Chapter 1: Analyze communication in plants and animals

Communication in plants and animals is fundamental to their survival, coordination, and interaction with the environment. Applied biology professionals in Kenya must understand the mechanisms by which organisms perceive and respond to stimuli, ensuring optimal health and productivity in sectors such as agriculture, wildlife management, and medical biology. This chapter explores the nervous system in animals, emphasizing its structure, the transmission of nervous impulses, and common diseases that affect it. These insights are crucial for managing animal health in farms, conservation areas, and clinical settings.

1.1 Nervous System

The nervous system detects changes inside and outside an animal, processes the information and sends commands to muscles and glands, often within milliseconds. It is built from neurons, which carry electrical impulses, and from supporting cells, and it is organised into a central part (the brain and spinal cord) and a peripheral part (the nerves). The same plan is found in the cattle and goats of a field station, the rats and rabbits of an animal house and the specimens used in a teaching laboratory, so this topic treats mammals in general and draws examples from each of these settings. Plants have no nervous system: their responses to stimuli are treated in Plant Responses and Growth.

1.1.1 Structure of the Nervous System

Information moves through the nervous system along a fixed chain: a receptor detects a stimulus, a sensory neuron carries the message to the central nervous system, the central nervous system processes it, a motor neuron carries the command out, and an effector (a muscle or a gland) responds. The structure of the system follows this chain. Two kinds of cell build it, and they are described first because every later part depends on them.

Neurons as functional units

A neuron is a cell specialised to conduct impulses. Its parts, and what each does, are:

• Cell body: contains the nucleus and most organelles, and maintains the cell.
• Dendrites: short, branched extensions that receive signals from other neurons or from receptors.
• Axon: a long extension that carries impulses away from the cell body to the next neuron or to an effector.
• Myelin sheath: a fatty insulating layer around many axons, interrupted at gaps called nodes of Ranvier.
• Axon terminals: the swollen ends of the axon, which release chemical messengers at a synapse.

Neurons are grouped by function into three types:

• Sensory (afferent) neurons: carry impulses from receptors to the central nervous system.
• Relay neurons (interneurons): lie wholly within the central nervous system and connect sensory neurons to motor neurons.
• Motor (efferent) neurons: carry impulses from the central nervous system to muscles and glands.

Example: in stained sections of a rat spinal cord examined in a teaching laboratory, the motor neurons stand out in the grey matter as large cells with prominent cell bodies and many processes.

Supporting cells and structures

Glial cells do not conduct impulses but are needed for neurons to work. Astrocytes support neurons, help form the blood-brain barrier and regulate the chemical surroundings of neurons. Oligodendrocytes make myelin in the central nervous system, while Schwann cells make it in the peripheral nervous system. Microglia remove debris and defend against infection, and ependymal cells line the fluid-filled spaces of the brain and spinal cord and help produce cerebrospinal fluid. Myelin matters because it greatly increases the speed of conduction, as explained under Transmission of Nervous Impulse.

1.1.1.1 Central Nervous System (CNS)

The central nervous system consists of the brain and the spinal cord. It receives sensory information, integrates it and sends out commands.

Protection comes from three layers. The skull encloses the brain and the vertebral column encloses the spinal cord. Three membranes, the meninges (dura mater, arachnoid mater and pia mater), cover both organs. Cerebrospinal fluid circulates between the arachnoid and pia mater and through the spaces of the brain, cushioning the tissue and exchanging nutrients and wastes.

The brain has three main regions:

• Forebrain: the cerebrum, whose folded outer cortex handles sensation, voluntary movement, learning and memory, together with the thalamus (a relay station for sensory messages) and the hypothalamus (control of temperature, thirst, hunger and the link to the pituitary gland, covered under Hypothalamus).
• Midbrain: relays visual and auditory reflexes and helps control eye movement.
• Hindbrain: the cerebellum, which coordinates movement, balance and posture; the pons, which links brain regions; and the medulla oblongata, which controls heart rate, breathing and swallowing.

Brain tissue is of two kinds. Grey matter is made mainly of cell bodies and lies on the outside of the cerebrum. White matter is made mainly of myelinated axons, which give it its pale colour.

The spinal cord runs inside the vertebral column from the brain stem. Its grey matter forms a butterfly shape in the centre and its white matter lies around it, carrying ascending tracts (sensory information up to the brain) and descending tracts (commands down from the brain). Sensory neurons enter the cord through the dorsal root, whose cell bodies lie in a swelling called the dorsal root ganglion, and motor neurons leave through the ventral root. The cord is also the centre for reflex actions.

Example: a teaching laboratory may use a preserved sheep brain cut in half along the midline to show the cerebrum, cerebellum, brain stem and the pale white matter beneath the grey cortex.

