Chapter 15: Coordination

Animals and Plants have internal communication system that allows information to pass between different parts of their bodies, and so help them to respond to change in their external and internal environments.

Nervous Communication

The mammalian nervous system is made up of the brain and spinal cord, which forms the central nervous system. The cranial and spinal nerve forms peripheral nervous system.

  • Central nervous system (CNS) – the brain and the spinal cord
  • Peripheral nervous system (PNS) – all of the nerves in the body

Neurons

The nerve called neurons play an important role in coordinating and communication within the nervous system. The stimulus is first detected by sensory receptor cell which is then transmitted to sensory neuron.  There are three types of neurone:

  1. Sensory neurones transmit impulses from receptors to CNS
  2. Intermediate Neurones/ Connector neurones transmit impulses from sensory neurones to motor neurones.
  3. Motor neurons transmit impulses from CNS to effectors.

The structure of neuron has a long fiber known as an axon. The axon is insulated by a fatty sheath with small uninsulated sections along its length (called nodes of Ranvier). The sheath is made of myelin, a substance made by specialized cells known as Schwann cells. Myelin is made when Schwann cells wrap themselves around the axon along its length.

The motor neurones have a large body at one end that lies within the spinal cord or brain and highly branched dendrites that extend from body cell.

The sensory neurones have the same basic structure with one long axon with cell body that branches off the middle of the axon.

Reflex arc

A reflex arc is a neural pathway that controls a reflex. There are two types: autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). Autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord.

During a somatic reflex, nerve signals travel along the following pathway:

  1. Somatic receptorsin the skin, muscles and tendons
  2. Afferent nerve fiberscarry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem
  3. An integrating center, the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse
  4. Efferent nerve fiberscarry motor nerve signals from the anterior horn to the muscles
  5. Effectormuscle innervated by the efferent nerve fiber carries out the response.

Example:

  • A pin (the stimulus) is detected by a pain receptor in the skin
  • The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
  • Electrical impulses are passed on to relay neurone in the spinal cord
  • The relay neurone connects to the motor neurone and passes the impulses on
  • The motor neurone carries the impulses to the muscle in the leg (the effector)
  • The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)

Transmission of Nerve Impulse

The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of electrical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized—that is, there is a difference in electrical charge between the outside and inside of the membrane.

  • Resting potential

An action potential is defined as a sudden, fast, transitory, and propagating change of the resting membrane potential. Only neurons and muscle cells are capable of generating an action potential; that property is called the excitability. In a resting axon (one that is not transmitting impulses), the inside of the axon always has a slightly negative electrical potential compared to outside the axon. The resting potential describes the unstimulated, polarized state of a neuron (at about –70 millivolts).

  • Action potential

The formation of an action potential can be divided into five steps.

 (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential.

(2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarize.

(3) At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close.

 (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire.

(5) The K+ channels close and the Na+/K+ transporter restores the resting potential.

Synapses

In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapse is junction between two neurones. Upon arrival of an action potential, the presynaptic membrane depolarizes therefore causing the calcium channels to open which subsequently allow calcium ions to enter the neurone.

When a nerve signal reaches the end of the neuron, it cannot simply continue to the next cell. Instead, it must trigger the release of neurotransmitters which can then carry the impulse across the synapse to the next neuron.

Once a nerve impulse has triggered the release of neurotransmitters, these chemical messengers cross the tiny synaptic gap and are taken up by receptors on the surface of the next cell. These receptors act much like a lock, while the neurotransmitters function much like keys. Neurotransmitters may excite or inhibit the neuron they bind to.

Parts of the Synapse

Synapses are composed of three main parts:

  • The presynaptic ending that contains neurotransmitters
  • The synaptic cleft between the two nerve cells
  • The postsynaptic ending that contains receptor sites

Types

There are two main types of synapses:

  • Chemical synapses
  • Electrical synapses

Chemical Synapses

  • Gap between: 20 nanometers
  • Speed: Several milliseconds
  • No loss of signal strength
  • Excitatory or inhibitory

Electrical Synapses

  • Gap between: 3.5 nanometers
  • Speed: Nearly instantaneous
  • Signal strength diminishes
  • Excitatory only

 

Muscle Contraction

Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick up something, or when you stretch or exercise with weights. Muscle contraction is often followed by muscle relaxation, when contracted muscles return to their normal state.

 

  • Striated muscle contracts when it receives an impulse from a motor neurone via the neuromuscular junction
  • When an impulse travelling along the axon of a motor neurone arrives at the presynaptic membrane, the action potential causes calcium ions to diffuse into the neurone
  • This stimulates vesicles containing the neurotransmitter acetylcholine (ACh) to fuse with the presynaptic membrane
  • The ACh that is released diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma (surface membrane of the muscle fibre cell)
  • This stimulates ion channels in the sarcolemma to open, allowing sodium ions to diffuse in
  • This depolarizes the sarcolemma, generating an action potential that passes down the T-tubules towards the centre of the muscle fibre
  • These action potentials cause voltage-gated calcium ion channel proteins in the membranes of the sarcoplasmic reticulum (which lie very close to the T-tubules) to open
  • Calcium ions diffuse out of the sarcoplasmic reticulum (SR) and into the sarcoplasm surrounding the myofibrils
  • Calcium ions bind to troponin molecules, stimulating them to change shape
  • This causes the troponin and tropomyosin proteins to change position on the thin (actin) filaments
  • The myosin-binding sites are exposed on the actin molecules
  • The process of muscle contraction (known as the sliding filament model) can now begin

Role of ATP in myofibril contraction

  • Allows actomyosin cross bridge to detach and is hydrolyzed so that the myosin can return to its original position.
  • Allows reabsorption of calcium ions.

 

Control and coordination in plants

It is the plant hormones that promote its growth. The movement of plant parts is due to various stimuli like gravity, water, light, chemical, touch, etc. So, the function of control and coordination in plants is performed by the chemical substances called plant hormones or phytohormones.

Plant Hormones

The control and coordination system in plants is done by plant hormones. They affect the growth of a plant in one or the other aspect. The growth of a plant is divided in three stages:

  1. i) Cell division
  2. ii) Cell enlargement

iii) Cell differentiation

The four types of plant hormones responsible for control and coordination in plants are:

1) Auxins

2) Gibberellins

3) Cytokinins

4) Abscisic acid (ABA)

While auxins, gibberellins and cytokinins promote the growth of a plant, abscisic acid prevents or hampers the growth of a plant.

The four types of plant hormones responsible for control and coordination in plants are:

1) Auxins

2) Gibberellins

3) Cytokinins

4) Abscisic acid (ABA)

While auxins, gibberellins and cytokinins promote the growth of a plant, abscisic acid prevents or hampers the growth of a plant.

Function of Plant Hormones

  • Germination of seeds
  • Growth of roots, stem and leaves
  • Movement of stomata
  • Flowering of plants
  • Ripening of fruits
  • Tropism and nastic movements