Control And Coordination
Different organs work together in an organism to carry out different functions, this is known as coordination. Proper control and coordination is necessary to carry out essential functions of the life.
Coordination is mainly of two types:
1. Nervous coordination
2. Chemical coordination
Nervous Coordination in animals: Nervous system
The structural and functional unit of nervous system.
Neuron (nerve cell) is the longest cell of human body (up to 100 cm)
Neuron is made up of -
(i) Cell body
(ii) Cell processes (axon and dendron)
(i) Cell body: or Cyton or Soma or Perikaryon
- It contains granular cytoplasm which is called neuroplasm.
- Many small fibrils are present in the neuroplasm called neurofibrils for the conduction of nerve impulses.
- Rough endoplasmic reticulum coils around the ribosome form a granule like structure called as Nissl's granule or Tigroid body.
- Nissl's granule is the centre of protein synthesis.
- Energy for conduction of nerve impulses is provided by numerous mitochondria.
- Except centriole, all other cell organelles are found in neuroplasm.
(ii) Cell process:
- It is longest cell process of cyton, its diameter is uniform and contains axoplasm.
- Axoplasm of axon contains only neurofibrils and mitochondria.
- Nissl's granules are absent.
- Axon is covered by axolemma.
- Axolemma may be covered by a layer of phospholipids which is called as medulla or myelin sheath.
- Myelin sheath acts as insulator and prevents leakage of ions.
- Myelin sheath is discontinuous around the axon. These interruptions where axon is uncovered by myelin sheath are called nodes of Ranvier.
- Axon produces centrifugal conduction i.e. nerve impulse travels away from the cell body.
- The terminal ends of axon are branched which are called telodendria.
- Each telodendron ends in a swollen knob called synaptic knob.
- Nerve fibres in which myelin sheath is present, are called medullated or myelinated nerve fibres and nerve fibres without myelin sheath, are called non-medullated or non-myelinated nerve fibres.
Axon is functional part of nerve cell, therefore term "nerve fibre" usually refers to axon.
- It is small cell process.
- It's fine branches are called 'dendrites'
- Dendron receive the stimuli and produce centripetal conduction i.e. nerve impulse travels towards the cell body.
- It is not covered by myelin sheath.
Differences between Axon and Dendron
|2||Number||Either absent or one||Either absent or one, mostly many|
|3||Diameter||Uniform||Non - Uniform|
|5||Terminal knobs (Telodendrin)||Present||Absent|
|8||Direction of nerve impulse||Away from cyton||Towards cyton|
Differences between medullated and non-medullated nerve fibre
|S.No.||Features||Medullated nerve fibre||Non-medullated nerve fibre|
|1||Occurence||White Matter||Grey Matter|
|2||Sheath||Two: inner medullary, outer neurilemma||Only neurilemma|
|3 ||Node of Ranvier||Present||Absent|
|4||Speed of nerve impulse||Faster||Slower|
Type of Neurons
Synapse: The junction between two adjacent neurons i.e. between the axon ending of one neuron and dendrites of the next.
Fig: Synapse between 2 neurons
Nerve impulse: It is an electro-chemical information (signal) passing through neuron.
Neurotransmitters or Neurohormones- Chemical substances which either transmit or inhibit the message from one neuron to another.
|Stimulatory Neurotransmitters||Inhibitory Neurotransmitters|
|Stimulate impulse at synapse||Inhibit impulse at synapse|
|e.g. - Acetyl choline (ACh)|
e.g. - GABA
(Gamma Amino Butyric Acid)
Fig: Neuro muscular junction
Fig: Transmission of nerve impulse across synapseWorking of neuron or Transmission of nerve impulse:
- Stimuli are detected by dendrites of receptor nerve cells located at our sense organs i.e. ear, eyes, nose, tongue and skin.
- A chemical reaction occurs and creates electric impulse.
- Impulse travels from dendrites and finally reaches axon endings (synaptic knobs)
- Impulse releases some chemicals like Acetylcholine from synaptic knob.
- By these chemicals, impulse transmits across synapse.
- This initiates similar electric impulse in a dendrite of next neuron and thus impulse is transferred from one nerve cell to another.
- Message is sent to CNS (brain & spinal cord) via sensory nerves.
- CNS sends message to muscles via motor nerves.
- Muscles of effector organ show response.
|Gustatoreceptors||taste||taste buds on tongue|
Physiology of Nerve Impulse
- At resting stage: in this stage, anions (negative ions) are present on inner surface of neuron membrane and cations (positive ions) are present on outer surface of neuron.
- This neuron membrane is said to be polarised.
- At exciting stage: As the neuron receive external stimuli, undergoes de-polarisation.
- The anions are now on outer surface and cations on inner surface.
- At this point, nerve impulse is initiated.
- Repolarisation: As the impulse conducts forward, repolarisation occurs at previous point.
- As the impulse reaches the nerve synapses, Acetylcholine is secreted by terminal end. Through this, impulse is transmitted to dendrites of the next nerve. Thus, the impulses are transmitted.