# Anatomy and Physiology of Animals/Nervous System

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## Objectives

After completing this section, you should know:

-   the role of the nervous system in coordinating an animal's response
    to the environment
-   that the nervous system gathers, sorts and stores information and
    initiates movement
-   the basic structure and functions of a neuron
-   the structure and function of a synapse and neurotransmitter
    chemicals
-   the nervous pathway known as a reflex with examples
-   that training can develop conditioned reflexes in animals
-   that the nervous system can be divided into the central and
    peripheral nervous systems
-   that the brain is surrounded by membranes called meninges
-   the basic parts of the brain and the function of the cerebral
    hemispheres, hypothalamus, pituitary, cerebellum and medulla
    oblongata
-   the structure and function of the spinal cord
-   that the peripheral nervous system consists of cranial and spinal
    nerves and the autonomic nervous system
-   that the autonomic nervous system consists of sympathetic and
    parasympathetic parts with different functions

## Coordination

Animals must be able to sense and respond to the environment in which
they live if they are to survive. They need to be able to sense the
temperature of their surroundings, for example, so they can avoid the
hot sun. They must also be able to identify food and escape predators.

The various systems and organs in the body must also be linked so they
work together. For example, once a predator has identified suitable prey
it has to catch it. This involves coordinating the contraction of the
muscle so the predator can run, there must then be an increased blood
supply to the muscles to provide them with oxygen and nutrients. At the
same time the respiration rate must increase to supply the oxygen and
remove the carbon dioxide produced as a result of this increased
activity. Once the prey has been caught and eaten, the digestive system
must be activated to digest it.

The adjustment of an animal's response to changes in the environment and
the complex linking of the various processes in the body that this
response involves are called **co-ordination**. Two systems are involved
in co-ordination in animals. These are the **nervous** and **endocrine
systems**. The first operates via electrical impulses along nerve fibres
and the second by releasing special chemicals or hormones into the
bloodstream from glands.

## Functions of the Nervous System

The nervous system has three basic functions:

:   1\. **Sensory function** - to sense changes (known as stimuli) both
    outside and within the body. For example the eyes sense changes in
    light and the ear responds to sound waves. Inside the body, stretch
    receptors in the stomach indicate when it is full and chemical
    receptors in the blood vessels monitor the acidity of the blood.

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:   2\. **Integrative function** - processing the information received
    from the sense organs. The impulses from these organs are analysed
    and stored as memory. The many different impulses from different
    sources are sorted, synchronised and co-ordinated and the
    appropriate response initiated. The power to integrate, remember and
    apply experience gives higher animals much of their superiority.

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:   3\. **Motor function** - The third function is the response to the
    stimuli that causes muscles to contract or glands to secrete.

All nervous tissue is made up of nerve cells or **neurons.** These
transmit high-speed signals called **nerve impulses**. Nerve impulses
can be thought of as being similar to an electric current.

## The Neuron

Neurons are cells that have been adapted to carry nerve impulses. A
typical neuron has a **cell body** containing a nucleus, one or more
branching filaments called **dendrites** which conduct nerve impulses
towards the cell body and one long fibre, an **axon**, that carries the
impulses away from it. Many axons have a sheath of fatty material called
**myelin** surrounding them. This speeds up the rate at which the nerve
impulses travel along the nerve (see diagram 14.1).

![](Anatomy_and_physiology_of_animals_Motor_neuron.jpg "Anatomy_and_physiology_of_animals_Motor_neuron.jpg")

Diagram 14.1 - A motor neuron

The cell body of neurons is usually located in the brain or spinal cord
while the axon extends the whole distance to the organ that it supplies.
The neuron carrying impulses from the spinal cord to the hind leg or
tail of a horse, for example, can be several feet long. A **nerve** is a
bundle of axons.

A **sensory neuron** is a nerve cell that transmits impulses from a
sense receptor such as those in the eye or ear to the brain or spinal
cord. A **motor neuron** is a nerve cell that transmits impulses from
the brain or spinal cord to a muscle or gland. A **relay neuron**
connects sensory and motor neurons and is found in the brain or spinal
cord (see diagrams 14.1 and 14.2).

