Biology Revision Notes - Reflex Actions And Arcs

AQA Biology, Energy and Continuity - Reflex Actions

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Biology Revision Notes:
Reflex actions and reflex arcs:
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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