a chemical signal binds to its receptor.
C. Allow substances to move into the cell but not out.
D. Are responsible for Resting Membrane Potential (K+ leak channels) in the resting plasma membrane.

D. Are responsible for Resting Membrane Potential (K+ leak channels) in the resting plasma membrane.

4 types of gated ion channels

1. Ligand-gated: change the permeability of membrane by opening/closing in response to a ligand (like Ach) binding to a receptor protein
2. Voltage-gated: change the permeability of membrane by opening/closing in response to small voltage changes
3. Touch receptors: change the permeability of membrane by opening/closing in response to mechanical stimulation of skin
4. Temperature receptors: change the permeability of membrane by opening/closing in response to temperature changes in skin

What is the most common voltage-gated ion channel?

Na+ & K+, however Ca2+ voltage gated are in cardiac/smooth muscle

Resting membrane potential (RMP)

-70 mV in nerves & -85 mV in skeletal muscle (but the concentration of K+ is higher on the inside & Na+ is higher on the outside for both)

Potential difference

electrical charge difference across the plasma membrane (the greater the charge difference, the greater the potential difference)

Which one (nerve or skeletal muscle) has the greater potential difference?

nerve (-70 mV to -90 mV)

The resting membrane potential is established when

the movement of K+ out of the cell is equal to their movement into the cell

Characteristics responsible for the resting membrane potential

(not 8)

How K+ concentration gradient depolarizes the RMP

If extracellular concentration of K+ increases, not as much K+ will want to go out, down it's concentration gradient. So, a less negative charge is needed inside the cell.

How K+ concentration gradient hyperpolarizes the RMP

If extracellular ion concentration decreases: steeper gradient (difference) between inside and outside.

Which of these terms are correctly matched?
A. depolarization: membrane potential becomes more negative.
B. hyperpolarization: membrane potential becomes more negative.
C. hyperpolarization: membrane potential becomes more positive.

B. hyperpolarization: membrane potential becomes more negative.

Changes in Resting Membrane Potential: Na+

1. Changes to the concentration of Na+ inside or outside the cell has very little effect (gates closed).
2. If gates open (like when ACh attaches to ligand receptors), Na+ diffuses into the cell, which results in depolarization of the membrane.

Changes in Resting Membrane Potential: Ca2+

1. Voltage-gated Na+ channels are sensitive to changes in extracellular Ca2+ concentrations, so if the extracellular Ca2+ concentration decreases, Na+ gates open, which results in depolarization.
2. If the extracellular concentration of Ca2+ increases, Na+ gates close and membrane repolarizes. (hyperpolarization).

Hypocalcemia

Uncontrolled contraction of skeletal muscles - tetany because of increased membrane permeability to Na+

Graded potential/Local Potential

A small change in the RMP confined to a small area of the plasma membrane

How are graded potentials and action potentials different?

An action potential spreads over the entire surface of the cell

Graded potentials result from (5 things)

1. Ligands binding
2. Changes in voltage
3. Mechanical stimulation
4. Temperature changes
5. Spontaneous change in permeability

Summation

when graded potentials add onto each other

Graded potentials can spread (conduct) over part of plasma membrane, but:

they rapidly decrease in magnitude

Characteristics of graded potentials (5 things)

Graded (or Local) potentials
A. Always cause an action potential.
B. Never summate.
C. Can travel long distances.
D. Are confined to a small region of the plasma membrane.

D. Are confined to a small region of the plasma membrane.

Action potential graph

6 characteristics of action potentials

1. Action potentials are produced when a graded potential reaches threshold
2. Actions potentials are all-or-none
3. Depolarization is a result of increased membrane permeability to Na+ and movement of Na+ into the cell. Activation gates of the voltage-gated Na+ channels open
4. Repolarization is a result of decreased membrane permeability to Na+ and increased membrane permeability to K+, which stops Na+ movement into the cell and increases K+ movement out of the cell. The inactivation gates of the voltage-gated Na+ channels close, and the voltage-gated K+ channels open.
5. Action potentials are propagated and, for a given axon or muscle fiber, the magnitude of the action potential is constant.
6. Stimulus strength determines the frequency of action potentials.

Absolute refractory period

from the beginning of the action potential to the near end of repolarization - no matter how large a stimulus, there can be no second action potential

Relative refractory period

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

Action potential frequency

Number of action potentials produced per unit of time to a stimulus

Supramaximal stimulus

any stimulus stronger than a maximal stimulus - these stimuli cannot produce a greater frequency of action potentials than a maximal stimulus

Action potential propagation

an action potential at one site causes an action potential at the next location by stimulating voltage gated Na+ channels in adjacent regions of the membrane