The influence of biochemistry on behaviour
The function of the endocrine system
The nervous system receives and sends information around the body using electrical and chemical means (see page 82). Another bodily messaging system is the endocrine system, a series of glands found throughout the body that release chemicals known as hormones via blood and other bodily fluids. This allows information to be sent to the organs of the body in order to affect their behaviour, rather like neurons do in the nervous system.
There are a number of specialist glands in the endocrine system. Each of these, when stimulated, releases particular hormones that produce specific behavioural effects. The pituitary gland is located behind the bridge of the nose and to the base of the brain just below the hypothalamus brain area (to which it is attached via nerve fibres). It is a pea-sized structure known as the ’master gland’, due to its important role within the endocrine system in regulating the functions of other glands — such as the ovaries in females, the testes in males and the thyroid and adrenal glands — and the hormones they secrete.
Fig 6.4 The endocrine system, showing the major glands in the human body
The pituitary gland
The pituitary gland has 2 lobes (parts): a front part known as the anterior pituitary and a back part known as the posterior pituitary. The anterior pituitary releases several important hormones: growth hormone, gonadotrophins (puberty hormones), thyroid-stimulating hormone and adrenocorticotrophic hormone (ACTH), which stimulates the adrenal stress hormone cortisol. The posterior pituitary releases the fluid-balance hormone ADH (anti-diuretic hormone).
To activate the pituitary gland the hypothalamus signals to it to either stimulate or inhibit hormone production. The anterior lobe releases hormones on receiving releasing or inhibiting hormones from the hypothalamus. These hypothalamic hormones inform the anterior lobe whether to release more of a specific hormone or cease its release. The hormones of the pituitary gland send signals to other endocrine glands to stimulate or inhibit their own hormone production. For instance, the anterior pituitary lobe will release ACTH to stimulate cortisol production from the adrenal glands (situated near the kidneys) when an individual is stressed. The adrenal glands are an important component of the fight-or-flight response (see opposite) as they facilitate the release of adrenaline.
Another important hormone that increasingly interests psychologists is oxytocin, which is released from the posterior lobe of the pituitary gland and is secreted during pleasurable activities like playing, cuddling or having sex. Oxytocin is especially important for females, as it causes contractions during labour and also causes the release of milk during breastfeeding. It is important in both males and females in creating bonds between individuals.
THE FIGHT-OR-FLIGHT RESPONSE
The fight-or-flight response is generated from the sympathetic nervous system branch of the autonomic nervous system (ANS) (see page 82). It is an innate reflex action (that is, one that requires no conscious thought) that has an evolutionary survival value in helping protect an individual confronted by potentially dangerous situations. The response is activated in times of stress when something is perceived to be a threat to safety, and helps an individual to react more quickly than usual, as well as optimising functioning so the body is able to effectively fight or run away from the threat.
The response can be broken down into separate steps:
1 The hypothalamus brain area perceives a threatening stressor and sends a signal to the adrenal glands.
2 The adrenal medulla within the adrenal glands triggers the release of the hormone adrenaline into the endocrine system and the neurotransmitter noradrenaline in the brain.
3 Physical changes are incurred within the body that help an individual to fight or run away from the threatening stressor, for example, an attacker:
• Increased heart rate — speeds up blood flow to vital organs and flow of adrenaline around the body
• Faster breathing rate — increases oxygen intake
• Muscle tension — improves reaction time and speed
• Pupil dilation — improves vision
• Sweat production — helps temperature control
• Reduced digestion and immune system functioning — targets energy towards prioritised functioning, such as fighting and running.
Oxytocin sprays have been used to treat autism, as oxytocin seems to stimulate areas of the brain associated with social interaction, something that autistic people often have difficulties with. The sprays are used to facilitate the effectiveness of therapies.