Anatomy slide set 1

Chapter 11: Functional Organization of Nervous Tissue

11.1 Learning Outcomes

Explain the functions of the nervous system.
List the divisions of the nervous system and describe the characteristics of each.
Differentiate between the somatic and the autonomic nervous systems.
Contrast the general functions of the CNS and the PNS.
Describe the structure of neurons.
Describe the functions of the components of a neuron.
Classify neurons based on structure.
Classify neurons based on function.
Describe the location, structure, and functions of CNS glial cells.
Describe the location, structure, and functions of PNS glial cells.
Discuss the function of the myelin sheath.
Describe the formation of myelin sheaths in the CNS and PNS.
Distinguish between gray matter and white matter.
Describe the components of gray matter in the CNS and PNS.
Describe the components of white matter in the CNS and PNS.
Define resting membrane potential.
Explain how resting membrane potential is created and maintained.
Explain the processes that can change the resting membrane potential.
List the three phases of neuron communication.
Describe the characteristics of a graded potential.
Describe the creation of an action potential.
Explain how an action potential is propagated.
Discuss the all-or-none principle as it applies to action potentials.
Explain the characteristics and purpose of the refractory period.
Explain the factors that determine action potential frequency.
Explain the five levels of stimulation.
Describe the effect of myelination on the speed of action potential propagation.
Describe other factors that affect the speed of action potential conduction.
Describe the general structure and function of a synapse.
Distinguish between electrical and chemical synapses as to mode of operation and types of tissues where they are found.
Describe the release of a neurotransmitter in a chemical synapse.
Describe the removal of a neurotransmitter in a chemical synapse.
Explain the effects of neurotransmitter binding to receptors in a chemical synapse.
Discuss the effects of neuromodulators in a chemical synapse.
Contrast excitatory and inhibitory postsynaptic potentials.
Explain the roles of presynaptic inhibition.
Define facilitation.
Describe the process of spatial summation.
Describe the process of temporal summation.
Contrast convergent and divergent neuron pathways.
Describe a reverberating circuit.
Explain a parallel after-discharge circuit.

Nervous System Overview

The nervous system is a communication system comprised of the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves, ganglia, and receptors).
The central nervous system is the master control system, responsible for all bodily functions.

Introduction to Nervous Tissue

Two main cell types in nervous tissue: neurons and glial cells.
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.
Glial cells: supportive cells with many functions.

11.1 Functions of the Nervous System

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.
Generating responses: Controlling muscles and glands.
Establishing and maintaining mental activity: Consciousness, thinking, memory, emotion.

Divisions of the Nervous System

The nervous system is divided into two principal parts:
- Central nervous system (CNS):
  - Brain and spinal cord of dorsal body cavity.
  - Integration and control center.
  - Interprets sensory input and dictates motor output.
- Peripheral nervous system (PNS):
  - The portion of the nervous system outside of the CNS.
  - Consists mainly of nerves that extend from the brain and spinal cord.
  - Spinal nerves to and from the spinal cord.
  - Cranial nerves to and from the brain.

11.2 Divisions of the Nervous System

Two major divisions:
- Central nervous system (CNS): brain and spinal cord.
- Peripheral nervous system (PNS): sensory receptors and nerves.
Branches of the Nervous System:
- Sensory (Afferent): Towards CNS
- Motor (Efferent): Away from CNS
Branches of PNS:
- Somatic:
  - Voluntary
  - Effectors: Skeletal muscle
  - Response: Stimulates contraction
- Autonomic:
  - Involuntary
  - Sympathetic:
    - Effectors: Cardiac and smooth muscle; glands
    - Response: Readies body for physical activity
  - Parasympathetic:
    - Effectors: Cardiac and smooth muscle; glands
    - Response: Regulates resting functions

Information Flow in the Nervous System

Sensory division of the PNS:
- Sensory input: Light, sound, taste, smell, touch, temperature, pain pressure, etc.
- Sensory receptors
CNS
Motor division of the PNS
- Somatic nervous system
  - Effectors: Skeletal muscle
  - Motor output: Movement
- Autonomic nervous system
  - Sympathetic division
  - Parasympathetic division
  - Effectors: cardiac and smooth muscle; glands
  - Motor output: changes in metabolism, heart rate, breathing rate, etc.

