Edexcel IGCSE Biology: Double Science 复习笔记 2.10.6 The Human Nervous System

Edexcel IGCSE Biology: Double Science 复习笔记 2.10.6 The Human Nervous System

The Human Nervous System: Structure

 

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

     

  • Information is sent through the nervous system as electrical impulses – these are electrical signals that pass along nerve cells known as neurones
    • bundle of neurones is known as a nerve

     

  • The nerves spread out from the central nervous system to all other regions of the body and importantly, to all of the sense organs
    • The CNS, therefore, acts as a central coordinating centre for the impulses that come in from (or are sent out to) any part of the body

     

 

 

The-human-nervous-system

 

The human nervous system is comprised of the CNS and the PNS

 

 

 

Adaptations of neurones

  • Neurones have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions from this body called axons and dendrites
    • The axon is the main long fibre of the neurone

     

  • Some human neurones have axons over a metre in length (but only 1 - 4 micrometres wide)
    • This is far more efficient than having multiple neurones to convey information from the CNS to effectors – less time is wasted transferring electrical impulses from one cell to another

     

  • The axon is insulated by a fatty myelin sheath with small uninsulated sections along its length (called nodes)
    • This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next

     

  • Many extensions called dendrites extend out from the cell body of the neurone and at the far end of the axon
    • This means neurones can connect to many other neurones and receive impulses from them, forming a network for easy communication

     

 

 

The-structure-of-a-myelinated-neuroneThe structure of a myelinated neurone.

 

 

Types of neurones

  • There are three main types of neurones: sensory neurones, relay neurones and motor neurones
    • Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord)
    • Relay neurones are found inside the CNS and connect sensory and motor neurones
    • Motor neurones carry impulses from the CNS to effectors (muscles or glands)

     

 

 

 

Identifying the three types of neurones

  • Sensory neurones are long and have a cell body branching off the middle of the axon
  • Relay neurones are short and have a small cell body at one end with many dendrites branching off it
  • Motor neurones are long and have a large cell body at one end with long dendrites branching off it

 

 

The-three-types-of-neurone

 

The three types of neurones

 

 

The Human Nervous System: Function

 

  • The pathway through the nervous system can be summarised as follows:

 

stimulus → sensory neurone → relay neurone → motor neurone → effector → response

 

 

 

  • First, a stimulus is received by a sensory (receptor) neurone
    • Most receptors are specialised to detect particular stimuli
    • When a receptor is stimulated, it produces electrical impulses

     

  • These impulses then travel along a sensory neurone to the central nervous system (the coordinator is either the brain or the spinal cord)
  • In the CNS, the impulses are passed on to a relay neurone
  • The relay neurone links to a motor neurone, along which the impulses travel until they reach the effector
  • The effector is what carries out the response (the effector may be a muscle or gland)

 

 

 

Nerve-pathway-stimulus-to-response_1

 

From stimulus to response: an example of a nerve pathway showing how an electrical impulse travels through sensory, relay and motor neurones

转载自savemyexams

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