The Influence of Genes
Genotype and Phenotype:
Genotype: The genetic code that is written in the DNA of an idividual's cells.
Phenotype: The physical appearance that results from the inherited information.
There is not always a direct relationship between the two, e.g. if someone inherits a recessive gene for blue eyes, this will not always be expressed. You cannot determine the genotype from the phenotype.
The Genetic Basis of Behaviour:
Heritability: The amount of variability in a trait in a population that can be attributed to genetic differences within individuals within that population.
Twin Studies - MZ and DZ Twins:
MZ twins share 100% of DNA whereas DZ twins only share 50% - the same amount as siblings. By assessing the concordance rate (the presence of the same trait in both members of a pair of twins) we can find whether the trait is genetic or a result of the environment.
We rarely find 100% concordance in MZ twins, suggesting that environmental factors are involved and that genes are predisposing rather than determining factors.
The Influence of Biological Structures:
Neurons transmit nerve impluses in the form of electrical signals to pass information beween the nervous systems.
The Nervous System:
Carries messages from one part of the body to another.
Central Nervous System (CNS): Brain and spinal chord. Transfer messages to and from the environment. All physiology is controlled here.
Peripheral Nervous System (PNS): Made of two more nervous systems: the Somatic Nervous System (SNS) which controls the muscles and information sent from the skin, and the Autonomic Nervous System which controls all the involuntary movements (e.g. breathing, heart beats etc.)
Cerebrum makes up 85% of the total mass of the brain - by far the largest part. The outer surface of this is called the Cerebral Cortex and is responsible for many 'higher order' functions, e.g. thought and language.
The cerebrum is divided into four hemispheres, and each hemisphere is further divided into four subsections, known as lobes.
The Influence of Neurochemistry
Chemicals released when a nerve impluse reaches the and of a neuron. Neurotransmitters travel across the synapse from neuron to neuron.
Neurotransmitters that trigger nerve impluses in the recieving neuron are exitatory neurotransmitters (e.g. Dopamine - associated with motivation) while those that inhibit nerve impulses are called inhibitatory neurotransmitters (e.g. Seratonin - associated with mood).
Chemicals produced in the endocrine glands (e.g. pituitary). In response to a signal from the brain, hormones are secreted directly into the bloodstream by the endorine glands, where they travel to their target cells and stimulate receptors on the surface of or inside cells.
The presence of a hormone causes a psysiological reaction in the cell, altering it's activity.
Evaluation of the Biological Approach
Nature/Nurture - negative:
Evidence: DZ twins ofen have higher concordance in studies despite sharing only 50% of DNA like siblings.
Explain: There is evidence for environmental factors being involved which is ignored.
Internal Validity - negative:
Evidence: Studies show seratonin reduces symptoms of depression, so it is assumed low levels of seratonin causes depression.
Explain: The approach claims to find causes where only associations exist.
Scientific Method - positive:
Evidence: The approach uses fMRI and EEG scans, family and twin studies and drug trials.
Explain: Accurately measures processes in a way that produces reliable data.
Applicability - positive:
Evidence: Psychoative drugs to treat serious mental illnesses due to increased understanding e.g. depression.
Explain: Sufferers are able to manage their condition and live a relatively normal life.
The Peripheral Nervous System (PNS)
The Somatic Nervous System:
The somatic nervous system is made of 12 pairs of cranial nerves, and 31 pairs of spinal nerves. These nerves both have sensory and motor neurons.
The somatic nervous system is also involved in relfex actions.
The Autonomic Nervous System:
Involuntary actions are regulated by the ANS. The ANS, too, is split into 2 parts: the Sympathetic and the Parasympathetic Nervous Systems. Both of these systems regulate the same organs but have opposite effects.
Sympathetic Nervous System:
Primarily involved in responses that help us deal with emergencies (for example, the flight or fight response). Neurons from the SNS trave to almost every organ and gland in the body.
Parasympathetic Nervous System:
As the SNS activates the body, the parasympathetic is designed to bring it back to its resting state. This is done by stabilising the heartbeat, signalling the end of release for adrenaline and reduces blood pressure.
Neuron Structure and Function
Dendrites are at one end of the cell and recieve signals from the other neurons or rom sesory receptors. They're connected to the cell body.
The cell body is the control centre of the neuron.
Impulses are carried along the axon from the cell body and terminated at the axon terminal. The myelin sheath insultates the axon and allows the nerve impulses to transmit more rapidly along the axon.
Carry nerve impulses from the sensory receptors to the spinal chord and the brain. They convert information from the receptors into neural impulses, which are translated into sensations so the organism can react appropriately.
Allow sensory and motor neurons to communicate with each other. These lie wholly in the brain and spinal chord.
Neurons located in the CNS and directly/indirectly control muscles. Motor neurons form synapses with muscles and control their contractions.
For an impulse to transfer from a presynaptic neuron to a postsynaptic neuron, it must be transferred across the synapse using neurotransmitters.
At the end of the axon are sacs known as synaptic vesicles which contain chemical messengers- neurotransmitters. As the action potential reaches the synaptic vesicles, it causes a release of the chemicals through exocytosis.
The released neurotransmitter diffuses across the gap between the pre- and postsynaptic cell where they bind to specialised receptors that recognise and activate by that particular neurotransmitter. Once activated, there are either excitatory or inhibitory effects.
The Endocrine System
The endocrine system is regulated by feedback. E.g: signal is sent from the hypothalamus to the pituitary gland in the form of a 'releasing hormone'. This causes the pituitary to secrete a 'stimulating hormone' into the bloodstream. This signals the target gland to secrete another hormone. As levels of this rise, the hypothalamus shuts down production of the releasing hormone, which in turn stops production of the stimulating hormone. The target gland's secretion stops, which cretaes a stable balance of hormones.
The Fight-or-Flight Response
- The hypothalamus (part of the brain ad CNS) activates the adrenal medulla, which is part of the ANS.
- The ANS is part of the peripheral nervous system which acts as a control system, maintaining homeostasis in the body. These activities are performed without conscious control.
- The adrenal medulla secretes the hormone adrenalie. This gets the body ready for a fight or flight response. The phisiological reaction includes increased heart rate.
- Adrenaline leads to the arousal of the sympathetic nervous system and reduced activity in the parasympathetic nervous system.
- Adrenaline creates changes in the body e.g. decreases in digestion and increases in sweating and blood pressure.
- Once the threat is over the parasympathetic branch takes control and bring the body back to a balanced state.
- This has no ill effects from this and has survival value in an evolutionary context.