Divisions of Peripheral Nervous System

Sensory (afferent): transmits action potentials from receptors toward the CNS.
- Sensory receptors: Can be neuron endings or specialized cells that detect external and internal stimuli; send input along nerves to brain or spinal cord.
Motor (efferent): transmits action potentials from CNS to effectors (muscles, glands).

Motor Division of Peripheral Nervous System

Somatic nervous system: from CNS to skeletal muscles.
- Voluntary; single neuron system.
- Involuntary; reflexes
Autonomic nervous system (ANS): 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.
- Divisions of ANS:
  - Sympathetic: Prepares body for physical activity.
  - Parasympathetic: Regulates resting functions such as digesting food or emptying of the urinary bladder.
  - Enteric: plexuses within the wall of the digestive tract.

11.3 Cells of the Nervous System

Neurons are electrically excitable cells of the nervous system.
Three major parts: Cell body, Dendrites, Axons
- Cell body:
  - Large nucleus and nucleolus.
  - Perikaryon - cytoplasm.
  - Rough ER / Nissl bodies - synthesizes proteins.
  - Mitochondria - produces energy.
- Dendrites: branched processes of cell body that receive information.
  - Dendritic spines: short branched extensions that receives information from other neurons and conduct currents toward the cell body.

11.3 Cells of the Nervous System (cont.)

Three major parts (cont):
- Axons: Conducting region of neuron
  - Generates nerve impulses and transmits along neuron cell membrane to axon terminal - action potential
  - Terminal: region that secretes neurotransmitters, which are released into the extracellular space
  - Axolemma: plasma membrane - covers axoplasm
  - Collaterals: branches of axon
  - Axon hillock: region of initial attachment to cell body

Types of Neurons

Functional classification – direction of action potential:
- Sensory or afferent: action potentials toward CNS.
- Motor or efferent: action potentials away from CNS.
- Interneurons: within CNS from one neuron to another.

Types of Neurons: Structural Classification

Structural classification – based on the number of dendrites.
- Multipolar: most neurons in CNS; motor neurons.
- Bipolar: sensory in the 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 the brain and retina where they only communicate using graded potentials.

Glial Cells of the CNS: Astrocytes

Star-shaped with processes that form feet that cover the surfaces of neurons, blood vessels, and the pia mater.
Regulate extracellular brain fluid composition.
Maintains blood-brain barrier
Release chemicals to promote the development of synapses and help regulate synaptic activity.
Repairs nervous tissue

Glial Cells of the CNS: Ependymal Cells

Line brain ventricles and spinal cord central canal.
Choroid plexus -specialized ependymal cell within certain regions of ventricles.
Secrete cerebrospinal fluid. Cilia help move fluid through the cavities of the brain.
Long processes on basal surface that extend into brain tissue

Glial Cells of the CNS: Microglia

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

Glial Cells of the CNS: Oligodendrocytes

Oligodendrocytes:
- Small bodies - few processes
- Form insulating myelin sheaths
- Increase speed of action potentials

Glial Cells of the PNS

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 and Unmyelinated Axons

Myelinated axons:
- Myelin protects and insulates axons from one another, speeds transmission, and functions in repair of axons.
- Not continuous; gaps called nodes of Ranvier.
- White matter
- Degeneration of myelin sheaths occurs in multiple sclerosis and some cases of diabetes mellitus.
Unmyelinated axons: rest in invaginations of Schwann cells or oligodendrocytes. Not wrapped around the axon; gray matter.

Nervous Tissue Response to Injury

A cut nerve either heals or is permanently damaged.
Degenerative changes:
- Within 3 to 5 days, axon distal from cut breaks into segments due to lack of nutrients; the distal portion dies.
- Schwann cells of the degenerating axon begin to degenerate.
- Macrophages invade the 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 reestablish contact with the nerve target.
- Regeneration in the CNS is limited because oligodendrocytes do not produce the columns of cells to guide growing axons.

11.4 Organization of Nervous Tissue

Gray Matter:
- Contains most of the brains neuronal cell bodies.
- 40% of the brain.
- Interprets sensory information from various parts of the body.
- Fully develops once a person reaches their 20's
White Matter:
- Made up of bundles which connect various gray matter areas
- 60% of the brain
- Conducts, processes, and sends information to various parts of the body.
- Develops throughout the 20's and peaks in middle age

11.4 Organization of Nervous Tissue Cont.

Gray matter: unmyelinated axons, cell bodies, and dendrites; involved in integrative functions. The cortex of the brain is gray matter.
White matter: myelinated axons; propagate action potentials.
CNS: clusters of neuron cell bodies are nuclei; bundles of myelinated axons are nerve tracts.
PNS: clusters of cell bodies are ganglia; bundles of axons with their connective tissue sheaths are nerves.

