This comprehensive IGCSE Biology presentation explains the nervous system, focusing on how the body coordinates and responds to stimuli. Learn about the central and peripheral nervous systems, reflex actions, neurons, synapses, and the role of neurotransmitters. Understand the differences between voluntary and involuntary responses and how the nervous system interacts with other body systems. Ideal for Cambridge IGCSE students preparing for exams!

2. The Nervous System
The nervous system in humans consists of 2 parts:
❑Central Nervous System (CNS) – the brain and spinal
cord
❑Peripheral Nervous System (PNS) – all of the nerves in
the body
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5. The Neuron
A nerve is a bundle of cells known as neurons, which are the longest cells in the body.
Neurons are the structural unit of the brain and spinal cord as well.
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6. Types of Neurons
Sensory Relay Motor
– Part of PNS
– Myelin sheath present
– Cyton not at end of cell
– Receives impulses
from a receptor
– Part of CNS
– No myelin sheath present
(so that speed of impulse is
slowed as they can’t jump
over the sheath so the brain
can intervene in the action)
– Cyton at end of cell
– Connects sensory and motor
neuron
– Part of PNS
– Myelin sheath present
– Cyton at end of cell
– Connects to an effector
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8. Synapses
A synapse is a junction between 2 neurons. The dendrites of 2 neurons don’t
actually touch at a synapse,but are very close.
They act like valves, so that impulses travel in 1 direction.
Note: drugs such as heroin act upon synapses.
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9. The Stages of Transmission of Information at a Synapse:
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10. 1. An impulse arrives at the end of the presynaptic neuron
2. When an electrical impulse arrives, the vesicles move towards and fuse with
the presynaptic membrane, and they burst to release neurotransmitters
(chemicals) into the synaptic cleft
3. Chemicals diffuse across the synaptic cleft (down a concentration gradient)
4. They will then get picked up by receptors of the postsynaptic neuron and
they will convert the chemical back into electricity
5. The impulse travels along the postsynaptic neuron
6. The chemicals are recycled or destroyed once the impulse is sent
This type of transmission of impulses is known as electrochemical
transmission.
The Stages of Transmission of Information at a Synapse:
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11. Voluntary and Involuntary Response
A voluntary response is one where you make a conscious
decision to carry out a particular action therefore it starts with your
brain
An involuntary (or reflex) response does not involve the brain
as the coordinator of the reaction and you are not aware you have
completed it until after you have carried it out
Involuntary actions are usually ones which are essential to basic
survival and are rapid, whereas voluntary responses often take
longer as we consider what the response might be before doing it
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12. Reflex Actions
A reflex action involves rapid and automatic integration of
coordination stimuli with the response of effectors.
Reflex actions are:
•Automatic
•Faster than any other nervous response
•Protective
•Involuntary
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13. The Reflex Arc
The nerve pathway involved in a reflex action is known as the
reflex arc.
This is the process:
1. Receptor in sense organ receives a stimulus and converts it
into an electrical impulse (there are different types of receptors
to receive different stimuli)
2. Sensory neuron carry these impulses to the CNS (spinal cord)
3. Relay neuron slowly carries the impulse across the CNS
4. Motor neuron carries the impulse from the CNS to an effector
(muscle or gland)
5.Effector carries out response to deal with the original stimulus
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17. Reflex Actions Non-Reflex Actions
Common reaction in all humans
that are carried out in the same
way and are evolved to help us
survive
Conditional/acquired reflex
which is done again and again
and becomes muscle memory
Blinking, shivering, swallowing Driving, playing an instrument,
typing
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