Introduction
As the complexity of the individuals, plants or animals increases the different cells and organs become separated from each other by greater distance. Thus it becomes necessary to have a system by which the different parts of the organisms can function as a single unit.
This is possible only if the different parts can coordinate with each other and carry out a particular function.
To carry out a simple function such as picking up an object from the ground there has to be coordination of the eyes, hands, legs and the vertebral column.
The eyes have to focus on the object, the hands have to pick it up and grasp it, the legs have to bend and so does the back bone (vertebral column).
All these actions have to be coordinated in such a manner that they follow a particular sequence and the action is completed. A similar mechanism is also needed for internal functions of the body.
The individuals also have to adjust to the changing conditions around them and vary their responses. At the same time, the internal conditions of the body should be maintained constant.
This is called homeostasis. Homeostasis is derived from 'homeo' meaning same and 'stasis' meaning standing still. The internal conditions of the body are maintained at a constant by controlling the physiology of the organism.
Just as in animals, plants also have to control and coordinate their various functions.
NERVOUS SYSTEM
The nervous system of an animal is composed of
i) Specialized cells called neurons or nerve cells which can detect, receive and transmit different kinds of stimuli.
ii) The nerve fibres which are certain bundles of extended processes of nerve cells.
NERVE CELLS: Nerve cells or neurons are the structural and functional units of the nervous system. Billions of nerve cells make up our brain.A nerve cell is a microscopic structure consisting of three major parts namely cell body, dendrites and axon.
CELL BODY: It is the cell structure irregular in shape or polyhedral structure, it is also called as cyton. Cell body contains cytoplasm with typical cell organelles and certain granular bodies are called Nissl granules.
DENDRITES: Dendrites or Dendrons are shorter fibres which branch repeatedly and project out of the cell body. Dendrites transmit electrical impulses towards the cyton.
AXON: One of the fibres arising from the cell body is very long with a branched distal end and it is called as Axon.
The distal branches terminate as bulb like structures called synaptic knob filled with chemicals called neuro transmitters. Axon contains axoplasm inside and is covered by a membrane called neurilemma. Neurilemma encloses the axon except at the branched distal ends. In some neurons called myelinated neurons an additional white fatty fibre called myelin sheath covers the neurilemma. Myelin sheath is not continuous over the neurilemma. The gaps left by the myelin sheath on the axon are called Nodes of Ranvier. Over the myelin sheath are found certain cells called Schwann cells.
TYPES OF NERVE CELLS
a) Myelinated or Medullated or White neurons: When the axon is enclosed by the white fatty myelin cover, it is called Myelinated or Medullated or White neurons. This forms the cerebral cortex of our brain.
b) Non- Myelinated or Non-Medullated or Grey neurons: This neuron is not enclosed by myelin sheath, so it appears greyish in colour.The axon is covered by only neurilemma and Schwann cells. This type of neuron is found in the white matter of cerebrum.
c) Unipolar neurons: The embryonic nervous tissue contains unipolar neurons. An unipolar neuron has a nerve cell body with a single process or fibre, which will act both as axon and dendron.
d) Bipolar neurons: The sensory hair cells of the sense organs like rods and cones of retina are made up of bipolar neurons. Each bipolar neuron has a cell body and two process at the ends, one acting as axon and the other acting as Dendron.
e) Multipolar neuron: The cerebral cortex contains the multipolar neurons; each multipolar neuron has a cell body with many dendrites and an axon.
Synapse: The dendrites and the synaptic knobs of the axons of neighbouring neurons are in physical contact with one another without fusing. This point of contact between the neighbouring nerve cells is called synapse.
NERVE IMPULSE
The conduction of stimuli by the nerve cells is called nerve impulse. The dendrites will receive the stimuli from the receptor (sense organ) and conduct the same as electrical impulse to the axon through the cyton. At the synapse, the synaptic knobs release out chemical substances called neuro transmitters which convert the electrical impulse into chemical impulse and pass it to the neighbouring neuron.
TYPES OF NEURON
a) Sensory neuron: These neurons receive signals from a sense organ.
b) Motor neuron: These neurons send signals to a muscle or a gland.
C) Association neuron: These neurons relay the signals between sensory neuron and motor neuron.
Pathways: From stimulus to response
In the holding stick activity you observed that there is coordination between eye and finger. Different pathways are taken by nerves to bring about this coordinated activity.
On the basis of pathways followed, nerves are classified mainly into three different types.
AFFERENT NEURONS: Afferent (or ferrying towards) which carry messages towards the central nervous system (spinal cord or brain) from nerve endings on the muscles of different sense organs that sense the change in surroundings are called stimulus detectors. These are also called ‘sensory’ nerves.
EFFERENT NEURON: Efferent (or ferrying away) which carry messages from the central nervous system to parts that shall carry out the response or the effectors (nerve endings). They are also called ‘motor’ nerves.
ASSOCIATION NEURON: Association neurons, which link together the afferent and efferent nerves.
NERVOUS SYSTEM IN HUMANS
The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else.
The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery—meaning beyond the brain and spinal cord.
Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.
The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The somatic nervous system (SNS) is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells “Boo!” you will be startled and you might scream or leap back. You didn’t decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as “habit learning” or “procedural memory”).
The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue.
The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.
There is another division of the nervous system that describes functional responses. The enteric nervous system (ENS) is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the PNS, and is not dependent on the CNS. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion.