11.5 Electrical Signals

Cells produce electrical signals called action potentials.
Allows perception of the environment, performance of complex mental activities, and responses to stimuli through the transfer of information from one part of the body to another.
Membrane potential is the result of ionic concentration differences across the plasma membrane and the permeability of the membrane.

Membrane Potential

Like all cells, neurons have a resting membrane potential.
Unlike most other cells, neurons can rapidly change the resting membrane potential.
Neurons are highly excitable.
Opposite charges are attracted to each other.
Energy is required to keep opposite charges separated across a membrane
Energy is liberated when the charges move toward one another
When opposite charges are separated, the system has potential energy.
Electrochemical gradient: electrical and chemical gradients combined
Ion flow creates an electrical current, and voltage changes across the membrane.

Ionic Concentration Differences Across the Plasma Membrane

Ion concentrations across a membrane are a result of two processes: the pump and membrane permeability.
High concentrations of $Na^+$ and $Cl^-$ ions outside of cell; high concentrations of $K^+$ and proteins on inside of the cell.
Steep concentration gradient of $Na^+$ but in opposite directions.

Permeability Characteristics of the Plasma Membrane

Proteins: synthesized inside cell. Large and negatively charged. Don't dissolve in phospholipids of membrane.
$Cl^-$ repelled by proteins and they exit thru open, non-gated $Cl^-$ channels.
Gated ion channels open and close in response to a stimulus. When they open, they change the permeability of the cell membrane.

Sodium-Potassium Pump

Neurons expend ATP to maintain the uneven distribution of ions across the membrane.
The pump actively pumps $Na^+$ out of the cell while simultaneously pumping $K^+$ into the cell for each ATP molecule used.

Leak Ion Channels

Also called non-gated ion channels; Always open and responsible for permeability when membrane is at rest.
Specific for one type of ion although not absolute.
Many more leak ion channels for $K^+$ than for $Na^+$ .
So, at rest, more $K^+$ are moving than $Na^+$ .
If leakage channels alone were responsible for resting membrane potential, in time $Na^+$ and $K^+$ ion concentrations would eventually equalize, but they are maintained by the pump.
For each ATP that is consumed, three $Na^+$ moved out, two $K^+$ moved in.
Outside of plasma membrane slightly positive.

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. For example, acetylcholine binds to acetylcholine receptor on a channel, Channel opens, $Na^+$ enters the cell.

Gated Ion Channels 2

Voltage-gated: open or close in response to specific, small voltage changes across the cell membrane.
- At rest, the 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, $Ca^{2+}$ are important.

Gated Ion Channels 3

Other gated ion channels
- Touch receptors: respond to mechanical stimulation of the skin.
- Temperature receptors: respond to temperature changes in the skin.

Establishing the Resting Membrane Potential

Intracellular fluid and extracellular fluid are electrically neutral, but there is a charge across the membrane due to the uneven distribution of anions/cations.
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 the membrane
- Differences in ion concentrations on each side of the membrane

Establishing the Resting Membrane Potential 2

Membrane more permeable to $K^+$ due to many leak channels, and diffuses from inside to outside the cell.
- Positive charges accumulate outside the membrane.
- Negatively charged proteins cannot diffuse and with $K^+$ diffuses out of cell, the inside of the membrane becomes more negative.
- $Na^+$ do not have a great affect on resting potential since there are very few leakage channels for these ions.
If leakage channels alone were responsible for resting membrane potential, in time ion concentrations would eventually equalize. But they are maintained by the pump.
For each ATP that is consumed, three moved out, two moved in.
Outside of plasma membrane slightly positive.

Establishing the Resting Membrane Potential 3

Distribution of ions and proteins across the plasma membrane. In a resting cell, there is a higher concentration of $K^+$ inside the plasma membrane and a higher concentration of $Na^+$ outside the plasma membrane. Negatively charged proteins are isolated to the cytoplasm because they cannot move easily across the plasma membrane.