![](Anatomy_and_physiology_of_animals_Relation_btw_sensory,_relay_&_motor_neurons.jpg "Anatomy_and_physiology_of_animals_Relation_btw_sensory,_relay_&_motor_neurons.jpg")

Diagram 14.2 - The relationship between sensory, relay and motor neurons

### Connections Between Neurons

The connection between adjacent neurons is called a **synapse**. The two
nerve cells do not actually touch here for there is a microscopic space
between them. The electrical impulse in the neurone before the synapse
stimulates the production of chemicals called **neurotransmitters**
(such as **acetylcholine**), which are secreted into the gap.

The neurotransmitter chemicals diffuse across the gap and when they
contact the membrane of the next nerve cell they stimulate a new nervous
impulse (see diagram 14.3). After the impulse has passed the chemical is
destroyed and the synapse is ready to receive the next nerve impulse.

![](Anatomy_and_physiology_of_animals_Magnification_of_a_synapse.jpg "Anatomy_and_physiology_of_animals_Magnification_of_a_synapse.jpg")

Diagram 14.3 - A nerve and magnification of a synapse

## Reflexes

A **reflex** is a rapid automatic response to a stimulus. When you
accidentally touch a hot object and automatically jerk your hand away,
this is a reflex action. It happens without you having to think about
it. Animals automatically blink when an object approaches the eye and
cats twist their bodies in the air when falling so they land on their
paws. (Please don't test this one at home with your pet cat!).

Swallowing, sneezing, and the constriction of the pupil of the eye in
bright light are also all reflex actions. Some other examples of reflex
actions in animals can be shivering with cold and the opening of the
mouth on hearing a sudden loud noise.

The path taken by the nerve impulses in a reflex is called a **reflex
arc**. Most reflex arcs involve only three neurons (see diagram 14.4).
The **stimulus** (a pin in the paw) stimulates the pain receptors of the
skin, which initiate an impulse in a sensory neuron. This travels to the
spinal cord where it passes, by means of a synapse, to a connecting
neuron called the relay neuron situated in the spinal cord. The relay
neuron in turn makes a synapse with one or more motor neurons that
transmit the impulse to the muscles of the limb causing them to contract
and remove the paw from the sharp object. Reflexes do not require
involvement of the brain although you are aware of what is happening and
can, in some instances, prevent them happening. Animals are born with
their reflexes. You can think of them as being wired in.

![](Anatomy_and_physiology_of_animals_A_reflex_arc.jpg "Anatomy_and_physiology_of_animals_A_reflex_arc.jpg")

Diagram 14.4 - A reflex arc

### Conditioned Reflexes

In most reflexes the stimulus and response are related. For example the
presence of food in the mouth causes the salivary glands to release
saliva. However, it is possible to train animals (and humans) to respond
to different and often quite irrelevant stimuli. This is called a
**conditioned reflex**.

A Russian biologist called Pavlov carried out the classic experiment to
demonstrate such a reflex when he conditioned dogs to salivate at the
sound of a bell ringing. Almost every pet owner can identify reflexes
they have conditioned in their animals. Perhaps you have trained your
cat to associate food with the opening of the fridge door or accustomed
your dog to the routines you go through before taking them for a walk.

## Parts of the Nervous System

When we describe the nervous system of vertebrates we usually divide it
into two parts (see diagram 14.5).

:   1\. The **central nervous system** (**CNS**) which consists of the
    brain and spinal cord.
:   2\. The **peripheral nervous system** (**PNS**) which consists of
    the nerves that connect to the brain and spinal cord (cranial and
    spinal nerves) as well as the **autonomic** (or involuntary) nervous
    system.

![](Horse_nervous_system_labelled.JPG "Horse_nervous_system_labelled.JPG")

Diagram 14.5 - The nervous system of a horse

### The Central Nervous System

The **central nervous system** consists of the brain and spinal cord. It
acts as a kind of 'telephone exchange' where a vast number of cross
connections are made.

When you look at the brain or spinal cord some regions appear creamy
white (**white matter**) and others appear grey (**grey matter**). White
matter consists of masses of nerve axons and the grey matter consists of
the cell bodies of the nerve cells. In the brain the grey matter is on
the outside and in the spinal cord it is on the inside (see diagram
14.2).

