General Senses
Objective
- Describe sensations and the classification of sensory receptors.
Is the water is the shower the right temperature? Ouch what stung me? We ask ourselves questions like this daily, but how do we know the answers? Being able to sense external stimuli, interpret, and respond accordingly is critical for continued survival; and is made possible by your nervous system and the process of sensation. Sensation is defined as a perception or feeling that results from a stimulus. This stimulus will come in a variety of forms and could arise from either the external or internal environment. We know this verbose definition of sensation as simply our senses.
If you were to ask someone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is equilibrium. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, itch, tickle, and hair-follicle position, on the basis of the type of receptors that perceive these touch sensations. Other overlooked senses include temperature perception, pain perception, and proprioception.
Contrasting General and Specific Senses
Senses can be classified as either general or specific. A general sense utilizes relatively simple receptors that are widely distributed throughout the body; meaning that you can tell the temperature of the water in the shower with your hand, foot, arm, face, etc. General senses often contribute to the sense of touch (as described above), or to proprioception (body position in space) and kinesthesia (body movement), or to a visceral sense (internal environment monitoring that stimulates the ANS). A special sense utilizes complex receptors and has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose. This means that you can only see with your eye, not the back of your head (sorry mom) because there are no receptors for light there. This module focuses on general senses, the special sense will be addressed in a later module. A quick comparison between general and special senses is listed below.
|
General Senses |
Special Senses |
|
Simple receptor and afferent pathway |
Complex receptor and afferent pathway |
|
Widely distributed all over the body |
Concentrated in the head and face |
|
Includes: pressure, pain, itch, tickle, vibration, temperature, and proprioception. |
Includes: vision, olfaction (smell), gustation (taste), audition (hearing), and equilibrium. |
Each of the senses is referred to as a sensory modality - modality refers to the way that information is encoded. Each modality has a specific receptor that is activated by a particular stimulus and a specific sensory neuron that carries the information from the receptor into the CNS. If you remember back to the module that addressed homoeostasis there was a flow of information from receptor into the CNS and then back out to the effector. The receptor senses the change in the external environment. This sensory information is then passed up the afferent pathway via a unipolar neuron into the posterior grey horn of the spinal cord. An interneuron then processes the sensory information deciding how to respond and creating a motor signal. This motor output is then passes down the efferent pathway via a multipolar neuron form the anterior grey horn of the spinal cord to the effector. The effector can then act allowing us to respond to the stimulus that set this chain of events in motion. See the diagram below for a visual representation of this pathway.
Figure 1: Testing the water: (1) the sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) The graded potential from the sensory endings, if strong enough, will initiate an action potential at the initial segment of the axon (which is immediately adjacent to the sensory endings in the skin). (3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. The contact is a synapse where another graded potential is caused by the release of a chemical signal from the axon terminals. (4) An action potential is initiated at the initial segment of this neuron and travels up the sensory pathway to a region of the brain called the thalamus. Another synapse passes the information along to the next neuron. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent from the precentral gyrus of the frontal cortex. (7) The upper motor neuron sends an action potential down to the spinal cord. The target of the upper motor neuron is the dendrites of the lower motor neuron in the gray matter of the spinal cord. (8) The axon of the lower motor neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle. CCBY: OpenStax college
View "General Senses" on YouTube (opens in a new window)
Helpful Study Fact #1: Words that go together – when you see one of these words being used to reference the general senses you know that the other 3 will soon follow. Group 1: sensory information, afferent pathway, unipolar neuron, and posterior/dorsal. Group 2: motor information, efferent pathway, multipolar neuron, and anterior/ventral.
The Withdrawl Reflex
The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscle movement. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. Let's now look at a simple reflex action - the withdrawal reflex. When you touch a hot stove, you pull your hand away. Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage (see section on selectivity – insert link to that portion of the module). This triggers an action potential, which travels along the sensory fiber from the skin, through the dorsal spinal root to the spinal cord, and directly activates a ventral horn motor neuron. That neuron sends a signal along its axon to excite the biceps brachii, causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove. This basic withdrawal reflex includes sensory input (the painful stimulus), central processing (the synapse in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the biceps brachii).
