Control and Coordination in Animals

 

Multicellular organisms have specialized organ system to coordinate their activities. Simple multicellular organism like Hydra consists of a network of nerve cells. Thus Hydra has only nervous system to coordinate its activities.

The control and coordination in higher animals (Human) takes place through combination of nervous system and hormonal system, i.e. neuro-endocrine system.

 

Nervous system of Hydra

 

Nervous System in Animals

 

Nerve cells are the fundamental unit of nervous system. They are the specialized cells responsible for carrying information across the body. A nervous system consist of millions of nerve cells communicating through neurotransmitters among themselves. It is the longest cell in the body.

Structure of Neuron

 

A neuron consist of two components

1. Cyton (Cell body): It is like a typical animal cell, which contains cytoplasm and nucleus. Small processes stretches out from the cell body called dendrons which may further branch into dendrites.

2. Axon: It is a cylindrical structure arising from cyton and branched at its terminal ends. It has an insulating and protective sheath of myelin (made of fat and protein) around it. It is also known as nerve fibre.

A Neuron (Nerve cell)

 

Receptor and Effector

 

    There are five sense organs in our body: eyes, ears, nose, tongue and skin. In a sense organ a receptor is present, which is a cell or group of cells sensitive to a particular type of stimulus (change in environment) such as light, heat, sound etc.

Types of receptors

 

The part of a body which can respond to stimulus according to the instruction sent from the nervous system is called effector. Effectors are mainly muscles and glands.

If we touch something hot (or smell something tasty) we need to detect it and this is done by specialized tips of some nerve cells located in our sense organs. This information is then transmitted to central nervous system (brain and spinal cord) from where the response of the same is conveyed to the effector organs (muscles and glands). The whole process is done by a set of 3 neurons.

Types of Neuron

 

There are three types of neurons

1. Sensory neuron: It transmits information from receptors (sense organs) towards central nervous system (brain and spinal cord).
2. Motor neuron: It transmits information from central nervous system to effectors (muscles or glands).
3. Inter or Relay neuron: It occurs in central nervous system and serves as link between other neurons.

Transmission of Nerve Impulse

 

The messages that are transmitted in the nervous system are in the form of electrical or chemical signals called nerve impulses. The nerve impulses are transmitted from one neuron to another in a nervous system. There is a microscopic gap between two neurons over which nerve impulses pass while going from one neuron to the next and it is called synapse. A chemical substance called neurotransmitter (e.g. Acetylcholine) helps in carrying nerve impulses over synapse.

Synapse

 

Steps for transmission of nerve impulse

• A chemical reaction followed by an electrical impulse sets off when a stimulus acts on a receptor.
• This impulse is transmitted from the dendrite of a sensory neuron to the terminal endings of its axon.
• At the synapse, the neurotransmitter is released which undergoes a chemical reaction resulting in initiation of a similar impulse in the next neuron.
• This impulse is again transmitted to the terminal endings of the next neuron and the process continues till it reaches the relay neuron in spinal cord and brain.
• From the brain and spinal cord arises a set of motor neurons which transmits electrical impulses in the similar way to the effectors like muscles and glands.

Action caused by Effectors due to Nervous Tissue

 

The nerve impulse received by the muscles cause action or movement (removing our hand away from a hot object). Muscles are made of muscle cells containing special proteins which change their arrangement when stimulated by electrical impulses. This causes the muscle to change shape and contract. When the muscles contract, they pull the bones and make it move.

The neuromuscular junction i.e., the area of contact between a nerve fibre and sarcolemma is similar to axon-dendron junction or synapse. When a nerve impulse reaches the end of a nerve fibre, a neurotransmitter is released and it creates a similar electrical disturbance in the muscle cell bringing about its contraction.

Neuromuscular Junction

 

To sum up, the transmission of nerve impulse and action by effectors can be shown as follows.

 

 

Human Nervous System

 

Human nervous system is one of the most highly developed nervous system among living organisms on this planet.

The human nervous system can be further divided.

 

 

(a) Central Nervous System

 

It occupies the central axis of the body and enables a person to give a suitable response to various situations.

It consists of the

1. Brain
2. Spinal Cord

 

(i)    Brain: The brain is located in a bony case (skull) called cranium or brain box. Inside the cranium, the brain is further surrounded by three membranes called meninges. The space between the membranes and brain is filled with cerebrospinal fluid (CSF). The cranium, meninges and cerebrospinal fluid protects the brain from mechanical shocks.

Human brain

 

The brain is broadly divided into three regions.

Fore Brain: It consists of

Olfactory lobes: A pair of bodies covered by cerebrum. It is not so developed in humans. It is concerned with olfaction (smell).

Cerebrum: It forms about two-third of the brain. Different areas of cerebrum perform different functions. Association areas control learning, reasoning, intelligence, personality, thinking, memory, etc. Sensory areas give us sensation by receiving information from eyes, ears, nose, tongue, skin. Motor areas give instructions to muscles for various voluntary actions.

Midbrain

It controls reflex movements of the head, neck, eye muscles, etc. in response to visual or auditory stimuli.

Hind brain: It is further sub divided

Pons: It takes part in regulating respiration.

Cerebellum: It helps in maintaining posture and balance. It also coordinates smooth body movements like walking, riding, etc.

Medulla Oblongata: It controls various involuntary actions such as heart beat, breathing, peristaltic movements, etc. Medulla also controls reflex actions like, swallowing, sneezing, vomiting, etc.

Different regions of brain showing different functions

(ii)    Spinal Cord: It is rod-like structure extending downwards in continuation with medulla. It is enclosed in a bony cage called vertebral column and is also surrounded by meninges.

It is concerned with reflex actions and conduction of nerve impulses to and from the brain.

Reflex Action and Reflex Arc

Reflex action: A reflex action may be defined as a spontaneous, automatic and mechanical response to a stimulus, acting on a specific receptor, without the will of an animal.

Blinking of eyes, moving our foot away when we step on something sharp, etc. are examples of reflex action. In reflex action we are not aware of the things which is going to happen to us. In reflex action, spinal cord is involved for quick response to specific stimulus. However for thinking process, the information also goes to the brain.

The involvement of brain in reflex action is to think about the dangerous situation and the possibility of reaction. Such reflex action which involves brain are called cerebral reflexes. For example, the contraction of pupil of our eye automatically in the presence of bright light.    

 

Reflex arc

 

Reflex arc: The path taken by nerve impulses in a reflex action is called reflex arc.

When we accidentally touch something hot, the heat is sensed by the receptors present in the skin. A nerve impulse is triggered for the same in the sensory neuron which transmits message to the spinal cord. In the spinal cord, impulse is passed to the connector neuron which in turn passes it to the motor neuron. The motor neuron transmits the instruction to a muscle of our arm. The arm muscles contracts and pulls other hand away from the hot object.

(b) Peripheral Nervous System

All the nerves arising from brain and spinal cord forms the peripheral nervous system. Nerves arising from the brain are called cranial nerves which are twelve pairs. There are 31 pairs of nerves arising from the spinal cord (spinal nerve). Cranial nerves are either sensory, motor, or mixed in nature. All the spinal nerves are mixed.

(c) Autonomic Nervous System

It comprises of a set of two (sympathetic and parasympathetic) network of nerves antagonistic to each other. These nerves are attached to the smooth muscles of internal organs and controls involuntary actions.

For example: Sympathetic nerves increase the rate of heart beat and parasympathetic decrease the rate of heart beat.