13.8 Reflexes

Question 1

What is a reflex action?

Answer 1

Where the body responds to a stimulus without making a conscious decision to respond.

This is because the pathway of communication doesn’t involve conscious parts of the brain — instead it goes to the unconscious parts of the brain or the spinal cord.

Reflex actions are protective — they help organisms to avoid damage to the body because the response happens so quickly.

Question 2

What does a reflex arc / pathway of communication involve?

Answer 2

• stimulus
• receptor: detects stimulus and creates an action potential in the sensory neurone
• sensory neurone: carries impulse to the spinal cord
• relay neurone: connects the sensory neurone to the motor neurone within the spinal cord or brain
• motor neurone: carries impulse to the effector to carry out the appropriate response

(Stimulus -> receptors -> CNS -> effectors -> response)

Question 3

Describe the spinal cord

Answer 3

The spinal cord is a column of nervous tissues running up the back
Is it surrounded by the spine for protection
At intervals along the spinal cord pairs of neurones emerge

Question 4

Describe the knee-jerk reflex

Answer 4

• it is a spinal reflex (this means that the neural circuit only goes up to spinal cord, not the brain)
• the knee-jerk reflex works to quickly strengthen your leg if the body detects your quadriceps is suddenly stretched
  It helps maintain posture and balance (e.g. helps prevent you from falling/ tripping)

Stimulus- you quadriceps muscle (patellar tendon) is stretched
Receptors - stretch receptors in the quadriceps muscle detect that the muscle is stretched. A nerve impulse is passed along a sensory neurone
CNS- the sensory neurone communicates directly with a motor neurone in the spinal cord (no relay neurone involved)
Effectors- the motor neurone carries the nerve impulse to the quadriceps muscle
Response- the quadriceps muscle contracts so the leg is extended once and come to rest

The absence of this reflex may indicate nervous problems and multiple oscillation of the leg may be a sign of a cerebellar disease

Question 5

Describe the blinking reflex

Answer 5

It occurs when the cornea is stimulated e.g. by being touched
It’s purpose is to keep the cornea safe from damage (e.g. bright light (optical reflex, dust)
The blinking reflex is a cranial reflex — it occurs in the brain not the spinal cord

Stimulus: something touches your eye
Receptors: sensory nerve endings in the cornea detect the touch stimulus. A nerve impulse is sent along the sensory neurone to a relay neurone in the CNS
CNS: the impulse is then passed from the relay neurone to motor neurones
Effectors: the motor neurones send impulses to the orbicularis oculi muscles that move your eyelids
Response: these muscles contract causing your eyelids to close quickly and prevent your eye from being damaged

Question 6

Ways in which reflex’s increase your chances of survival?

Answer 6

• being involuntary responses: the decision-making regions of the brain are not involved, therefore the brain is able to deal with more complex responses. It prevents the brain from being overloaded with situations in which the response is always the same
• not having to be learnt: they are present at birth and therefore provide immediate protection
• extremely fast: the reflex arc is very short (usually only involves one or two synapses, which are the slowest part of nervous transmission)