Homeostasis and Neurophysiology (unit 8)

Question 1

Define physiology

Answer 1

• the study of biological function of how the body works
• from molecular mechanisms within cells, to the actions of tissue, organs and systems, and how the organism as a whole accomplishes tasks essential for life

Question 2

State the relationship between physiology and anatomy

Answer 2

• study of physiology focuses on mechanisms of action, looking at how the body acts
• whereas anatomy is concerned with the description of structures within the body.
• however, physiology and anatomy can be combined to provide a bigger picture of the body overall

Question 3

Define and give examples of homeostasis.

Answer 3

• term coined by Walter Cannon
• used to describe the dynamic constancy of the internal environment as compared to variations in the external environment
• Examples: our ability to maintain body temperature, blood pressure, and glucose metabolism

Question 4

Discuss the relationships between the external and internal environments as they relate to homeostasis.

Answer 4

• homeostasis involves keeping internal environment constant, while external environment changes
• Example: body temp is kept internally around 37 C, while the external temperature varies

Question 5

Identify the components of a homeostatic feedback loop.

Answer 5

• negative homeostatic feedback loop maintains homeostasis
• positive feedback loop pushes organism further out of homeostasis, but may be necessary for life to occur
• components of the feedback loop include 1) a sensor, 2) an integrating centre, and 3) an effector
• sensors send info to the integrating centre to allow for detection in changes from a set point
• changes from set point then send signals from the integrating centre to the effector to counter the deviation from the set point

Question 6

Discuss the relationship between the nervous and endocrine systems and their control over homeostasis.

Answer 6

• both endocrine an d nervous systems are able to extrinsically maintain homeostatic regulation
• endocrine regulation is through chemical regulators called hormones. hormones act on given organ to produce a change
• nervous system regulation is done via innervation of a target organ
• endocrine and nervous sys can interact since the nervous sys can control hormone release and some hormones can affect the nervous sys function

Question 7

Discuss and give examples of negative and positive feedback loops.

Answer 7

• negative feedback loop = maintains a state of dynamic constancy around a given set point
• neg loops are important to maintain body temp, blood glucose levels.
• positive feedback loop= aim to amplify a response.
• pos loops are important for blood clotting as well as preovulatory surge in luteinizing hormone in females

Question 8

Identify the major divisions of the nervous system including: CNS, PNS, SNS, ANS and ENS.

Answer 8

• central nervous system (CNS): brain and spinal cord
• peripheral nervous system (PNS): everything outside the CNS (nerves, ganglia, nerve plexuses)
• somatic nervous system (SNS): a division of the nervous system responsible for the control of skeletal muscles
• autonomic nervous system (ANS): a division of the nervous sys that is responsible for the control of involuntary effectors such as smooth muscle, cardiac muscle and glands. The ANS is further divided into the sympathetic and parasympathetic nervous systems
• enteric nervous system (ENS): a complex network of neurons involved in the intrinsic control of the gastrointestinal system

Question 9

Discuss the structural classification of neurons and identify anatomical features of a neuron.

Answer 9

anatomical features of a neuron:

-neurons contain 1) a cell body, 2) dendrites and 3) an axon

• the cell body is the nutritional centre of the neuron
• dendrites are thin, branched processes that transmits signals from their ends to the cell body
• the axon is a long process that conducts impulses away from the cell body

3 structural classifications:

1) Pseudounipolar neurons: have a single short process that branches like a T to form a pair of longer processes
2) Bipolar neurons: have two processes, one at either end
3) Multipolar neurons have several dendrites and one axon extending from the cell body

Question 10

Define nucleus, ganglion, tract, somatic neuron, motor neuron, sensory neuron, afferent, efferent and nerve as they relate to the nervous system.

