Part Seven: The Nervous System
Biology of the Chicken, Part 7
Not unlike our own human body, the chicken’s body needs a control center with a communication network. The nervous system inside our Hank and Henrietta integrates and directs the various functions of their body. It is comprised of two major parts: the central nervous system (CNS), and the peripheral nervous system (PNS). Additional stimuli are received through the senses and interpreted by the brain to alert our fowl of the constantly changing environmental conditions.
The central nervous system is composed of the brain, spinal cord and nerves. Within this system, the brain acts as the “main office” by processing the information it is given through various stimuli and returning a decision for an appropriate response. The spinal cord collects micro-electric responses from the nerve endings, and like a major phone line, transfers the messages to the brain. Both of these organs are encased by a protective boney structure. In the case of the spinal cord it also has a myelin (fatty) sheath for additional protection.
As the name implies, the peripheral nervous system interprets the periphery or area around the CNS. The PNS includes the senses and telegraphs its environmental stimuli, such as a tug on Hank’s tail, to the sensory neuron (nerve cell). This neuron sends an immediate message to the brain by way of the spinal cord at a speed of more than 120 meters per second. Hank’s squawk seems almost instantaneous as the brain sends the response to use muscles stimulated by a motor neuron to escape the danger.
Within the chicken’s nervous system, individual nerve responses may be either voluntary or involuntary. Voluntary control functions occur when the chicken consciously responds to some activity or stimulus. The nerves that initiate these types of responses are called somatic nerves. For example, Henrietta may use her taste bud receptors to avoid a bitter tasting treat and choose instead something sour. Something as simple as to walk or to fly is based on somatic or voluntary nerve responses.
Involuntary nerves perform their function without the chicken’s conscious control or choice of action or event. The vital actions of regulation of heartbeat, process of digestion and breathing in and out cannot be afforded to conscious thought. These critical functions are regulated by the autonomic or involuntary nervous system. How long would we stay alive, let alone our chicken friends, if we had to think about every beat of our heart, where that burger (or kernel of corn) is in our food tube, or remember to breathe? And all at the same time?
A different type of involuntary response to external stimuli is a reflex. Reflexes are “short cuts” in an already expedient nervous system built in for protection. In the peripheral network of nerves covering the body of the chicken, certain actions need to be taken immediately without including the thought process of the brain. The sensory signal of the reflex reaction only travels as far as the spinal cord to initiate the appropriate response. Life and death decisions such as ducking from a hawk or flying from the fox cannot be afforded any thought process, only immediate physical responses in the form of a reflex action.
As in humans, there are five basic senses. The senses of sight, hearing, smell, taste and touch appear in most animals but vary in degree of strength. As we have mentioned in the past, the ability of flight has influenced the chicken’s biological systems. A chicken brain is highly developed for coordination, eyesight with better visual acuity, and a sense of touch that can detect the slightest change in air pressure. These senses are imperative for flight.
By far, sight is the chicken’s strongest sense. A bird’s eyes are the largest relative to their body in comparison of all animals. The location of the eyes on the face afford binocular vision (both eyes see an object); this placement is important for distance perception. Though similar to our mammalian eye, our fowl’s eye has a higher threshold of light intensity. Therefore chickens are diurnal or active only during the daylight hours. It is the reason they seek to roost at night for protection from nocturnal predators. As a prey animal, their sight affords them a tremendous field of view of almost 360 degrees or a full circle. It makes it difficult for a predator to sneak up on them.
Hearing ranks a close second to sight in the senses of our Hank and Henrietta. Their keen sense of hearing is, however, not as good as our own. The ear of the chicken is located on each side of the face behind the eye. Unlike the human ear there is no ear flap or lobe to direct sound waves. Ears are also covered by a tuft of feathers to protect the ear canal from dust and other harmful materials. Because birds interact with varying altitudes during flight, they have a special duct (tube) that connects the middle ear with the roof of the mouth to regulate air pressure and prevent injury to the tympanic membrane (eardrum).
The sense of taste is first interpreted by the taste buds located on the base of the tongue. These stimuli are transferred to the appropriate receptors in the brain. Chickens have a low tolerance to sodium chloride (table salt, NaCl) while being more accepting of sour food. Hank and Henrietta tend to be sensitive to a bitter taste, but unlike humans, have little preference for sugars.
A sense of touch is present in our bird friends but is not as extensive as it is in humans. As a creature of flight our chickens are very sensitive to changes in air pressure and wind velocity. Those stimuli transfer through feathers to the skin, resulting in expedient adjustments while in flight. The feet and legs contain very few nerves, however, to afford tolerances to cold weather conditions. Pressure and pain sensors also help to protect the comb and wattles of our Hank and Henrietta.
The sense of smell is received and interpreted in the olfactory lobes of the chicken’s forebrain. Birds in general have little use for a sense of smell and have comparatively smaller olfactory lobes than mammals.
Motor neurons cause muscles to respond and take action when needed. Reflexes protect without thought. Involuntary nerve responses “take care of business” (such as heartbeat) that any organism could not remember to do voluntarily. The nervous system of our Hank and Henrietta controls the reactions and activities necessary to sustain life and respond to an ever-changing environment. Just remember that a chicken’s “field of view” can always see you coming. The best plan is to catch them at night!