Answers to review questions – chapter 28

1.Describe the basis of the resting membrane potential. (pp. 662–667)

A current will travel between two points only when there is a difference in voltage between the two points. In the neuron, electrical communication depends on a voltage difference occurring across the plasma membrane, with the inside of the neuron being negative with respect to the outside. Ion pumps, using ATP as their energy source, indirectly establish this voltage difference. These pumps transport Na+ to the outside and K+ inside, thus establishing a concentration gradient for these ions (but not an electrical gradient). However, the membrane is more leaky to K+ than Na+, and so K+ moves down its concentration gradient to the outside much more rapidly than Na+ moves in, leaving the inside of the neuron negatively charged. This charge, or resting potential, lies between –70 and –80 mV.

2.How is an action potential transmitted along an axon? (pp. 667–670)

While local responses in electrical potential can occur in a neuron, and are called active responses, these are not suitable for conducting information any distance as, depending on the cell involved, they can die away quite rapidly. Long-distance communication is dependent on the formation of an action potential. In neuronal cells that can generate action potentials, the number of voltage-dependent Na+ channels opening at the threshold voltage increases rapidly, creating a positive feedback loop so that the potential inside the cell rises very rapidly (called rather confusingly depolarisation). This loop is broken only when very high depolarisation levels are reached and the Na+ channels close again and K+ open. This electrical excitement passes into adjacent areas of the cell, causing their Na+ channels to open and so on, thus propagating the signal along the axon.

3.What are the differences between an electrical synapse and a chemical synapse? (pp. 667–670)

Neurons transmit information to other cells through the synapse, which is a small area of close contact to other neurons or muscles. Chemical synapses are the most common form of synapse and comprise a narrow gap between 20 and 50 nm wide into which chemical signalling substances, called neurotransmitters, are released. An action potential arriving at a chemical synapse causes the opening of Ca2+ channels, which allows Ca2+ to rush into the neuron. This in turn triggers a series of biochemical events that culminate in the release of previously stored chemical signalling substances, called neurotransmitters, into the synaptic gap, or cleft to give the gap its proper title, by exocytosis. These substances then diffuse across the cleft, bind to receptors on the target cell and trigger a post-synaptic response.

In contrast at an electrical synapse, the membranes of the pre- and post-synaptic cells are very closely opposed and communicating junctions are present, allowing electrical signals to pass across directly.

4.How do synaptic transmitters excite or inhibit post-synaptic cells? (pp. 662–667)

A neurotransmitter can affect the target cell in a variety of different ways. The neurotransmitter usually binds to a receptor on the target cell membrane (the exception being nitric oxide which passes through the membrane to act intracellularly) and can either cause a conformational change directly to the receptor or nearby, or may trigger the release of secondary messengers inside the cell. In both cases, a neurotransmitter can change the post-synaptic membrane potential. If the change is to make the membrane potential become more negative, that is the electrical difference between the inside of the cell and the outside increases, the neuron is said to be hyperpolarised, becomes more difficult to activate and is thus inhibited. If the membrane potential is increased, that is the electrical difference between the inside and the outside is decreased, the neuron is said to be depolarised, the membrane is brought closer to its threshold potential and the neuron is excited. Which way the target cell goes, inhibited or excited, is dependent purely on the nature of the target cell and its receptors.

5.What factors determine the influence a particular dendritic synapse has on the generation of an action potential at the axon hillock where an action potential is initiated? (pp. 667–670)

The activation of a neuron is inhibited when the neuron is hyperpolarised (becomes more negative), whereas when the neuron is excited is when it is depolarised (becomes more positive). There are many different molecules that act as neurotransmitters which have the ability to inhibit or excite neurotransmitters. Inhibitory inputs at the axon hillock can block the propagation of an action potential and cancel out the effects of large excitatory inputs on the dendrites.

6.What are the main changes observed during the evolution of more complex nervous systems and why are they thought to have occurred? (pp. 671–673)

At the simplest levels, time and space limit the distances over which nervous transmission can occur efficiently and so there has been an aggregation of nerve cells into bundles along pathways down each side of the animal, increasing the efficiency of processing. For animals that move with one end consistently forwards, further increases in efficiency are achieved through a preferential aggregation at the anterior end of the body, particularly of the nerve groups associated with the co-ordination of body movement, feeding structures and the detection of light. This progressive trend is called encephalisation.

As animals have become more complex and larger, they have developed peripheral ganglia involved in the control of local systems such as the heart and digestive system, as well as an increasingly complex array of both internal and external sensors. As such, there has been a need for the central processor, the brain, to expand accordingly to co-ordinate the reporting and activity of these peripheral systems. The expansion of the brain has had the additional benefit of allowing more complex behaviours such as the development of thought and emotion, which in turn has led to the development of learning and language. This in turn has allowed the animal to display a far more complex range of actions in response to changes in its environment.

7.Is the neural processing associated with particular sensory modalities spread diffusely throughout the brain or localised in particular areas? Provide some examples to support your answer. (pp. 673–676)

Sensory modalities are located in particular areas of the brain. For example, in humans, if part of the left temporal lobe is injured, speech may be lost entirely, whereas if damage is sustained on the right side in the same region, damage to speech is far less obvious. Refer to Figure 28.10.

8.What is meant by the higher functions of the brain? In what way are animals that evolve higher functions advantaged? (p. 676)

Higher brain functions are usually divided into two groups: those of thought (cognition) and those of emotion (affect). Possession of these abilities allows behaviours involving complex computations such as those required for learning and language. Animals with such higher brain function can interact with their environment in a far more complex manner than those without.

9.What are the main sensory modalities in mammals and where are they located in the body? (pp. 679–684)

Vision—eyes

Hearing—ears

Chemoreception—nose, mouth

Mechanoreception—body surfaces, joints, tension in the walls of viscera, hairs

Pain—body surfaces

10.What are the two divisions of the autonomic nervous system? List some of the ways in which some body functions are affected by each. (pp. 684–686)

The vertebrate autonomic (or automatic) nervous system innervates the visceral organs of the body, and as the name implies its functions are not consciously controlled. Conventionally, it has two major divisions, the sympathetic and parasympathetic divisions, though somewhat confusingly as far as this question is concerned, this text also deals with the enteric nerves as a separate division. The sympathetic division is that part of the system whose motor pathways emerge from the thoracic and lumbar parts of the spinal cord, and though it is not made particularly clear in the text, have ganglia located close to the spinal cord and thus short preganglionic and long postganglionic fibres. In contrast, the parasympathetic division originates in the brain and sacral regions of the spinal cord, has ganglia located in the visceral effector organs and has long preganglionic and short postganglionic fibres.

Again though not clear in the text, the two divisions serve the same visceral organs but cause essentially the opposite effects. The system as a whole has a very wide range of effects throughout the body but the following are mentioned in particular: the cardiovascular system, the digestive system, the respiratory system, the eye, the excretory system, the reproductive system and metabolic system and the temperature control system.