Section 0.1.1

Question 1

What do afferent neurons do?

Answer 1

carry nerve impulses from the peripheral receptors and special sense organs to the CNS

also called receptors or sensory neurons

Question 2

describe the structure of afferent neurons

Answer 2

small, round cell body with a single long dendrite and a short axon

the dendrite extends to the periphery and functionally acts as an axon meaning that once an appropriate stimulus is received, it conducts action potentials towards the cell body

Question 3

where are afferent nerve cell bodies found?

Answer 3

In clusters called ganglia immediately external to the spine.

Question 4

Where do the axons of efferent neurons extend?

Answer 4

their axons extend into the dorsal horn of the spinal cord

Question 5

What is transduction?

Answer 5

the conversion of an environmental signal to an electrical signal.

this occurs in afferent neurons; once a threshold stimulus has been reached and a receptor is activated, afferent neurons use action potentials to propagate this signal

Question 6

What four different properties do afferent neurons use to allow the CNS to accurately differentiate incoming signals from the PNS?

Answer 6

• Modality
• Intensity
• Location
• Duration

Question 7

Describe how afferent neurons use modality to allow the CNS to differentiate signals.

Answer 7

Each type of receptor is specialized to respond to a different type of energy or stimulus. This is what we
mean when we use the term modality.

Photoreceptors: responsive to visible wavelengths of light

Mechanoreceptors: responsive to mechanical energy. For example, skeletal muscle receptors are sensitive to stretch and
the receptors in the ear contain fine hairs cells that bend in response to sound waves.
Mechanoreceptors can also respond to vibration and acceleration.

Thermoreceptors: sensitive to heat

Chemoreceptors: sensitive to specific chemicals and include the receptors for taste, smell, contents of the digestive tract,
as well as things like the presence of oxygen in the blood

Question 8

Describe how afferent neurons use intensity to allow the CNS to differentiate signals.

Answer 8

action potentials are all-or-none. This means that afferent neurons cannot encode the strength of the stimulus by simply changing the strength of the action potential - this is constant. A strong signal, however, can trigger an increased frequency of action potentials. Thus, nerve cells code the intensity of information by the frequency of action potential

Question 9

Describe how afferent neurons use location to allow the CNS to differentiate signals.

Answer 9

The brain is able to identify the site of sensory stimulation using the location of activated afferent
fibres.

The ways in which neurons are able to encode the location of a stimulus:

Receptive Field: Each neuron has a region of the environment to which it is sensitive, called a receptive field. If a stimulus appears in a neuron’s receptive field, the neuron will fire and location is communicated to the brain.

Multiple Sensors: Our brain can compare inputs from more than one sensor. For example, we have two eyes and two ears.

Gradients: With smell, we can determine location based on gradients. For example, we smell something and then move in some direction. If the smell has become more intense, we know we’re going toward its source

Question 10

Describe how afferent neurons use duration to allow the CNS to differentiate signals.

Answer 10

Afferent neurons also encode the duration of a stimulus and communicate this to the brain for processing. Some cells fire as long as the stimulus is present, and some fire briefly as the stimulus goes on, then stop, and then fire briefly when the stimulus goes off.

Question 11

True or false: The receptors can be a separate receptor cell closely associated with the peripheral ending of a neuron

Answer 11

True

Question 12

Describe the process of the beginning of transduction

Answer 12

The receptors can either be the specialized ending of an afferent neuron or a separate receptor cell closely associated with the peripheral ending of a neuron. It is in these receptors that transduction occurs.

In either case, stimulation of a receptor alters its membrane
permeability, causing the opening of nonselective cation channels (depolarizing the membrane potential)

This change in potential due to an incoming signal is known as a receptor potential in specialized receptor cells or a generator potential in the ending of an afferent neuron

Question 13

Describe the mechanism by which specialized afferent ending receptors create graded potentials.

Answer 13

In the case of a specialized afferent nerve ending, the receptor potential itself can cause the afferent nerve fibre to reach threshold and trigger an action potential

Question 14

Describe the mechanism by which separate receptor cells create graded potentials.

Answer 14

In the case of a separate receptor, when the receptor potential is strong enough it will release a
chemical messenger that diffuses to the afferent neuron and opens chemically gated sodium channels.

If threshold is achieved, then the afferent nerve fibre will initiate and propagate an action potential

Question 15

How are afferent neurons able to encode the intensity of a signal to transmit this information
to the CNS

Answer 15

A larger intensity stimulus can increase the size of the receptor/generator potential. This cannot bring about a larger action potential due to the all-or-none law, but it can induce rapid firing of action potentials in the afferent neuron. Stronger stimuli can also affect many neighbouring receptors, which further communicates to the CNS the intensity of the stimulus

Question 16

What is receptor adaptation?

Answer 16

The ability of the receptors themselves to regulate their responses.

A stimulus of a same intensity does not always bring about the same magnitude of receptor potential. In some situations, the frequency of action potentials generated decreases or adapts to the signal by lessening its response.

Question 17

What are two different types of receptors that vary in their speed of adaptation? Provide examples

Answer 17

Tonic receptors: are generally slowly adapting or do not adapt at all. They are important in situations where a near constant signal from a stimulus is necessary. An example of this is muscle stretch receptors as the CNS constantly requires knowledge of the state of contraction of all skeletal muscles in order to maintain posture and balance. Similarly, pain receptors are tonic receptors as the CNS
requires knowledge of painful and potentially dangerous stimuli.

Phasic receptors: are rapidly adapting such that upon initiation of a stimulus action potentials are
generated, however, the receptor will stop generating action potentials rapidly, even in the presence of the stimulus. However, once the stimulus is removed phasic receptors will again respond with a depolarization called an off response. This type of receptor is important for monitoring changes in stimulus intensity

Question 18

Compare and contrast receptor potentials and action potentials.

Answer 18

Receptor potentials are graded, local changes in membrane potential in sensory receptor cells in response to a stimulus. They are generated at the receptor cell’s peripheral endings. In contrast, action potentials are all-or-nothing electrical signals that travel along axons to transmit information to the central nervous system. Action potentials result from the summation of receptor potentials.

Question 19

Describe the types of receptors and what stimulates them.

Answer 19

Receptors can be categorized into various types, including chemoreceptors (stimulated by chemicals), mechanoreceptors (stimulated by mechanical pressure or vibrations), thermoreceptors (stimulated by temperature changes), photoreceptors (stimulated by light), and nociceptors (stimulated by noxious stimuli or pain).

Question 20

Describe a tonic receptor and how it differs from a phasic receptor.

Answer 20

Tonic receptors generate a continuous stream of receptor potentials as long as the stimulus persists. They provide information about the duration and intensity of a stimulus. Phasic receptors, on the other hand, adapt quickly to a constant stimulus, ceasing to generate receptor potentials unless the stimulus changes. They are sensitive to changes in stimulus intensity or onset.

Question 21

Describe the function of sensory receptors.

Answer 21

Sensory receptors are specialized cells or structures that detect various environmental stimuli and convert them into electrical signals (receptor potentials). These signals are then transmitted to the central nervous system, where they are processed and interpreted. The function of sensory receptors is to provide the brain with information about the external and internal environments, allowing organisms to perceive and respond to their surroundings.