Download Chapter 10 Introduction

Chapter 10
Special Senses and Functional
Aspects of the Nervous System
Introduction
• The experiences we have as we move through
life are a result of sensing the world around us
• These experiences are called sensations,
which are a state of awareness of the external
or internal condition of the body
• Provide the brain with information necessary
to maintain homeostasis
How do sensations help us
maintain homeostasis?
• Hunger – get some food
• Cold – find a warmer place
• Pain – something’s wrong – FIX IT
Sensory Pathways
• All sensory pathways begin with a stimulus, or
a change in the environment that is great
enough to initiate a nerve impulse
• The stimulus is converted to a nerve impulse
by a receptor, then conducted along a sensory
neuron to the CNS
• All receptors are excitable (have the ability to
generate an action potential)
• The threshold of stimulus for each type of
receptor is typically very low for one type of
stimulus yet very high for all other types of
stimuli
• For example, receptors in the eye have a low
threshold of stimulus for light and will initiate
a nerve impulse in response to it, but have a
very high threshold to slight changes in
temperature – receptors are stimulus-specific
• In many receptors, the threshold level for a
particular stimulus may rise after continuous
stimulation; this is called sensory adaptation
• Impulses are generated at decreasing rates
until they stop completely and the sensation
ends
• This prevents the brain from being overloaded
by information that is no longer important
Types of Receptors
• Mechanoreceptors – detect mechanical or
physical change in the receptor or nearby
cells; sensitive to touch, pressure muscle
tension, hearing, equilibrium and blood
pressure
• Thermoreceptors- detect temperature
changes
• Nociceptors- detect pain, usually from physical
or chemical damage to nearby cells
• Photoreceptors – sensitive to changes in the
amount of light (present only in retina)
• Chemoreceptors—detect chemicals dissolved
in fluid providing smell and taste; also detect
levels of oxygen and carbon dioxide in blood
General Sensory Pathways
• Conduction pathways that carry impulses from
a simple receptor (found in skin, visceral
organs, and muscles) to the brain are known
as General sensory pathways
• From the receptor, impulse is conducted along
three sensory neurons before in reaches its
destination in the brain
• First-order neuron—connects receptor to the
spinal cord; may extend to the medulla
oblongata
• Second-order neuron—carries the impulse
toward the thalamus
• Third-order neuron—conducts the impulse to
the cerebral cortex for processing
In the cerebral cortex, sensations are
interpreted. Interestingly, these
sensations aren’t always interpreted
correctly…..
• Assumptions Video
Special Sensory Pathways
• Carry impulses from complex receptors like
those in the ear and eye
• Exhibit more variation in the number of
neurons than general sensory pathways
• Include at least 3 sensory neurons that
connect to a specific region of the cerebral
cortex
• Located in the head, travel along cranial
nerves to the thalamus then to the cerebral
cortex (except for smell)