Science ; biology<3

• Reflex action – rapid, automatic, involuntary response to a stimulus. They are not under conscious control and have survival value.
• Reflex arc – the specific pathway taken by the nerve impulses in a reflex action.
• Characteristics of reflexes:
• Involve a receptor and an effecter
• They are rapid, because they only involve few neurones and synapses
• They are automatic and involuntary, the same stimulus produces the same response each time without the need for a conscious decision
• Roles of reflexes:
• Reduce/avoid damage to tissues
• Escape from predators
• Vital roles in homeostasis
• Posture/balance
• Finding food/mates
• Types of reflexes and reflex arcs:
• Spinal reflexes – do not involve brain, impulse passes to spinal cord and back out e.g. knee-jerk, withdrawal reflex
• Cranial reflexes – use cranial nerves which run to and from involuntary part of the brain e.g. pupil size, blinking, coughing, sneezing
• Monosynaptic reflex arc – knee jerk, monosynaptic reflex arcs only involve one synapse and two neurones.
• Polysynaptic – withdrawal reflex, polysynaptic reflex arcs involve more than one synapse and three neurones
• The Autonomic Nervous System:
• Consists of two sets of antagonistic nerves:
• Sympathetic nerves – which stimulates the body and prepare it for action
• Parasympathetic nerves – which bring the body, back down to normal.
• In an excited state the sympathetic system dominates and in a relaxed state the parasympathetic system dominates
• Parasympathetic nerves use acetylcholine at the effecter
• Sympathetic nerves use nor-adrenaline at the effecter.

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