Answer 10

Nucleus: A grouping of neuron cell bodies within the CNS

Ganglion: A grouping of neuron cell bodies located outside the CNS

Tract: A grouping of axons that interconnect regions of the CNS

Somatic (motor) neuron: A nerve the stimulates contraction of skeletal muscles

Motor neuron: conduct impulses out of the CNS to effector organs (ie. Muscle)

Sensory neuron: conduct impulses from sensory receptors into the CNS

Afferent: conducts nerve impulses from organ into CNS (ie. Sensory neuron)

Efferent: transmits impulses from CNS to effector organ (ie. Motor neuron)

Nerve: Cable-like collection of many axons in the PNS, can contain both sensory and motor fibers

Question 11

Identify and briefly describe the function of specialty receptors located on dendrites.

Answer 11

• specialty receptors are located on the dendrites of the postsynaptic cell* for neurotransmitters
• neurotransmitters are released from the axon of the presynaptic cell into the synapse

-the function of these receptors is to allow for signals to be sent from one cell (presynaptic) to another cell (postsynaptic)

Question 12

Identify the functions of the supporting cells types of the nervous system including: oligodendrocytes, Schwann cells, astrocytes, microglia, and ependymal cells.

Answer 12

Oligodendrocytes: Responsible for the myelin sheath around axons in the CNS

Schwann cells: Also known as neurolemmocytes, are responsible for the myelin sheath around myelinated axons in the PNS

Astrocytes: A cell type in the CNS that covers capillaries and induces the blood-brain barrier as well as interacts metabolically with neurons

Microglia: A cell type in the CNS that phagocytoses pathogens and cellular debris

Ependymal cells: A cell type in the CNS that forms the epithelial lining of brain cavities (ventricles) and the central canal of the spinal cord; covers tufts of the choroid plexuses

Question 13

Describe myelin and identify its role in the nervous physiology.

Answer 13

The myelin sheath encases the axon and is formed by Schwann cells in the PNS and oligodendrocytes in the CNS. Myelinated axons are able to conduct nerve impulses more rapidly than unmyelinated axons.

Question 14

Define ‘node’ and ‘antinode’ and describe their formation.

Answer 14

A node is a site of no myelination on a myelinated axon. An antinode is the opposite of a node and is the region of the axon that is myelinated. Schwann cells in the PNS and oligodendrocytes in the CNS form antinodes. Nodes are formed where gaps are left between the myelin sheath.

Question 15

Describe simple diffusion, ion channels, gating of integral membrane proteins, facilitated diffusion, primary active transport, secondary active transport, pinocytosis, exocytosis and endocytosis.

Answer 15

Simple diffusion: The net movement of ions or molecules from regions of higher concentration to regions of lower concentration

Ion channels: Channels in the membrane that permit the movement of ions; channels can be open or gated

Gating of integral membrane proteins: Many channels have gates that can open or close and channel. In this way, particular stimuli can open an otherwise closed channel

Facilitated diffusion: the net movement of ions or molecules from regions of high to regions of low concentration through the aid of a transmembrane protein (carrier-mediated transport)

Primary active transport: The movement of molecules and ions against their concentration gradients where hydrolysis of ATP is the source of energy

Secondary active transport: The movement of an ion or molecule against its concentration gradient where the energy is obtained through the movement of an ion or molecule with its concentration gradient (coupled transport)

Pinocytosis: Also known as ‘cell drinking’; invagination of the cell membrane to form narrow channels that pinch off into vacuoles. This permits cellular intake of extracellular fluid and dissolved molecules.

Exocytosis: Bulk transport of large molecules out of the cell through the fusion of a membrane-bound vesicle with the plasma membrane

Endocytosis: Bulk transport of large molecules into the cell through the formation of a membrane-bound vesicle from the plasma membrane

Question 16

Describe the Nernst equation (DO NOT HAVE TO DEFINE EQUATION)

Answer 16

The Nernst equation allows for the theoretical equilibrium potential to be calculated for a particular ion when its concentrations are known.

Question 17

Define the resting membrane potential. List conditions that result in a resting membrane potential.

Answer 17

The resting membrane potential is the membrane potential of a cell that is not producing impulses. The resting membrane potential depends on 1) the ratio of the concentrations of each ion on the two sides of the plasma membrane and 2) the specific permeability of the membrane to each different ion.