#### The Brain

The major part of the brain lies protected within the sturdy "box" of
skull called the **cranium**. Surrounding the fragile brain tissue (and
spinal cord) are protective membranes called the **meninges** (see
diagram 14.6), and a crystal-clear fluid called **cerebrospinal fluid**,
which protects and nourishes the brain tissue. This fluid also fills
four cavities or **ventricles** that lie within the brain.

Brain tissue is extremely active and, even when an animal is resting, it
uses up to 20% of the oxygen taken into the body by the lungs. The
**carotid artery**, a branch off the dorsal aorta, supplies it with the
oxygen and nutrients it requires. Brain damage occurs if brain tissue is
deprived of oxygen for only 4--8 minutes.

The brain consists of three major regions:

:   1\. the **fore brain** which includes the **cerebral hemispheres**,
    **hypothalamus** and **pituitary gland**;
:   2\. the **hind brain** or **brain stem**, contains the **medulla
    oblongata** and **pons** and
:   3\. the **cerebellum** or "little brain" (see diagram 14.6).

![](Anatomy_and_physiology_of_animals_Longitudinal_section_through_brain_of_a_dog.jpg "Anatomy_and_physiology_of_animals_Longitudinal_section_through_brain_of_a_dog.jpg")

Diagram 14.6 - Longitudinal section through the brain of a dog

##### Mapping the brain

In humans and some animals the functions of the different regions of the
cerebral cortex have been mapped (see diagram 14.7).

Diagram 14.7 - The functions of the regions of the human cerebral cortex

##### The Forebrain

The **cerebral hemispheres** are the masses of brain tissue that sit on
the top of the brain. The surface is folded into ridges and furrows
called **sulci** (singular sulcus). They make this part of the brain
look rather like a very large walnut kernel. The two hemispheres are
separated by a deep groove although they are connected internally by a
thick bundle of nerve fibres. The outer layer of each hemisphere is
called the **cerebral cortex** and this is where the main functions of
the cerebral hemispheres are carried out.

The cerebral cortex is large and convoluted in mammals compared to other
vertebrates and largest of all in humans because this is where the
so-called "higher centres" concerned with memory, learning, reasoning
and intelligence are situated.

Nerves from the eyes, ears, nose and skin bring sensory impulses to the
cortex where they are interpreted. Appropriate voluntary movements are
initiated here in the light of the memories of past events.

Different regions of the cortex are responsible for particular sensory
and motor functions, e.g. vision, hearing, taste, smell, or moving the
fore-limbs, hind-limbs or tail. For example, when a dog sniffs a scent,
sensory impulses from the organ of smell in the nose pass via the
olfactory (smelling) nerve to the olfactory centres of the cerebral
hemispheres where the impulses are interpreted and co-ordinated.

In humans and some animals the functions of the different regions of the
cerebral cortex have been mapped (see diagram 14.8).

![](Anatomy_and_physiology_of_animals_Functions_of_the_regions_of_the_cerebral_cortex.jpg "Anatomy_and_physiology_of_animals_Functions_of_the_regions_of_the_cerebral_cortex.jpg")

Diagram 14.8 - The functions of the regions of the cerebral cortex

The **hypothalamus** is situated at the base of the brain and is
connected by a "stalk" to the **pituitary gland**, the "master"
hormone-producing gland (see chapter 16). The hypothalamus can be
thought of as the bridge between the nervous and endocrine (hormone
producing) systems. It produces some of the hormones that are released
from the pituitary gland and controls the release of others from it.

It is also an important centre for controlling the internal environment
of the animal and therefore maintaining homeostasis. For example, it
helps regulate the movement of food through the gut and the temperature,
blood pressure and concentration of the blood. It is also responsible
for the feeling of being hungry or thirsty and it controls sleep
patterns and sex drive.

##### The Hindbrain

The **medulla oblongata** is at the base of the brain and is a
continuation of the spinal cord. It carries all signals between the
spinal cord and the brain and contains centres that control vital body
functions like the basic rhythm of breathing, the rate of the heartbeat
and the activities of the gut. The medulla oblongata also co-ordinates
swallowing, vomiting, coughing and sneezing.

##### The Cerebellum

The **cerebellum** (little brain) looks rather like a smaller version of
the cerebral hemispheres attached to the back of the brain. It receives
impulses from the organ of balance (vestibular organ) in the inner ear
and from stretch receptors in the muscles and tendons. By co-ordinating
these it regulates muscle contraction during walking and running and
helps maintain the posture and balance of the animal. When the
cerebellum malfunctions it causes a tremor and uncoordinated movement.

