Coordination and control in animals

Coordination and control in animals

Coordination means to cause the part to function together or in proper order. Coordination and control in animals is performed by the nervous system and endocrine system.

The nervous system.

The nervous system is composed of highly differentiated cells called nerve cells or neurons. Those that carry impulses from receptors to the control nervous system are called sensory neurons while those that carry impulses from the CNS to effector are called effector neurons

Difference between effector and nervous 

WATCH  THIS VIDEO ABOUT THE NERVOUS SYSTEM 

Structure of the nerve cell

1. The cytoplasm contains the same organelles as the other body cell; mitochondria, nucleus cell membrane and ribosome grouped in Nissl’s granules.

2. Along nerve fibre axon extend from the cell body in effector neurons or either sides of the cell body in sensory neuron to transmits impulses. The axon enclosed within a fatty myelin sheath which is not part of the neuron but another cell Schwann cell which wraps itself repeatedly round the axon. The myelin sheath protects the axon, but also insulates the axon and speeds up transmission of impulse.

Transmission of impulse.

The resting potential of axon A potential difference is maintained between the inside and outside of undisturbed axon. The inside being negatively charged [about- 70 ml] with respect to the outside. In this state the membrane is said to be polarized. The resting membrane potential is maintained by the sodium / potassium pump using energy derived from ATP and it pumps ions against their concentration gradient pumping sodium ions are pumped out of the neuron in exchange for potassium ion (K+). K+ ions and organic ions is higher inside the neuron whereas Na+ and Cl- ion concentration is higher outside the neuron

The impulse An impulse or action potential is a temporary and local reversal of the resting membrane potential, arising when an axon is stimulated. The term used during the electrical change which occur during the passage of action potential is depolarization, and axon is said to be depolarized. The action potential is short- lived lasting about a millisecond, after which the resting membrane potential is restored. Events occurring into axon during the transmission of action potential are show in the figure below.

THE IMPULSE

Information is transmitted through the nervous system as a series of impulse which travel as action potential.

Properties of nerves and impulses

Stimulation In normal circumstance impulse are set up in nerve cell as a result of excitation of receptor. But an impulse can be set up in nerve by applying any stimulus which opens the sodium channels and cause depolarization of the membrane. In general nerve can be stimulate by mechanical, osmotic, chemical, thermal and electrical stimuli.

All or nothing law An excitable tissue will only be excited by a stimulus above a certain threshold stimulus intensity. For any given neuron the amplitude of the action potential is always constant and increasing the strength or number of stimuli has no effect on this. For this reason, potentials are described as all or nothing event. The all- or-nothing law states that the response of excitable unit (axon) is independent of the intensity.

Refractory period.

After an axon has transmitted an impulse, it cannot transmit another one straight way. The axon has to be recovered first. The membrane has to be repolarized and first. The period of under which the nerve cannot transmit an impulse following the transmission of first one is called refractor period and typically lasts about 3 milliseconds. It can be divided into absolute refractory period during which the axon is totally incapable of transmitting an impulse, followed by a somewhat longer relative refractory period during which it’s possible to generate an impulse in the axon provided that the stimulus is stronger than usual. The importance of the refractory period, together with transmission speed, it determines the maximum frequency at which the axons can transmit impulses. For most axons the maximum frequency is about 500 per second, though some neurons can reach 1000 per second.

Factors that affect nerve conduction velocity

1. Axon diameter:

(i) Impulses move faster in an axon with larger diameter because longitudinal resistance of exoplasm decreases with increasing diameter of axon, which increases the length of the membrane influenced by local circuit as the distance between adjacent depolarizations increases; causing increased conduction velocity.

(ii) Small cells or cells with large surface area: volume ratio or ion leakage weakens membrane.

(iii) Myelin sheath stops ion leakage; therefore, large diameter only important for unmyelinated neurons

2. Myelination and saltatory conduction:

(i) Myelination speeds up conduction. In a myelinated neuron, the conduction velocity is directly proportional to the fiber diameter.