There are some differences between the two, but for our purposes here there will be a good bit of overlap. See Figure for examples of where these divisions of the nervous system can be found.
REFLEX ACTION
Reflex action is a special case of involuntary movement in voluntary organs. When a voluntary organ is in the vicinity of a sudden danger, it is immediately pulled away from the danger to save itself.
For example; when your hand touches a very hot electric iron, you move away your hand in a jerk. All of this happens in flash and your hand is saved from the imminent injury. This is an example of reflex action.
Reflex Arc: The path through which nerves signals, involved in a reflex action, travel is called the reflex arc. The following flow chart shows the flow of signal in a reflex arc.
The receptor is the organ which comes in the danger zone. The sensory neurons pick signals from the receptor and send them to the relay neuron. The relay neuron is present in the spinal cord. The spinal cord sends signals to the effector via the motor neuron. The effector comes in action moves the receptor away from the danger.
The reflex arc passes at the level of the spinal cord and the signals involved in reflex action do not travel up to the brain. This is important because sending signals to the brain would involve more time. Although every action is ultimately controlled by the brain, the reflex action is mainly controlled at the level of spinal cord.
Muscular Movements and Nervous Control: Muscle tissues have special filaments; called actin and myosin. When a muscle receives a nerve signal; a series of events is triggered in the muscle. Calcium ions enter the muscle cells. It results in actin and myosin filaments sliding towards each other and that is how a muscle contracts. Contraction in a muscle brings movement in the related organ.
HUMAN BRAIN
Human brain is a highly complex organ; which is mainly composed of the nervous tissue. The tissues are highly folded to accommodate a larger surface area in less space. The brain is covered by a three layered system of membranes, called meninges.
Cerebrospinal fluid is filled between the meninges. The CSF provides cushion to the brain against mechanical shocks.
Furthermore, the brain is housed inside the skull for optimum protection.
The human brain is be divided into three regions, viz. forebrain, midbrain and hindbrain.
PARTS OF HUMAN BRAIN
👉Forebrain: It is composed of the cerebrum.
👉🏿Midbrain: It is composed of the hypothalamus.
👉🏻Hindbrain: It is composed of the cerebellum, pons and medulla oblongata.
Some main structures of the human brain are explained below.
Cerebrum: The cerebrum is the largest part in the human brain. It is divided into two hemispheres, called cerebral hemispheres.
FUNCTIONS OF CEREBRUM:
a) The cerebrum controls the voluntary motor actions.
b) It is the site of sensory perceptions; like tactile and auditory perceptions.
c) It is the seat of learning and memory.
Hypothalamus: The hypothalamus lies at the base of the cerebrum. It controls sleep and wake cycle (circadian rhythm) of the body. It also controls the urges for eating and drinking.
Cerebellum: Cerebellum lies below the cerebrum and at the back of the whole structure. It coordinates the motor functions. When you are riding your bicycle; the perfect coordination between your pedaling and steering control is achieved by the cerebellum.
Medulla: Medulla forms the brain stem; along with the pons. It lies at the base of the brain and continues into the spinal cord. Medulla controls various involuntary functions like heart beat, respiration, etc.
COORDINATION IN PLANTS
Unlike animals, plants do not have a nervous system. Plants use chemical means for control and coordination. Many plant hormones are responsible for various kinds of movements in plants. Movements in plants can be divided into two main types, viz. tropic movement and nastic movement.
TROPIC MOVEMENT: The movements which are in a particular direction in relation to the stimulus are called tropic movements. Tropic movements happen as a result of growth of a plant part in a particular direction. There are four types of tropic movements, viz. geotropic, phototropic, hydrotropic and thigmotropic.
👉🏻Geotropic Movement: The growth in a plant part in response to the gravity is called geotropic movement. Roots usually show positive geotropic movement, i.e. they grow in the direction of the gravity. Stems usually show negative geotropic movement.
👉🏾Phototropic Movement: The growth in a plant part in response to light is called phototropic movement. Stems usually show positive phototropic movement, while roots usually show negative phototropic movement.
If a plant is kept in a container in which no sunlight reaches and a hole in the container allows some sunlight, the stem finally grows in the direction of the sunlight. This happens because of a higher rate of cell division in the part of stem which is away from the sunlight. As a result, the stem bends towards the light. The heightened rate of cell division is attained by increased secretion of the plant hormone auxin in the part which is away from sunlight.
👉🏿Hydrotropic Movement: When roots grow in the soil, they usually grow towards the nearest source of water. This shows a positive hydrotropic movement.
👉Thigmotropic Movement: The growth in a plant part in response to touch is called thigmotropic movement. Such movements are seen in tendrils of climbers. The tendril grows in a way so as it can coil around a support. The differential rate of cell division in different parts of the tendril happens due to action of auxin.
NASTIC MOVEMENT
The movements which do not depend on the direction from the stimulus acts are called nastic movement.
For example: when someone touches the leaves of mimosa, the leaves droop. The drooping is independent of the direction from which the leaves are touched.
Such movements usually happen because of changing water balance in the cells. When leaves of mimosa are touched, the cells in the leaves lose water and become flaccid, resulting in drooping of leaves.
Some Plant Hormones: Auxin, gibberellins and cytokinin promote growth in plant parts.
Abscissic acid inhibits growth in a particular plant part.