#### The Spinal Cord

The spinal cord is a cable of nerve tissue that passes down the channel
in the vertebrae from the hindbrain to the end of the tail. It becomes
progressively smaller as paired **spinal nerves** pass out of the cord
to parts of the body. Protective membranes or meninges cover the cord
and these enclose cerebral spinal fluid (see diagram 14.9).

![](Anatomy_and_physiology_of_animals_The_spinal_cord.jpg "Anatomy_and_physiology_of_animals_The_spinal_cord.jpg")

Diagram 14.9 - The spinal cord

If you cut across the spinal cord you can see that it consists of white
matter on the outside and grey matter in the shape of an H or butterfly
on the inside.

### The Peripheral Nervous System

The **peripheral nervous system** consists of nerves that are connected
to the brain (**cranial nerves**), and nerves that are connected to the
spinal cord (**spinal nerves**). The **autonomic nervous system** is
also part of the peripheral nervous system.

#### Cranial Nerves

There are twelve pairs of cranial nerves that come from the brain. Each
passes through a hole in the cranium (brain case). The most important of
these are the olfactory, optic, acoustic and vagus nerves.

The **olfactory nerves** - (smell) carry impulses from the olfactory
organ of the nose to the brain.

The **optic nerves** - (sight) carry impulses from the retina of the eye
to the brain.

The **auditory (acoustic) nerves** - (hearing) carry impulses from the
cochlear of the inner ear to the brain.

The **vagus nerve** - controls the muscles that bring about swallowing.
It also controls the muscles of the heart, airways, lungs, stomach and
intestines (see diagram 14.5).

#### Spinal Nerves

**Spinal nerves** connect the spinal cord to sense organs, muscles and
glands in the body. Pairs of spinal nerves leave the spinal cord and
emerge between each pair of adjacent vertebrae (see diagram 14.9).

The **sciatic nerve** is the largest spinal nerve in the body (see
diagram 14.5). It leaves the spinal cord as several nerves that join to
form a flat band of nervous tissue. It passes down the thigh towards the
hind leg where it gives off branches to the various muscles of this
limb.

#### The Autonomic Nervous System

The **autonomic nervous system** controls internal body functions that
are not under conscious control. For example when a prey animal is
chased by a predator the autonomic nervous system automatically
increases the rate of breathing and the heartbeat. It dilates the blood
vessels that carry blood to the muscles, releases glucose from the
liver, and makes other adjustments to provide for the sudden increase in
activity. When the animal has escaped and is safe once again the nervous
system slows down all these processes and resumes all the normal body
activities like the digestion of food.

The nerves of the autonomic nervous system originate in the spinal cord
and pass out between the vertebrae to serve the various organs (see
diagram 14.10).There are two main parts to the autonomic nervous
system---the **sympathetic system** and the **parasympathetic system**.

The **sympathetic system** stimulates the "flight, fright, fight"
response that allows an animal to face up to an attacker or make a rapid
departure. It increases the heart and respiratory rates, as well as the
amount of blood flowing to the skeletal muscles while blood flow to less
critical regions like the gut and skin is reduced. It also causes the
pupils of the eyes to dilate. Note that the effects of the sympathetic
system are similar to the effects of the hormone adrenaline (see Chapter
16).

The **parasympathetic system** does the opposite to the sympathetic
system. It maintains the normal functions of the relaxed body. These are
sometimes known as the "housekeeping" functions. It promotes effective
digestion, stimulates defaecation and urination and maintains a regular
heartbeat and rate of breathing.

![](Anatomy_and_physiology_of_animals_Function_of_the_sympathetic_&_parasympathetic_nervous_systems.jpg "Anatomy_and_physiology_of_animals_Function_of_the_sympathetic_&_parasympathetic_nervous_systems.jpg")

Diagram 14.10 - The function of the sympathetic and parasympathetic
nervous systems

