Nervous Control

• Simplest type of response controlled by the nervous system is a reflex arc.

• To be able to understand how a reflex arc works, it is first necessary to understand how the nervous system is organised.

Nervous organisation

• Central Nervous System (CNS) – made up of the brain and spinal cord.

• Peripheral Nervous System (PNS) – made up of nerves that run to/from the nervous system

Peripheral Nervous System

• Sensory Neurones – carry nerve impulses from receptors towards the CNS

• Motor Neurones – carry nerve impulses away from the CNS to effectors.

Motor Nervous System

• Voluntary Nervous System – carries nerve impulses to body muscles and is under conscious control.

• Autonomic Nervous System – carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under conscious control. It is involuntary.

Autonomic Nervous SystemSympathetic Nervous System – triggers the “fight or flight” response. Involved in adrenaline release. Aids in the control of most of the internal organs. Regulates homeostatic mechanisms. It will be involved in increasing heart rate, sweat production, pupil dilation.It stimulates effectors so speeds up activity. Acts like an emergency controller and helps cope with stressful situations.

Parasympathetic Nervous System – promotes the maintenance of the body whilst at rest. Involved in digestion, tear formation, sexual arousal, salivation etc.Inhibits effectors so slows down activity. Involved with conserving energy.

Spinal Cord

• 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.

Dorsal root of spinal cord

Ventral root of spinal nerve

The Reflex Arc• Totally involuntary response

• Example of withdrawing hand away from a hot or sharp object.

• By the time the CNS (usually spinal cord) has received nerve impulses from the receptor in hand, muscles in arm have already pulled hand clear of danger.

• This type of response is a reflex

• Pathway of neurones involved - reflex arc

Reflex Arc – Sequence of Events

• Stimulus – Heat from hot object.• Receptor – Temperature receptors in skin. These create

a nervous impulse in a sensory neurone.• Sensory Neurone – Passes the nerve impulse to the

spinal cord.• Intermediate Neurone – Links sensory neurone to

motor neurone in the spinal cord.• Motor Neurone – Carries nerve impulse from the spinal

cord to the effector.• Effector – Muscle in the upper arm, which is stimulated

to contract.• Response – Pulling the hand away from the hot object.

Importance of Reflex Arcs• Involuntary actions designed at increasing chances of

survival. Effective from birth. Do not need to be learned.

• Do not require decision-making powers of the brain. This leaves other areas of brain free to coordinate complex responses.

• Brain not overloaded with information. Although some information is sent to brain to give it option of over-riding reflex if necessary.

• They are very fast because pathway is short and simple.