Reflex arc

Question 1

What is the nervous system divided into?

Answer 1

• the central nervous system (CNS) which is made up of the brain and spinal cord
• the peripheral nervous system (PNS) which is made up of pairs of nervous that originate from either the brain or the spinal cord

Question 2

What is the peripheral nervous system divided into?

Answer 2

• sensory neurones, which carry nerve impulses (electrical signals) from receptors towards the central nervous system
• motor neurones which carry nerve impulses away from the central nervous system to effectors

Question 3

What are the sub divisions of the motor nervous system?

Answer 3

• the voluntary nervous system, which carries nerve impulses to body muscles and is under voluntary conscious control
• the autonomic nervous system which carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control that is it is involuntary

Question 4

What is the spinal cord?

Answer 4

• the spinal cord is a column of nervous tissue that runs along the back and lies inside the vertebral column for protection
• emerging at intervals along the spinal cord are pairs of nerves

Question 5

What is a reflex arc?

Answer 5

• you will have noticed that you immediately withdraw your hand if you place it on a hot or sharp object
• you do not stop to consider any alternative actions
• the response is rapid, short-lived, localised and totally involuntary
• indeed by the time the brain has received nerve impulses from the receptors in the hand, the muscles in the arm have already pulled the hand clear of the danger
• this type of involuntary response to a sensory stimulus is called a rifles
• the pathway of neurones involved in a reflex is known as a reflex arc

Question 6

What are the stages of he spinal reflex arc in the context of withdrawing the hand from a hot object?

Answer 6

• the stimulus-heat from the hot object
• a receptor-temperature receptors in the skin on the back of the hand, which generates nerve impulses in the sensory neurone
• a sensory neurone - passes nerve impulses to the spinal cord
• a coordinator (intermediate neurone) - links the sensory neurone to the motor neurone in the spinal cord
• a motor neurone -carries nerve impulses from the spinal cord to a muscle in the upper arm
• an effector - the muscle in the upper arm, which is stimulated to contract
• the response -pulling the hand away from the hot object

Question 7

What is the importance of reflex arcs?

Answer 7

• any action that makes survival more likely is clearly of value
• reflexes are involuntary - the actions they control do not need to be considered, because there is only one obvious course of action, that is, to remove the hand from the hot object

Question 8

Why are reflex actions important?

Answer 8

• they are involuntary and therefore do not require the decision-making powers of the brain, thus leaving it free to carry out more complex reasons, in this way, the brain is not overloaded with situations in which the response is always the same, some impulses are nevertheless sent to the brain, so that it is informed of what is happening and can sometimes override the reflex if necessary
• they protect the body from harm, they are effective from birth and do not have to be learnt
• they are fast, because the neurone pathway is short with very few, typically one or two, synapses where neurones communicate with each other (synapses are the slowest link in a neurone pathway) which is important in withdrawal reflexes
• the absence of any decision-making process also means the action is rapid