Fish Nervous System: How The Brain & Nerves Work

 

Fish have a relatively simple nervous system as compared to humans.

Having said that, their nervous system has helped them evolve and thrive on earth’s oceans for over 500 million years.

The nervous system of fish is composed of the brain and a series of nerves that run along the length of the body.

Like any creature, it is the central operating unit and helps the fish coordinate its actions.

In this article, we will show you everything you need to know about the fish nervous system.

From its basic anatomy to how it works, we have got you covered!

The Brain and Nervous System of Fish

The nervous system of fish (much like ours) is composed of a central coordinating brain, a spinal cord, and many, many nerves.

These nerves send electrical impulses to and from the brain, telling the fish what to do.

The brain of a fish is relatively small when compared to the human brain. In fact, the brain of a goldfish is roughly the same size as its eye.

You would think that such a small brain wouldn’t be able to control the complicated body of a fish very well, but it does a pretty good job.

The Fish Brain

Generally speaking, fish have small brains in relation to their overall body weight.

While the brains of some fish (like the great white shark brain) can weigh up to 35 grams (0.008% of their total body weight), the average fish’s brain only weighs about 0.0014% of their total body weight.

Different fish have different-sized brains. However, does brain size matter when it comes to fish and intelligence?

Knowing the answer to this question can help us understand how fish think and process information.

Does the Brain Size of a Fish Determine its Intelligence?

When comparing Elasmobranchs (Sharks and Rays) to Teleosts (Bony Fish), Elasmobranchs tend to have a larger brain for the same body mass.

While this doesn’t necessarily mean they are more intelligent, it does suggest they have a more complex nervous system.

A Pike (Essox lucius) has a brain that amounts to only 0.077 percent of its total body mass.

This means that when you catch a 5-kilogram Pike, you have outsmarted a fish with a brain weighing 3.85 grams – or one-seventh of an ounce.

In comparison, a Burbot (Lota lota) is a little smarter because it has a brain that is 0.13 percent of its body weight.

Meaning a 5-kilo specimen would have a 6.5-gram brain. However, these fish specimens aren’t really that intelligent and their behavior is mostly driven by instinct.

Most fish have brains that are less than 1 percent of their body weight, but not all.

At the other end of the continuum of fish intelligence is the electric Elephantnose fish. With a brain that is 3.1 percent of its body weight, it is a lot smarter than the Pike.

If it grew to weigh 5 kilos, its brain would weigh 155 grams, which would make it 40 times as large as that of a Pike of similar weight.

Humans (on average) have a brain that weighs in at around 2.3 percent of their body weight. A five-kilo human would have a brain of only 115 grams, which is smaller than that of the Elephantnose Fish.

However, does this mean Elephantnose Fish are more intelligent than humans? Of course not.

Intelligence is more than just brain mass or even brain-to-body ratio.

Like humans, mice brains also weigh about 2% of their body weight, and some ant species’ brains weigh about 15% of their body weight.

While large brains do make it easier to be smart, it is the complexity of the brain and the relative proportion of the forebrain that determines intelligence.

The most complex and most forebrain-dominated brains on this planet belong to dolphins and humans.

Nevertheless, we can see that some fish are much smarter than others, and this is due to the complexity of their brain structures.

Fish Brain Parts and Functions

You may not think that fish have a very complicated brain, but they do. In fact, the fish brain is divided into several regions, each of which controls specific functions.

The Medulla

Among the fish brain parts, the medulla is the most primitive. It controls basic functions such as heartbeat and respiration.

Blood pressure, digestion, and even waste disposal are all regulated by the medulla.

It is also a relay center for many nerves, sending messages to and from the mid and/or forebrain.

The Cerebellum

This part of the brain controls motor coordination (but it does not initiate motor activities).

This means it controls the timing and interaction of muscles once a muscular action has been initiated.

The cerebellum is also important in maintaining equilibrium. Motor nerves from the brain pass through the cerebellum on their way to the spinal cord.

The Mid-brain

A fish’s midbrain is mostly made up of optic lobes, which vary greatly in size depending on the species’ dependence on sight.

This means that fish that hunt in murky waters or at night will have larger optic lobes than those that don’t.