Establishing the Resting Membrane Potential 4

Movement of ions through leak channels. The plasma membrane is more permeable to $K^+$ because of a higher proportion of $K^+$ leak ion channels compared with leak channels for other ions. Positively charged $K^+$ can therefore diffuse down its concentration gradient from inside to outside the cell. As $K^+$ diffuses out of the cell, the loss of positive charges makes the inside of the plasma membrane more negative. Because opposite charges attract, $K^+$ is attracted back toward the cell. The accumulation of $K^+$ outside the plasma membrane makes the outside of the plasma membrane positive relative to the inside. The resting membrane potential is an equilibrium. This equilibrium is established when the tendency for $K^+$ to diffuse out of the cell is equal to the tendency for $K^+$ to move into the cell. To reiterate, the concentration gradient for $K^+$ is toward the outside of the cell, but because of the negative charge inside the cell, $K^+$ tends to be pulled back toward the interior of the cell.

Establishing the Resting Membrane Potential 5

Sodium-potassium pump maintains resting levels of ions across the plasma membrane. The sodium-potassium pump maintains the uneven distribution of across the plasma membrane. The pump is also responsible for a small portion of the resting membrane potential, usually less than 15 mV, because it transports approximately three out of the cell and two into the cell for each ATP molecule used. The outside of the plasma membrane becomes more positively charged than the inside because more positively charged ions are pumped out of the cell than are pumped into it.

Characteristics Responsible for the Resting Membrane Potential

The concentration of K+ is higher inside the cell than outside, and the concentration of Na+ is higher outside the cell than inside.
Due to the K+ leak channels, the plasma membrane is 50–100 times more permeable to K+ than to other positively charged ions, such as Na+.
The plasma membrane is impermeable to large, cytoplasmic, negatively charged molecules, such as proteins. In other words, these anions are “trapped” inside the cell.
Potassium ions tend to diffuse across the plasma membrane from the inside to the outside of the cell.
Because negatively charged molecules cannot follow the positively charged K+, a small negative charge develops inside the plasma membrane.
The negative charge inside the cell attracts positively charged K+. When the negative charge inside the cell is great enough to prevent additional K+ from diffusing out of the cell through the plasma membrane, an equilibrium is established.
The charge difference across the plasma membrane at equilibrium is reflected as a difference in potential, which is measured in millivolts (mV).
The resting membrane potential is proportional to the potential for K+ to diffuse out of the cell but not to the actual rate of flow for K+.
At equilibrium, very little movement of K+ or other ions takes place across the plasma membrane.

Changing the Resting Membrane Potential: Depolarization

Depolarization: inside of cell becomes more positive; for example, from $-70mV$ to $-55mV$ .
Factors leading to depolarization:
- Sodium ions:
  - Most common way neurons become depolarized.
  - If gated sodium channels open, sodium diffuses into cell down its concentration gradient, and inside of cell becomes more positive.

Changing the Resting Membrane Potential: Depolarization

Calcium ions:
- Calcium entry also causes depolarization, as seen in cardiac muscle.
Other roles:
- Regulation of voltage-gated sodium channels: closed voltage-gated sodium channels stabilized by calcium; low levels of extracellular calcium can cause channels to open spontaneously.
- Regulation of neurotransmitter secretion at presynaptic terminal.

Changing the Resting Membrane Potential: Depolarization

Potassium ions:
- Normally, potassium diffuses out of the cell, but changes in the extracellular concentration of potassium can affect the resting membrane potential.
- If extracellular potassium levels increase, intracellular stays inside cell (less concentration gradient), and membrane becomes depolarized.

Changing the Resting Membrane Potential: Hyperpolarization 1

Hyperpolarization: inside of cell becomes more negative; for example, from $-70mV$ to $-90mV$ .
Two major ways to hyperpolarize:
- Potassium ions:
  - Most common way neurons become hyperpolarized.
  - If gated potassium channels open, potassium diffuses out of cell down its concentration gradient, and inside of cell becomes more negative.

Changing the Resting Membrane Potential: Hyperpolarization 2

Chloride ions:
- Chloride is in higher concentration outside cell, so opening of ligand- gated chloride channels causes chloride to diffuse into cell.
- Adding negative charges into the cell hyperpolarizes it.
- Mechanism for some inhibitory neurotransmitters.

Neuron Communication

Three phases:
1. Generation of axon potential
2. Action potential propagation along the axon
3. Communication with target cell at the synapse