(ii) Schwann cells prevent diffusion of ions; flow of current occurs only between adjacent nodes of Ranvier (iii) Therefore, depolarization only at nodes of Ranvier because action potential jumps from node to node

3. Temperature:

Homoiotherms with steady body temperature have faster impulse propagation than poikilotherms which have fluctuating body temperature.

4. Resting membrane potential:

Effect of RMP changes on conduction velocity is quite variable.

Usually, any change in the RMP in either direction (hyper polarization or depolarization) slows down the conduction velocity.

The Synapse.

This a functional area, where an axon come into contact with another for the purpose of transferring information. These are two types of synapse, electrical and chemical, depending on the nature of transfer of information across the synapse, A structurally dissimilar but functionally similar synapse exists between the terminal of a motor neuron and the surface of a muscle fibre and this is called neuromuscular junction.

Structure of the chemical synapse

Transmission across a synapse.

(i) Arrival of an impulse at the synaptic causes an influx of Ca2+ ions into the knob from the synaptic cleft. (ii) The Ca2+ ion causes the synaptic vesicles to move towards the pre-synaptic membrane.

(iii) The vesicles fuse with the pre- synaptic membrane and release a transmitter substance into the synaptic cleft by exocytosis.

(iv) The transmitter substance diffuses across the synaptic cleft and attaches to specific receptor sites on the post synaptic membrane.

(v) This causes an influx of Na+ ion into post- synaptic membrane, resulting in local depolarization of the membrane. If the Na+ ion surge is large enough, an action potential (impulse) is generated in the post –synaptic neuron.

The transmitter substance.

The transmitter substance at the majority of synapse is acetylcholine an ammonium base which has an effect on the permeability of the nerve membrane. After its effect, it’s inactivated by an enzyme cholinesterase to enable successive impulse to be kept separate. The products of this hydrolysis pass back into the synaptic knob where they are resynthesized into Acetylcholine using energy from ATP.

The Nerve- muscle junction or Neuromuscular.

Transmission across the neural muscular junction is the same as in any other chemical synapse. When an impulse arrives at the nerve muscular junction, acetylcholine is discharged from synaptic vesicle into synaptic cleft. The acetylcholine diffuses across the gap and depolarize the muscle end plate.

The action of drugs and poisons.

Chemicals that destroys acetylcholine, inhibits it’s formation or prevents it’s action, will stop synaptic transmission.

Atropine, for example doesn’t prevent acetylcholine being formed but it stops it depolarizing the post- synaptic membrane and therefore cause synaptic block.

Curare, the poison used on the tips of arrows by south Americans Indians has a similar effect specially on nerve muscle junction.

On the other hand chemicals (e.g. serine) nerve gases that destroy or prevent the formation or action of cholinesterase will be expected to enhance and prolong the effects of acetylcholine.

Strychine also enhance synaptic transmission to such an extent that a person suffering from strychnine poisoning will give convulsive muscular contraction upon slightest stimulation.

Noradrenaline This another neural transmitter substance in sympathetic nervous system of vertebrates. Nerve that produce acetylcholine are called cholinergic nerve and those producing noradrenaline are called adrenergic.

Function of synapse

1. Ensure unidirectional flow of impulse from neuron to the next.

2. Amplifies the impulses by producing sufficient acetylcholine to excite.

3. Adaptation or accommodation;

The amount of transmitter substances released by a synapse steadily falls off in the response to constant stimulation until the supply of transmitter substance is exhausted and the synapse is described as fatigued. Further information passing along this pathway is inhibited and the adaptive significance of fatigue in the prevention of damage effect due to over stimulation.

4. Integration;

A post synaptic neuron receives stimuli from a variety of source and integrates them and produce a coordinated response.

5. Discrimination;

Temporal summation at synapse enable weak background stimuli to be filtered out before it reaches the brain.eg information from receptor in the skin, the eyes and ears receive constant stimuli from the environment which has little immediate importance for the nervous system. Only changes in the intensity of stimuli are significant to the nervous system and these increase the frequency of stimuli and pass across the synapse and evoke a response.