1.1.1.2 Peripheral Nervous System (PNS)

The peripheral nervous system is made of all the nerves and ganglia outside the brain and spinal cord. A nerve is a bundle of axons wrapped in connective tissue. The nerves are the cranial nerves, which leave the brain (12 pairs in mammals), and the spinal nerves, which leave the spinal cord (the number of pairs depends on the species and its vertebrae).

Functionally the peripheral nervous system has a sensory division, which carries impulses to the central nervous system, and a motor division, which carries commands out. The motor division has two parts:

• Somatic nervous system: controls voluntary movement of skeletal muscle, although it also carries out reflexes.
• Autonomic nervous system: controls involuntary activity of smooth muscle, cardiac muscle and glands. It has two branches that usually act in opposition.

Example: a goat restrained for handling in a field station shows a fast heartbeat and wide pupils because the sympathetic branch is active, while a goat chewing the cud quietly shows the parasympathetic branch at work, since the vagus nerve drives the contractions of the stomach compartments.

1.1.2 Transmission of Nervous Impulse

A nervous impulse is an electrical signal that travels along a neuron. Between neurons, and between a neuron and an effector, the signal is passed on chemically at a synapse. Both stages depend on the movement of ions across the cell membrane.

Resting membrane potential

A neuron that is not carrying an impulse is at rest, and its inside is negative compared with its outside by about 70 millivolts, written as about −70 mV. Two things set this up. The sodium-potassium pump uses ATP to move three sodium ions out of the cell for every two potassium ions it brings in. In addition, the membrane is more permeable to potassium ions, which leak out through open channels, than to sodium ions. The result is a high sodium concentration outside and a high potassium concentration inside, and a charge difference across the membrane.

Action potential

An action potential is a brief reversal of the membrane potential that travels along the axon. Its stages are:

1. A stimulus makes the membrane slightly less negative at one point.
2. If the change reaches the threshold, about −55 mV, voltage-gated sodium channels open.
3. Sodium ions rush in and the inside becomes positive, up to about +30 mV. This is depolarisation.
4. The sodium channels close and voltage-gated potassium channels open.
5. Potassium ions leave the cell and the inside becomes negative again. This is repolarisation.
6. The membrane briefly overshoots to a more negative value (hyperpolarisation) before the pump and leak channels restore the resting potential.

An action potential follows the all-or-nothing rule: below the threshold nothing happens, and above it every impulse has the same size. A stronger stimulus does not make a bigger impulse but a higher frequency of impulses. During the refractory period just after an impulse the channels cannot reopen, so the impulse can only move forward along the axon and not back.

Conduction along the axon

The depolarisation at one point sets up small electric currents that bring the next stretch of membrane to threshold, so the impulse moves along the axon. In an unmyelinated axon this happens along the whole length of the membrane. In a myelinated axon the myelin blocks ion movement, so the action potential occurs only at the nodes of Ranvier and the impulse jumps from node to node. This is saltatory conduction, and it is much faster and uses less energy.

Speed also depends on axon diameter (wider axons conduct faster) and on temperature. Mammals keep a steady body temperature, so temperature matters more in animals that do not: the nerve of a frog prepared for a demonstration in a physiology practical conducts more slowly when it is cooled.

Synaptic transmission

A synapse is the junction between the axon terminal of one neuron and the next neuron or effector, with a narrow gap, the synaptic cleft, between them. Transmission across it runs as follows:

1. The impulse reaches the axon terminal and opens voltage-gated calcium channels.
2. Calcium ions enter and make vesicles holding a neurotransmitter fuse with the membrane and release their contents by exocytosis.
3. The neurotransmitter diffuses across the cleft.
4. It binds to receptors on the membrane of the next cell and opens ion channels.
5. Depending on the neurotransmitter, the next cell is excited (nearer the threshold) or inhibited (further from it).
6. The neurotransmitter is removed, either by an enzyme such as acetylcholinesterase, which breaks down acetylcholine, or by being taken back into the terminal.

Transmission is one-way, because only the terminal holds vesicles of neurotransmitter and only the next cell has the receptors. Acetylcholine is the messenger at the junction between a motor neuron and a skeletal muscle fibre and in the parasympathetic branch. Noradrenaline, dopamine, glutamate and gamma-aminobutyric acid (GABA) are other neurotransmitters. A single synapse often receives signals from many neurons, and the next cell fires only when their combined effect reaches the threshold.

Reflex arcs

A reflex is a rapid, automatic response to a stimulus that does not wait for a decision by the brain. The pathway is called a reflex arc: receptor, sensory neuron, relay neuron in the spinal cord, motor neuron and effector. Because only a few synapses are involved, the response is quick, and the brain is told about it afterwards.

