Anatomy Physiology

Chicken Nervous System 101

And The Five Senses

So far, I’ve posted about chicken digestive, respiratory and reproductive systems. I’ve found that this one, on the nervous system, is a bit more difficult to translate into layman’s terms. Bear with me: I hope that I can make it interesting and informative.

The nervous system and sensory organs play a critical role in everyday life of any animal, by integrating and controlling the various functions of the body. The things that are voluntarily controlled, meaning an animal has some choice over any action they might make (i.e. moving part of the body), involve voluntary, or somatic nerves.

Involuntary, or automatic, nerves control activities such as: heart beat and circulation, digestion, and respiration – all things that are essential for survival that don’t require conscious decision-making.

Sensory organs (i.e. beak, eyes, ears, taste buds) detect the various stimuli in the bird’s environment that it reacts to.

The Brain 

Think of the brain as the control centre for all the functions and activities of the various systems, organs and tissues in the body. It consists of many parts that contain special cells able to detect, recognize, remember and direct.

Without going into great detail the brain contains nerves involved in sight and hearing, as well as glands that regulate systems, organs and tissues in the rest of the body.

Spinal cord 

The spinal cord contains nerve tissue that extends from the brain inside the length of the vertebral column. Together with the brain, the spinal cord makes up the Central Nervous System (CNS). Both are well protected by fluids, sheaths and bones. Nerves leave the spinal cord through openings in the joints between the different vertebrae and control the systems, organs and tissues of the body.

Chicken Embryo Nervous System
Chicken Embryo Nervous System

If the spinal cord is broken, the connection between the brain and the parts that it controls will also be broken. A break of the spinal cord in the lower back results in paralysis of the legs and other functions that take place below that break. The most effective way to euthanize a chicken is to sever the spinal cord at the neck.

Nerves

Nerve endings, or receptors, carry the sensors that respond to stimuli and transfer that information to the brain. Nerve cells carry messages, via very weak electrical currents, involving specific tasks such as smell, sound, sight or touch. A chicken reacts to stimuli and their response is directed through nerves, organs or tissue. Nerves that send messages to muscles are called “motor nerves” and transmit information associated with pain, touch, cold, heat and pressure.

The Senses
The Senses (Credit: Wardhenline)

The Five Senses 

The sensory organs receive the various stimuli from the bird’s environment and include: beak (touch); eyes (sight); ears (hearing and balance); olfactory organ (smell); and taste buds.

Touch

Sensory nerves are located throughout the beak and help chickens negotiate fine movements when feeding, preening, building nests and in social interaction. Their beak is critical for functions of touch and contain sensory nerves. When birds are de-beaked they lose a significant part of their sensory system, which is comparable to losing one’s fingertips.

Conditions: Crossbeak, Split Beak 

Sight

Like other prey animals, chickens’ eyes are on the side of their head allowing them to have panoramic vision of about 300 degrees. They can see in two completely different ways: binocular vision – both eyes combine to view an object with enhanced depth perception (useful when foraging), or monocular vision to look at separate objects on either side of their head (e.g. watching out for predators).

They’ve got two main eyelids and a nictitating membrane, which is located in the front corner of each eye. While the eye is open, the membrane is only partially visible and is capable of covering the eyeball with a very fast movement to protect the eyes.

Nictitating Membrane
Nictitating Membrane

Chickens have a higher threshold of light intensity at which they see than do some other birds and mammals, which makes them active during the day (diurnal) rather than at night (nocturnal). In nature, or in a free ranging situation, chickens seek the protection of trees to sleep at night.

It’s believed they see colours and have sharp vision, even after long periods of fixed staring.

Chickens produce tears that drain from the eye into the nasal cavity via a tear duct. A very fine, sensitive, transparent membrane called the conjunctiva covers the surface of the eye ball and the inside of the eye socket and can become inflamed and/or infected if the bird contracts conjunctivitis.

Their body postures and displays are forms of visual communication that play a vital role in the social order of chicken flocks.

Conditions: Marek’s Disease, Blindness, Cataracts, Infectious Coryza

Hearing 

Their ears are located on each side of the face behind the eye. The outer ear consists of the ear canal, covered by a tuft of feathers to protect it from dust and other potentially harmful materials. Unlike mammals, birds don’t have an external ear flap or lobe and the three middle ear bones in mammals have been replaced by a single structure of bone and cartilage.

Their ears consist of three main segments: the outer ear; the middle ear, separated from the outer by the tympanic membrane (ear drum) and the inner ear which consists of the cochlea and three ducts.

Chickens hear by sound waves that enter the outer ear canal and apply pressure on the tympanic membrane. Nerve endings receive and transmit them via the auditory nerve to the brain where it is recognized as sound.

They have a good sense of hearing and are able to perceive sounds more quickly than we are, which is a necessity for prey animals. If you watch your flock in action out in the yard you’ll see they can distinguish between different kinds of sounds and vocalizations. They are adept at pinpointing the source of a sound by assessing the lag time between the sound’s arrival at either side of the head

Conditions: Ear Infections, Fowl Cholera

Smell 

Obviously chickens don’t have a nose – their olfactory receptors are located in their upper jaw and can respond to particular scents which is helpful when making food choices. It’s believed that smell is of secondary importance to the other senses in the location and choice of food. The loss of a chicken’s ability to smell well does not seem to influence either its selection of food or other behaviour.

Conditions: Sinusitis

Chicken Skull

Taste 

Their taste buds are located at the base of the tongue and on the floor of the mouth. Small quantities of chemicals are recognized by the taste buds and this information is transferred to the appropriate receptors on the brain. Their sense of taste is more acute when consuming liquid, rather than dry, food. If you are giving strong tasting medications it is better to mix them into dry food, rather than water.

An important feature is their sensitivity to the temperature of drinking water. They are more likely to reject water with a temperature above 32c/90f. An important strategy to reduce the effects of heat stress is to provide your birds with cool water in hot weather. Interestingly, cold water in low temperatures doesn’t seem to bother them.

Conditions: Candida, Wet Fowl Pox, Canker (Trichomonosis)

The respiratory and nervous systems are connected and many diseases affect them both.

Diseases that affect the Nervous System:

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