6. Inhibition;

The transmission of information across a synapse and neuromuscular function may be prevented post-synaptically by the activity of certain chemical blocking agents or presynaptically.

The vertebrate nervous system. The nervous system of vertebrate is characterized by the structural and functional diversity of neurons and their complex organization with the body. The nervous system is subdivided into two main parts, the central nervous system [CNS] and peripheral nervous system.

The central nervous system consists of brain and spinal cord while peripheral nervous system consists of numerous nerves which link the CNS with the receptors and effectors.

WATCH THIS VIDEO TO GET MORE ABOUT SYNAPSE STRUCTURE AND FUCNTION

Reflex action and reflex arch.

The reflex action in a rapid, automatic stereotyped response to a stimulus which is not under the conscious control of the brain. It’s also described as involuntary action. The neurons forming the pathway taken by the nerve impulse in reflex action is referred to as reflex arch. Illustrated below

The reflex arc

Conditioned reflex arch This is a form of reflex action where the response is modified by the past experience, it’s coordinated by the brain. Learning for the basis of all conditioned reflexes such as salivation at sight or smell of food.

The autonomic nervous system

The autonomic nervous system is the part of the peripheral nervous controlling activities of the internal environment that normally involuntary such as heart rate, peristalsis and sweating.

The autonomic system is divided into two parts;

The sympathetic and parasympathetic system. Both contain nerve fibres serving structures over which the body has little or no voluntary control. In both cases nerve fibre emerge from the brain or spinal cord and pass to the organs concerned. There are many pathways in each of the two systems. Along the course of each pathway there is a complex set of synapses constituting a ganglion. The nerve fibres on the proximal side of the ganglion are called pre- ganglionic fibre, those on the distal side post- ganglionic fibres,

The main structural difference between the parasympathetic and sympathetic system relate the position the ganglion and are given below.

The brain

The brain is swollen anterior end of the vertebrate neural tube which has the over role of the coordination and control of the activities of the whole nervous system. To accomplish this there are special centers or nuclei in the different parts of the brain for dealing with specific functions such as locomotion, balancing and so on

Functions of the brain

1.Recieves impulses from receptors

2.Intergrates these impulses

3.sends out new impulses to the appropriate effect.

Function of the main parts of the brain the human brain.

1. Medulla oblongata is the most posterior part of the brain. It contains centers controlling breathing, circulation, swallowing salivation and vomiting.

2. The cerebellum coordinates voluntary movements such as posture, balance, coordination, and speech, resulting in smooth and balanced muscular activity. It is also important for learning motor behaviors

3. The thalamus and associated structures: contain centers controlling such function as sleep, aggression, feeding, drinking, osmoregulation, temperature regulation, speech and sexual activity.

4. Pituitary is an endocrine gland that secretes a wide range of hormones controlling such function as water and salt balance, growth, metabolism and sexual development.

5. The cerebral hemispheres coordinate all voluntary response.

WATCH THIS VIDEO DETAILED ABOUT THE BRAIN

Hormonal communication.

Hormones are organic compounds produced in one part of the body, from which is transported usually in the blood stream – to another part when it evokes a response. In the human and other vertebrate hormones are secreted into the blood stream by endocrine glands.

Position of the main endocrine gland in a human body

How hormones control cells When a hormone molecule reaches a target cell, it binds to the plasm membrane at a specific receptor site. The receptor site is located on the outer surface of the membrane and is associated with a molecule of adenylate cyclase. The binding of hormone to the membrane increases the activity of adenylate cyclase, causing ATP to convert into cyclic AMP. The cyclic AMP [Adenosine monophosphate] then activate enzymes which bring about the appropriate response with in the cell. The extent of the particular response is determined by the concentration of cyclic AMP which in turn depends on a delicate balance between adenylate cyclase responsible for synthesis, and another enzyme phosphodiesterase – which destroys it.