Outline the roles of sensory receptor cells in the mammalian nervous system.

1. (a)Outline the roles of sensory receptor cells in the mammalian nervous system.[3]
2. (b)[Fig. 8.1] shows the changes in potential difference (p.d.) across the membrane of a receptor cell over a period of time. The membrane was stimulated at time A and at time B with stimuli of different intensities. [Fig. 8.1: graph of membrane potential / mV vs time / ms, showing resting potential, a small depolarization at stimulus A, and a larger depolarization followed by an action potential (C-D-E) at stimulus B. Threshold potential is indicated.]
3. (b)(i)State which of the letters C, D and E on Fig. 8.1 correspond to each of these events. You may use each of the letters C, D or E once, more than once or not at all. The Na+/K+ pump is operating ... The voltage-gated Na+ channels are open ... The voltage-gated K+ channels are open ...[3]
4. (b)(ii)Explain why stimulus A did not result in an action potential being produced whereas stimulus B did.[2]
5. (b)(iii)Describe the importance of the refractory period in the transmission of action potentials.[2]
6. (b)(iv)Describe how action potentials are transmitted along a myelinated axon.[4]

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