• Withdrawal reflex: a calf that treads on a sharp stone lifts its foot before it feels pain. The arc includes a relay neuron.
• Knee-jerk (patellar) reflex: tapping the tendon below the kneecap stretches a muscle and makes it contract. The sensory neuron synapses directly with the motor neuron, so the arc has no relay neuron.

A technician in a teaching practical can use the patellar reflex of a restrained goat to show how reflex testing checks whether the spinal cord and nerves of the leg are working.

1.1.3 Nervous System Diseases and Disorders

Diseases of the nervous system arise when neurons, myelin, the meninges or the supply of nutrients are damaged. Signs depend on the part affected and may include paralysis, loss of sensation, tremors, convulsions, circling, loss of balance and changes in behaviour. The main groups of cause are described below, with examples from livestock, laboratory animals and humans.

Infectious diseases

• Rabies: a virus that enters through a bite and travels along peripheral nerves to the brain. Signs include behaviour change, excess salivation and paralysis, and the disease is fatal once signs appear. It also affects humans, so vaccination of dogs and of staff who handle suspect animals is the main control.
• Meningitis: inflammation of the meninges, caused by bacteria or viruses, giving fever, stiff neck and sensitivity to light in humans.
• Newcastle disease: a viral disease of poultry whose nervous form causes twisted necks and paralysis.
• Listeriosis: a bacterial infection of the brain stem of cattle, sheep and goats that makes affected animals circle and droop one side of the face.
• Tetanus: a toxin from Clostridium tetani blocks inhibitory signals in the spinal cord and causes stiff, spasmodic muscles. Vaccination prevents it.
• Botulism: the toxin of Clostridium botulinum stops the release of acetylcholine and causes floppy paralysis. Animals acquire it from spoiled feed or decaying carcasses.

Nutritional and metabolic disorders

• Polioencephalomalacia: softening of the brain in cattle, sheep and goats linked to thiamine (vitamin B1) deficiency, with blindness, head pressing and convulsions.
• Swayback: a copper-deficiency disease of lambs and kids in which poor myelin formation causes weakness and a lack of coordination.
• Hypomagnesaemic tetany: a fall in blood magnesium, often in cattle grazing lush young grass, which causes nervousness, muscle twitching and collapse.

Toxic disorders

Organophosphate pesticides block acetylcholinesterase, so acetylcholine builds up at synapses and causes salivation, muscle tremors and breathing difficulty. Plants containing neurotoxins can cause tremors or staggering in grazing animals. In both cases the signs show that the normal ion and neurotransmitter balance has been disturbed.

Traumatic injuries

Blows to the head, falls and fractures of the vertebral column can bruise or cut the brain or spinal cord. Damage to the spinal cord stops impulses crossing the injury, so there is paralysis and loss of sensation below it. Neurons in the central nervous system repair poorly, so recovery is often limited.

Degenerative and autoimmune disorders

• Parkinson's disease: loss of dopamine-producing neurons gives tremor, stiffness and slow movement in humans.
• Alzheimer's disease: progressive loss of neurons causes memory loss in humans.
• Multiple sclerosis: the immune system attacks myelin, which slows or blocks impulses.
• Epilepsy: abnormal bursts of electrical activity in the brain cause seizures, in dogs as in humans.

Congenital and genetic disorders

Some animals are born with faults of the nervous system, such as hydrocephalus, in which excess cerebrospinal fluid enlarges the brain spaces, or underdevelopment of the cerebellum, which gives a lack of coordination from birth. Breeding records help a breeder to see which parents repeatedly produce affected young so that they are not used again.

Laboratory diagnosis and handling

A diagnostic laboratory confirms rabies by examining brain tissue, using a fluorescent antibody test. Technicians handling such material wear gloves, a mask and eye protection, work in a safety cabinet where available, and should have rabies vaccination. Other investigations include neurological examination of reflexes, analysis of cerebrospinal fluid and microscopic examination of sections of brain or spinal cord.

Chapter Summary

This chapter provided a comprehensive analysis of communication mechanisms in both plants and animals, beginning with the nervous system in animals. It detailed the structure of the nervous system, distinguishing between the central nervous system and the peripheral nervous system, followed by an explanation of how nervous impulses are transmitted. The chapter also addressed common diseases and disorders affecting the nervous system. Moving on to sensory organs, the structure and functions of these organs were explored, highlighting their role in detecting environmental stimuli. The endocrine system was examined next, with an overview of its function in hormonal communication and regulation, and a detailed look at the major endocrine glands including the pituitary, hypothalamus, adrenal, pineal, thyroid, parathyroid, pancreas, and thymus. Finally, the chapter discussed plant responses and growth, explaining plant growth curves, differentiating between primary and secondary growth, and describing tropic and tactic growth responses that enable plants to adapt to their environment. Together, these topics illustrate the complexity and diversity of communication systems essential for survival and adaptation in living organisms.