Animals Nervous System: Introduction, Classification, Spinal Cord, Neurons

Nervous System Definition

The nervous system is the network of nerves and cells that integrates and transmits action and signals between parts of an animal's body. It is an important part or organ serving a principal function in controlling and coordinating body activities, from simple to very complex, sensory perception, and maintaining homeostasis. As such, it enables animals to monitor, adapt, and react to both sets of internal and external stimulations.

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This Story also Contains

1. Nervous System Definition
2. Structure Of The Nervous System
3. Neurons: The Functional Units
4. Neuroglia: The Supporting Cells
5. Transmission Of Nerve Impulses
6. Reflex Actions And Reflex Arc
7. Nervous System In Different Animal Phyla
8. Disorders Of The Nervous System

Structure Of The Nervous System

The broad division of the nervous system is classified into two parts: the Central Nervous System and the Peripheral Nervous System.

Central Nervous System (CNS)

The Central Nervous System consists of two major divisions:

Brain: Structure And Functions

• The structure and functions of the cerebrum include higher brain functions such as thought, action, and sensory processing.

• The structure and functions of the cerebellum include control over balance, coordination, and fine muscle control.

• The structure and functions of the brainstem include regulating vital functions of the body such as heartbeat, breathing, and sleep cycles.

Spinal Cord: Structure And Functions

• A cylindrical bundle of nerve fibres extending from the brainstem down the vertebral column.

• It transmits information between the brain and the rest of the body and mediates reflex actions.

Peripheral Nervous System (PNS)

The Peripheral Nervous System has the following components:

Somatic Nervous System

• It is responsible for controlling voluntary movements and transmitting sensory information towards the CNS.

• This comprises nerves connecting to muscles and sensory receptors.

Autonomic Nervous System

• Sympathetic Division: This prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Serves 'rest and digest' activities.

Neurons: The Functional Units

Neurons are the simplest units of nervous systems that carry nerve impulses.

Types Of Neurons

• Sensory Neurons: Transmit impulses from sensory receptors to the CNS.

• Motor Neurons: Carry instructions from the CNS to muscles and glands.

• Interneurons: Integrate neurons within the CNS and interconnect sensory and motor functions.

Structure Of Neurons

• Cell Body (Soma): Nucleus and organelles.

• Dendrite: Receive signals from other neurons.

• Axon: Conducts the electrical impulse away from the cell body.

• Synapse: Gap between neurons, where neurotransmitters are released.

Diagram: Structure of Neuron

Neuroglia: The Supporting Cells

• Astrocytes: Provide mechanical support and protection for neurons, and maintain the blood-brain barrier.

• Oligodendrocytes: Generate myelin in the CNS.

• Microglia: Serve as immune cells within the CNS.

• Ependymal Cells: Line ventricles and produce CSF.

Transmission Of Nerve Impulses

Nerve impulses are transmitted by both electrical and chemical means.

Resting Membrane Potential

• Electric charge difference across the neuronal membrane when it is at rest.

Action Potential

• A rapid change in membrane potential that travels along the axon.

• The action potential would be produced as a result of fast depolarisation by the influx of sodium ions and repolarisation by the efflux of potassium ions.

Synaptic Transmission

• Electrical Synapses: The communication between the neurons directly occurs electrically.

• Chemical synapses: Neurotransmitters transmit the signal across the synaptic cleft.

Neurotransmitters And Functions

• These chemical messengers modulate different CNS functions, as seen in dopamine, serotonin, and acetylcholine.

Reflex Actions And Reflex Arc

• These reflex actions are responses to stimuli that are automatic and help protect the body.

• Involuntary and almost instant response of the body to stimuli.

• It consists of a sensory neuron, an interneuron, and a motor neuron.

Examples Of Reflex Actions

• Knee jerk reflex and withdrawal reflex.

Nervous System In Different Animal Phyla

Nervous systems in different animal phyla vary from simple nerve nets to complex centralised systems.

Simple Nervous Systems

• Cnidarians: Nerve Nets

• The simplest form of the nervous system can be found in jellyfish.

• Platyhelminthes: Ladder-Like Nervous System

• Found in flatworms, it consists of two nerve cords joined by transverse nerves.

Advanced Nervous Systems

• Annelids: Ventral Nerve Cord and Ganglia

• Segmented worms with a chain of connected nerve centres.

• Arthropods: Brain and Ventral Nerve Cord

• Insects and crustaceans have a more developed nervous system that uses a brain.

• Molluscs: Varied Nervous System Complexity

• Range from simple systems in snails to complex brains in octopuses.

• Vertebrates: Highly Organised CNS and PNS

• Includes a well-developed brain and spinal cord with complex peripheral networks.

Disorders Of The Nervous System

Neurological disorders can seriously debilitate health and the quality of life.

Parkinson's Disease: Degenerative disorder that affects motor control.

Alzheimer's Disease: Progressive dementia that affects memory and cognition.

Multiple Sclerosis: Autoimmune disorder that affects the myelin sheath in the CNS.