Sensory Neurophysiology “One does not see anything until one sees its beauty. Then, and then only, does it come into existence” Oscar Wilde

In this lecture, we will deal with stimuli which reach the conscious level of perception.

Sensory Receptors Some organs such as the eye and ear are complex, containing over 126 million and 16,000 receptors, respectively.

Internal and external stimuli are converted into a graded potential upon activation of sensory receptors. If graded potentials exceed the threshold for firing, they generate APs along the axon, which in turn elicit neurotransmitter release from axon terminals onto postsynaptic neurons in the CNS. The specificity of a receptor for a particular type of stimulus is called the law of specific nerve energies. At each synapse along the way to the cerebral cortex, sensory information can be modified. Some stimuli come into our conscious perception, and others are acted upon without our awareness. Perceptual threshold refers to the level of stimulus intensity needed to become aware of a particular sensation. Our CNS avoids processing information below the perception threshold to avoid becoming overwhelmed with information. Eg. turning down the radio when studying sets the stimulus below perception threshold. Primary sensory neurons may perceive the sound, but higher order neurons dampen the intensity by the time it reaches the auditory cortex.

Sensory Pathways Sensory projections ascend from the spinal cord into the brain through the brain stem. They travel through the thalamus, which acts as a relay processing station of signals to other brain regions. Olfactory signals project directly to the olfactory cortex. The olfactory cortex is close to the temporal lobes which process memory. Thus, particular smells trigger a long-stored memory.

Sensory Neuron Receptor Fields The receptor of each primary neuron can pick up information from a specific area called the receptive field of the receptor. Receptive fields can be irregular in shape and overlap with receptive fields of other neurons. If receptive fields from primary neurons converge onto a single secondary neuron, the individual receptive fields merge into a single receptive field. This allows subthreshold stimuli to be summed by the secondary neuron.

Two-point Discrimination Helps to Determine The Size of the Secondary Receptive Field The sensitivity to touch is demonstrated by the two-point discrimination test. The smaller the distance between two points of a divider, the smaller the size of the receptive field, and greater the touch sensitivity.

The Amount of Space Devoted to Different Body Parts In The Primary Somatosensory Cortex The somatotopic map (homunculus) is not scaled like the human body.

The Body Needs to Distinguish Four Properties of a Stimulus 1.Modality (nature) of the stimulus. The brain associates a signal coming from a specific group of receptors with a specific modality. Eg. stimulation of a cold receptor is always perceived as cold. The 1:1 association of a receptor to a sensation is called labeled line coding. 2.Localization of the stimulus. The organization of receptors in the periphery is preserved in the brain. Eg. Touch-pressure receptors on the hand project to a specific area on the cortex. Stimulation of the specific area of that cortex would be interpreted as a touch to the hand, eventhough there was no actual touch. 3.Magnitude of the stimulus. 4.Duration of the stimulus.

The Brain Uses Timing Differences to Localize Sound and Smell Auditory and olfactory stimuli are exceptions to the localization rule. Neurons in the ear and nose are sensitive different sound frequencies and different odors, respectively. However, their activation provides no information about the location of the stimulus. Instead, the brain uses timing differences in receptor activation to compute the localization of sound or odors.

Localization of a Stimulus Can Be Isolated Through Lateral Inhibition Receptors close to the sensation activate their respective primary neurons. This in turn would normally activate their respective secondary neurons. However, the primary neuron whose receptors are closest to the stimulus will signal the secondary neuron, which in turn suppresses activity of adjacent secondary neurons. The inhibition of neurons farthest from the stimulus enhances the contrast between the centre and side of the receptive field. This aids to localize the site of the sensation more easily. This is termed lateral inhibition.

Intensity of the Stimulus is Coded in the AP Frequency Stimulus intensity is coded by the number of activated receptors, and the resulting frequency of AP propagation along the axon. The amount of released transmitter from axon terminals is proportional to the frequency of APs.

Stimulus Duration Affects The Speed of Adaptation of the Receptors The duration of the stimulus is coded by the duration of the APs. As the stimulus persists, some receptors turn off and cease to respond. Tonic receptors are slowly adapting receptors which continue to transmit signals to the CNS as long as they are stimulated. Stimuli which activate tonic receptors are those which need to be continually monitored by the body. Eg. pain receptors are tonic receptors. Phasic receptors are rapidly adapting receptors which fire when they first receive a stimulus, but cease firing if the stimulus strength remains the same. Once a stimulus reaches a steady intensity, phasic receptors adapt to a new steady state and turn off. These type of responses allows the body to ignore information which is not a threat to its well-being or homeostasis. Eg. the smell of cologne.

Once a phasic receptor adapts, the only way to create a new signal is to change the intensity of the stimulus. Adaptation could occur in a number of ways depending on the receptor type: 1) it could involve activating additional channels in the receptor membrane to cause the membrane to repolarize; b) accessory structures decrease the amount of stimulus reaching the receptor. Eg. in the ear, the vibration of small bones is decreased upon hearing loud noises.

Touch-pressure Receptors Touch receptors are the most common receptors in the body. They are found in the superficial layer of the skin and deep layers of the body such as viscera.

Sensory Projections to the Somatosensory Cortex Neurons from one side of the body project to the opposite part of the brain.

Summary Sensory pathways begin with a stimulus which is converted by the receptor into an electrical potential. There are 5 kinds of sensory receptors: chemoreceptors; mechanoreceptors, thermoreceptors, photoreceptors & nociceptors. Primary sensory neurons carry the stimulus to higher order neurons in the CNS. Sensory projections through the spinal cord input into the thalamus. The perceptual threshold is the level of stimulus needed for awareness of a particular sensation. The area over which a receptor picks up information is called a receptive field. Primary neurons projecting to a single secondary neuron can create a single large receptive field. The body needs to distinguish four properties of a stimulus: modality, location, magnitude and duration. Sensory projections carrying touch-pressure signals are processed in the primary somatosensory cortex. Dermatomes map the area of skin innervated by a single dorsal root ganglion.

References 1.Tortora, G.J. & Grabowski, S.R (2003). Principles of Anatomy & Physiology.New Jersey: John Wiley & Sons. Ch.10, pp.. 2.Silverthorn, D.U (1998). Human Physiology: An Integrated Approach. New Jersey: Prentice Hall. Ch.12, pp