Question 18

Describe the movement of K+ and Cl- ion across the plasma membrane to restore resting membrane potentials.

Answer 18

Potassium (K+): If the cell potential were to become more negative than its equilibrium potential (-90mV), K+ would be draw into the cell to restore the potential. If the cell potential were to become less negative, K+ would diffuse out of the cell to restore the membrane potential.

Chloride (Cl-): Chloride remains in the cell as a fixed anion population. It is responsible for drawing positively charged ions into the cell to restore membrane potential if it becomes more negative than the resting potential.

Question 19

Describe the ‘patch clamp’ technique.

Answer 19

Patching clamping of a neuron allows for detection of the cell’s membrane potential. One electrode is placed outside the cell membrane near the area being recorded and a second electrode is placed inside the cell. The voltage difference can be visualized by connecting the electrodes to a computer. Deflections up or down represent changes in membrane potential with an upward deflection indicating the inside of the cell has become more positive compared to the outside and a downward deflection indicating the cell has become more negative compared to outside.

Question 20

Define: graded potential, action potential, resting potential, all or none law, threshold potential, depolarization, repolarization and hyperpolarization.

Answer 20

Graded potential: A change in membrane potential with amplitudes that are varied, or graded, by gradations in stimulus intensity. Ie. The amount of neurotransmitter released determines the amount of depolarization or hyperpolarization on the postsynaptic neuron.

Action potential: An all-or-none electrical event in an axon or muscle fiber in which the polarity of the membrane is rapidly reversed and reestablished.

Resting potential: The potential across a plasma membrane when the cell is in an unstimulated state.

All-or-none law: Action potentials are an all-or-none event: if the threshold potential is not met, no action potential will occur, but if the threshold potential is exceeded an action potential will occur. The size of the action potential dose not change depending on how the threshold potential is exceeded.

Threshold potential: The potential at which an action potential will occur. Generally -55 mV in neurons.

Depolarization: The loss of membrane potential in which the inside of the cell becomes less negative compared to the outside of the membrane.

Repolarization: The reestablishment of the resting membrane potential after depolarization has occurred.

Hyperpolarization: An increase in the negatively of the inside of the cell membrane with respect to the resting membrane potential.

Question 21

Describe the three states of the Na+ channel during rest, depolarization and repolarization of an excitable cell.

Answer 21

At rest, the Na+ channels are closed.

During depolarization, Na+ channels are open allowing the membrane to become permeable to Na+ and Na+ flows into the cell.

During repolarization, the Na+ channels are inactivated.

Question 22

Describe how an action potential is propagated down the length of an axon and explain the refractory period.

Answer 22

Action potentials (APs) are conducted down the length on an axon due to the cable properties. Depolarization in one region of the axon allows for the depolarization of the region next to it. The AP can’t “bounce backwards” up the axon because of the refractory period, which immediately follows. Axons also make use of the myelin sheath to allow for conduction over long distances. A refractory period is the period of time during which a region of axon (or muscle) cell membrane cannot be stimulated to produce an action potential (absolute refractory period), or when it can only be stimulated by a very strong stimulus (relative refractory period)

Question 23

Define and describe saltatory conduction.

Answer 23

Saltatory conduction involves the “leaping” of action potentials from one node to the next in myelinated axons. Instead of depolarization along the whole length of the axon, depolarization only occurs at the nodes of Ranvier allowing for faster conduction of action potentials

Question 24

Describe the differences in myocyte action potentials.

Answer 24

In skeletal muscle, rapid opening of sodium channels causes the action potential, but the resting membrane potential and recovery are driven largely by chloride permeability. This is why defects in chloride channels can give rise to myotonia, associated with a failure of the muscle to relax after contraction. (Notes: Propagation of the action potential)

Question 25

Describe the differences in impulse characteristics between large and small axon and myelinated and unmyelinated axons.

Answer 25

1) Action potentials are conducted faster in large axons versus small axons due to a reduction in resistance in the large axon.
2) Action potentials are conducted faster in myelinated axons versus unmyelinated axons because the myelin sheath allows for saltatory conduction of action potentials