#87 Simple reflex arc

A reflex action is an automatic response to a stimulus. 

A reflex arc describes the pathway of an electrical impulse in response to a stimulus.

Relay neurones are found in the spinal cord, connecting sensosy neurones to motor neurones. 

On the picture below, the stimulus is a drawing-pin sticking in the finger. The response is the withdrawal of the arm due to contraction of the biceps. 

The sequence of events is: 

Reflex: automatically and rapidly integrating and coordinating stimuli with responses

Neurones do not connect directly with each other: there is a gap called a synapse. The impulse is 'transmitted' across the synapse by means of a chemical called acetylcholine. 

Try this 

Figure below shows a nerve cell.

a. i) Name the type of nerve cell shown in the figure [1 mark]

    ii) State two features that distinguish it from other types of nerve cell [2 mark]

   iii) Where, in the nervous system, is this cell located? [1 mark]

b. Nerve cells are specialised cells. Suggest how the following parts of the nerve cell, labelled in the figure, enable the nerve cell to function successfully: 

cytoplasm; myelin sheath. [4 mark]

c. Reflex involve a response to a stimulus. 

  i) Copy and completes the flowchart by putting the following terms in the boxes to show the correct sequence in a reflex. [2 marks]

 ii) For the pupil reflex , identify each of the parts of the sequence by copying and completing the table below. The first has been done for you. [ 4 marks]

Part of sequence	Part in pupil reflex
Coordinator	Brain
Effector
Receptor
Response
Stimulus

Answer

a. i) Motor neurone 

   

    ii) Two features from: 

      - presence of motor end plates 

      - the cell body is at the beginning of the cell

      - the cell body has dendrites on

      - there is no dendron (only on axon).

    

     iii) Peripheral nervous system. 

b. Cytoplasm: two suggestions from: 

   

    - is elongated

   - passes electrical signals along

   - connects different parts of the body 

   - is modified to form dendrites.

   Myelin sheath: two suggestions from: 

   - acts as insulating material

   - so prevents leakage of electrical signal from axon

   - allows faster transmission of impulses. 

c. i) flowchart

 ii) 

Part of sequence	Part in pupil reflex
Coordinator	Brain
Effector	Iris (muscle)
Receptor	Retina or rods or cones
Response	Pupil changes diameter or iris muscles contract
Stimulus	Bright light or change in light intensity

Video: Reflex Arc

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