The midbrain is also home to the corpora quadrigemina, a group of structures that help the fish process visual information.

The mid-brain is responsible for sorting out incoming information and it is also the main center of learning (whereas in mammals, it is the forebrain that is the main center of learning).

In some species, the optic lobes may be so large that they completely cover the forebrain.

The Forebrain

The forebrain of fish is dominated by the olfactory lobes or bulbs. These are much larger in fish that hunt by smell (such as sharks) than in those that don’t.

Fishes are attracted to certain kinds of smell, and their heightened sense of smell allows them to hunt for food.

The forebrain is also responsible for higher functions such as memory, learning, and decision-making.

Teleosts or bony fish have smaller olfactory lobes because sight is often the most important sense for them.

A cerebrum or pair of cerebral hemispheres exists in many elasmobranchs and some teleosts.

These appear to be primarily concerned with the sense of smell as well (in mammals, the cerebrum is much larger and involved in planning and learning).

The pituitary gland, which also arises from the forebrain, plays an important role in the regulation of the metabolism of fish.

The Meninges

Finally, the brain (along with the gel) is surrounded by a membrane that helps prevent foreign matter and microorganisms from entering the brain.

This membrane is called the meninges. Without this protective barrier, the brain would be susceptible to infection.

The Fish Spinal Cord

The spinal cord is similar in function to the brain stem in that it controls basic functions such as heartbeat and respiration. It also relays messages to and from the brain.

The main difference between the two is that the spinal cord is much simpler than the brain stem.

When it comes to fishes, the spinal cord, or nerve cord, is similar in all species.

It is a thick sheath of nervous material that runs from the base of the brain, back along the fish’s body, through – and protected by – the neural canal of the spinal column.

Normally it extends the full length of the fish’s body, but a notable exception to this is the giant Sunfish (Mola mola), wherein the spinal cord is actually shorter than the brain.

It serves as the basis of many simple responses and as the major link to the brain for sensory input and brain-mediated responses.

Internal organs such as the heart and gills are also controlled by the spinal cord, with messages relayed to and from these organs via the sympathetic and parasympathetic nervous systems.

In addition, some fish have specialized cells in the spinal cord that can generate an electrical impulse known as a “spinal reflex.”

This is an involuntary response to a stimulus that does not require input from the brain and can be useful in allowing the fish to swim quickly away from predators or other threats.

Nerve Pairs in Fish

Apart from the brain and the spinal cord, the fish body is supplied with a vast network of nerves: the electric wires of the body along which messages travel.

Fish pain receptors, for instance, are located along the length of the body, and when they are stimulated (by a bump or a cut, for example), they send a message to the brain via the spinal cord telling it that something is wrong.

Nerves are made up of many neurons, and neurons are a one-way system. Messages travel along a specific neuronal path to or from the brain (or spinal cord), but never both ways.

The nerves that emerge from the spinal cord are known as spinal nerves, while those that emerge from the brain are known as cranial nerves.

Normally there is one pair of spinal nerves (left and right) for each vertebra.

Thus, long thin fish with many vertebrae, such as eels, will have many more pairs of spinal nerves than a much shorter fish such as a gobi.

In fish, there are 10 pairs of cranial nerves, all with well-defined roles.

Below, we have listed the 10 pairs of cranial nerves in fish and their primary functions:

  • Nerve Pair 1: Sensory connecting the nasal organs to the olfactory lobes.
  • Nerve Pair 2: Sensory connecting the eyes with the optic lobes.
  • Nerve Pair 3: Connecting to muscles.
  • Nerve Pair 4: Connecting to muscles.
  • Nerve Pair 5: Mixed, part sensory part muscles.
  • Nerve Pair 6: Connecting to muscles.
  • Nerve Pair 7: Mixed, part sensory part fish’s muscles.
  • Nerve Pair 8: Sensory, connects the brain with the inner ear (see hearing in fish), important for balance.
  • Nerve Pair 9: Sensory, connects the brain with the gills and the palate of the mouth.
  • Nerve Pair 10: Mixed, intestines, gills, heart, and lateral line.

 

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Gordon Ramel

Gordon is an ecologist with two degrees from Exeter University. He's also a teacher, a poet and the owner of 1,152 books. Oh - and he wrote this website.

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