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In this lesson we’ll take a look at the structure and function of the nervous system of the bony fish. Bony fish, as opposed to cartilaginous fish, are within the Class Osteichthyes.

How Do Osteichthyes Sense Their Environment?

Think of a fish, any fish at all (well, with the exception of a shark, a skate, or a ray).

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Odds are you are thinking of a member of the group Osteichthyes.Osteichthyes (oss-tee-ICK-thees), or bony fish, are a major group of fish that possess a bony skeleton. The Greek root ‘ostei-‘ means ‘bone’. Many modern fish fall within this group, though they are distantly related to their cousins the cartilaginous fish, or Chondrichthyes (con-DRICK-thees), which include sharks, skates, and rays.Just like humans and most vertebrates, bony fish have a nervous system comprised of a central brain and spinal cord, as well as many branching nerves. This is what allows them to sense the things around them.Let’s delve into the structure and function of the central nervous system in bony fish – the brain and spinal cord – as well as the peripheral nervous system – branching nerves that extend throughout the body.

Central Nervous System Structure and Function

The central nervous system of osteichthyes is comprised of a brain and a spinal cord, just like our own central nervous system.Most fish brains are very small compared to overall body size, about 1/15th the mass of a similarly-sized mammal or bird.Other bony fish, such as the freshwater elephant fish (Family Mormyridae), have exceptionally large brains in comparison to overall body size.Regardless of brain size, osteichthyes’ brains all follow a basic plan.

The basic plan of the brain of osteichthyes.

Osteichthyes brain

At the rostral, or nose, end of the fish lie olfactory lobes, which provide the sense of smell. Some species of bony fish have exceptionally large olfactory lobes, particularly catfish and other predators that hunt by smell.

Behind the olfactory lobes sits the telencephalon, which is equivalent to the cerebrum in most other vertebrates. Our cerebrum is the part of our brain that is allowing us to read this sentence. Together, the olfactory lobes and the telencephalon comprise the forebrain of osteichthyes fish.The forebrain connects to the midbrain by the diencephalon, a hormone-balancing structure. The diencephalon is associated with the pineal body, which detects light and dark and coordinates color changes.

The midbrain (mesencephalon) itself is comprised of two optic lobes, which are especially well-developed in osteichthyes that hunt by sight.At the caudal, or back, end of the brain lies the hindbrain, or metencephalon. The hindbrain contains the cerebellum, the part of the brain that controls movement and balance in humans. In osteichthyes fish the cerebellum has a similar function, coordinating balance and controlling the movements that help fish swim. In many fish, the cerebellum is the largest part of the brain.The hindbrain connects to the spinal cord via the myelencephalon, which functions in osmoregulation – water balance – and respiration.

A fish’s spinal cord transmits motor messages to its peripheral nerves, and sends sensory messages back to the brain. As we will see, the various nerves of the peripheral nervous system branch throughout the body.

Peripheral Nervous System Structure and Function

A fish in the group Osteichthyes has a number of peripheral nerve adaptations that allow it to delicately sense the watery environment in which it lives.

There are many nerves that branch off from the spinal cord, bringing sensory messages in from the skin surface (somatic sensory) and sending motor messages outward to move muscles (somatic motor). In the same way, visceral sensory and visceral motor neurons connect to the various viscera, or organs, of the fish.

Sensory Organs

Osteichthyes have specialized taste buds, located either in their mouth or along whisker-like barbels.

Barbels on a koi fish, a member of Osteichthyes
barbels on koi osteichthyes fish

The lateral line system of nerves provides osteichthyes with a sense of hearing, in the form of vibrations in the water, which can help osteichthyes avoid predators and detect prey.

Lateral line system in a trout, a member of Osteichthyes
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