## Summary

-   The **neuron** is the basic unit of the nervous system. It consists
    of a **cell body** with a nucleus, filaments known as **dendrites**
    and a long fibre known as the **axon** often surrounded by a
    **myelin sheath**.
-   A **nerve** is a bundle of axons.
-   **Grey matter** in the brain and spinal cord consists mainly of
    brain cells while **white matte**r consists of masses of axons.
-   **Nerve Impulses** travel along axons.
-   Adjacent neurons connect with each other at **synapses**.
-   **Reflexes** are automatic responses to stimuli. The path taken by
    nerve impulses involved in reflexes is a **reflex arc**. Most reflex
    arcs involve 3 neurons - a **sensory neuron**, a **relay neuron**
    and a **motor neuron**. A stimulus, a pin in the paw for example,
    initiates an impulse in the sensory neuron that passes via a synapse
    to the relay neuron situated in the spinal cord and then via another
    synapse to the motor neurone. This transmits the impulse to the
    muscle causing it to contract and remove the paw from the pin.
-   The nervous system is divided into 2 parts: the **central nervous
    system**, consisting of the brain and spinal cord and the
    **peripheral nervous system** consisting of nerves connected to the
    brain and spinal cord. The **autonomic nervous system** is
    considered to be part of the peripheral nervous system.
-   The brain consists of three major regions: 1. the **fore brain**
    which includes the **cerebral hemispheres** (or **cerebrum**),
    **hypothalamus** and **pituitary gland**; 2. the **hindbrain** or
    **brain stem** containing the **medulla oblongata** and 3. the
    **cerebellum**.
-   Protective membranes known as the **meninges** surround the brain
    and spinal cord.
-   There are 12 pairs of cranial nerves that include the optic,
    olfactory, acoustic and **vagus** nerves.
-   The **spinal cord** is a cable of nerve tissue surrounded by
    meninges passing from the brain to the end of the tail. **Spinal
    nerves** emerge by a **ventral** and **dorsal root** between each
    vertebra and connect the spinal cord with organs and muscles.
-   The **autonomic nervous system** controls internal body functions
    not under conscious control. It is divided into 2 parts with 2
    different functions: the **sympathetic nervous system** that is
    involved in the flight and fight response including increased heart
    rate, bronchial dilation, dilation of the pupil and decreased gut
    activity. The **parasympathetic nervous system** is associated with
    decreased heart rate, pupil constriction and increased gut activity.

## Worksheet

Nervous System
Worksheet

## Test Yourself

1\. Add the following labels to this diagram of a motor neuron.

:   cell body \| nucleus \| axon \| dendrites \| myelin sheath \| muscle
    fibres

![](Anatomy_and_physiology_unlabeled_neuron.jpg "Anatomy_and_physiology_unlabeled_neuron.jpg")

2\. What is a synapse?

3\. What is a reflex?

4\. Rearrange the parts of a reflex arc given below in the order in
which the nerve impulse travels from the sense organ to the muscle.

:   sense organ \| relay neuron \| motor neuron \| sensory neuron \|
    muscle fibres

5\. Add the following labels to the diagram of the dog's brain shown
below.

:   cerebellum \| cerebral hemisphere \| cerebral cortex \| pituitary
    gland \| medulla oblongata

![](Anatomy_and_physiology_unlabeled_LS_dog's_brain.jpg "Anatomy_and_physiology_unlabeled_LS_dog's_brain.jpg")

6\. What is the function of the meninges that cover the brain and spinal
cord

7\. Give 3 effects of the action of the sympathetic nervous system.

/Test Yourself Answers/

## Websites

-   <http://en.wikipedia.org/wiki/Neuron> Wikipedia. Lots of good
    information here but as usual a warning that there are terms and
    concepts that are beyond the scope of this course. Also try 'reflex
    action' ; 'autonomic nervous system' ;
-   <http://images.google.co.nz/imgres?imgurl=http://static.howstuffworks.com/gif/brain-neuron.gif&imgrefurl=http://science.howstuffworks.com/brain1.htm&h=296&w=394&sz=17&hl=en&start=5&tbnid=LWLRI9lW_5PZhM:&tbnh=93&tbnw=124&prev=/images%3Fq%3Dneuron%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN>
    How Stuff Works. This site is for the neuron but try 'neuron types',
    'brain parts' and 'balancing act' too.
-   <http://web.archive.org/web/20060821134839/http://www.bbc.co.uk/schools/gcsebitesize/flash/bireflexarc.swf>
    Reflex Arc. Nice clear and simple animation of a reflex arc.

## Glossary

-   Link to
    Glossary

