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The Nervous System

Biology (Year 12) - Homeostasis

Ben Whitten

The nervous system comprises two different divisions, including:

  • The central nervous system (CNS), which is made up of the brain and spinal cord, and is responsible for the processing, storing and coordinating of information

  • and the peripheral nervous system (PNS), which is made up of all other neurons, and is responsible for transmitting information to and from the CNS

An important aspect of the nervous system are neurotransmitters, which are a type of chemical that transmits signals between neurons once they pass a synapse, which is a gap between neurons. Neurotransmitters includes chemicals such as noradrenaline, acetylcholine or dopamine.

What is a neuron? Neurons are the basic unit (cells) of the nervous system, and there are different types of neurons in terms of structure and in terms of function. Characteristics of Neurons

  • Neurons have a tubular extension of the cell body (which contains the nucleus of the cell), called an axon

  • Neurons are wrapped around in myelin, which is the fatty coating provided by axon-wrapping Schwann cells, and they help to increase the speed of nerve impulses along sensory and motor neurons; they act as an insulator

  • Neurons have extensions called nerve fibres, along which nerve impulses travel

  • A bunch of nerve fibres comprises a nerve, and each nerve is wrapped in a tube of connective tissue.

Functional Types of Neurons

  • Sensory neurons carry impulses to the interneurons of the CNS

  • Interconnector neurons or interneurons relay information between sensory and motor neurons

  • Motor neurons carry impulses to effectors such as muscles and glands.

Structural Types of Neurons

  • Unipolar neurons have a single long axon that is responsible for sending electrical signals (impulses); the axon is myelinated, allowing for saltatory conduction (rapid transmission)

  • Bipolar neurons send signals and receive information from the world

  • Multipolar neurons are able to receive impulses from multiple neurons through dendrites; dendrites transmit the signals through the neuron via impulses sent down the axon

  • Pseudounipolar neurons relay signals from the skin and muscles to the spinal cord; the primary neurons responsible for coordinating the movement of limbs using brain input

When one of the regions of the brain receives a message about a detected change, it coordinates any response necessary to counteract the change and sends messages to the effector organs via the motor neurons in the spinal cord and then the motor neurons in the PNS. Afferent and efferent are words that you will come across, which are directional words. Messages sent in afferent direction travel from the receptor cells to the CNS along afferent sensory neurons, and messages sent in efferent direction travel from the CNS to the effector along efferent motor neurons. Just remember, afferent is arriving to the CNS, and efferent is exiting the CNS. How are stimuli detected in the nervous system? Signals (stimuli) may from from the external environment, from other parts of the orgnaism or from within the cells (internal environment). For example, stimuli may be physical things such as light, heat, pressure, or they may be chemical things such as hormones, neurotransmitters etc. There are millions of internal and external receptors which allow an organism to respond to stimuli. The types of receptors which you need to know are;

  • Chemoreceptors

  • Mechanoreceptors

  • Photoreceptors

  • Pain receptors

  • Thermoreceptors

How do internal and external receptors work? Internal Receptors Internal receptors receive signals from the body regarding the internal environment. For example, an increase in carbon dioxide concentrations leads to a decrease in the blood pH. The interstitial fluid (the fluid which bathes the cells) and the blood plasma are the medium of the internal environment, and cells exchange substances with the interstitial fluid across membranes. External Receptors External receptors of organisms detect changes in their external environment, interpret the signals and coordinate a response for either survival or development. It is important to note that external receptors may work individually or as a group. These receptors can be distributed evenly over the body, such as pain receptors in specialised areas such as the taste buds (taste receptors) or concentrated in organs such as the eye (photoreceptors).

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