Biopsychology

The Nervous System

Divisions of the Nervous System

The human nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems.

Parts of the Nervous System

The brain is made up of two hemispheres, and its outer layer is called the cerebral cortex. The spinal cord passes messages to and from the brain and the peripheral nervous system and is also responsible for reflex actions.

The somatic nervous system transmits information from sensory receptors to the central nervous system and directs voluntary movement of muscles.

The autonomic nervous system transmits automatic signals such as breathing. The sympathetic nervous system prepares the body for the fight or flight response, while the parasympathetic nervous system returns the body to its resting state.

 

Neurons

Structure of Neurons

Neurons consist of several key components. The cell body contains the nucleus, which holds genetic information. Dendrites extend from the cell body and carry electrical impulses from other neurons towards the cell body. The axon carries impulses away from the cell body.

The axon is covered by a myelin sheath, which is a fatty layer that increases the speed of transmission. There are gaps in the myelin sheath called nodes of Ranvier, which allow impulses to jump along the axon and further increase transmission speed.

Types of Neuron

Motor neurons carry signals from the central nervous system to effectors such as muscles and glands. They have short dendrites and long axons.

Sensory neurons carry signals from sensory receptors to the central nervous system. They have long dendrites and short axons.

Relay neurons connect sensory neurons to motor neurons or other relay neurons within the central nervous system and typically have short dendrites and short axons.

Synaptic Transmission

Neurons do not touch directly but are separated by a small gap called a synapse. Signals cross this gap through the action of chemicals called neurotransmitters.

An electrical impulse, known as an action potential, travels along the axon of the presynaptic neuron. This triggers the release of neurotransmitters from vesicles into the synaptic gap. These neurotransmitters then diffuse across the synapse and bind to receptors on the postsynaptic neuron. This binding stimulates the postsynaptic neuron to generate an electrical impulse.

Some neurotransmitters are reabsorbed into the presynaptic neuron or broken down by enzymes, which is known as reuptake.

Summation

Neurotransmitters can be excitatory or inhibitory. Excitatory neurotransmitters increase the likelihood that a neuron will fire, while inhibitory neurotransmitters decrease this likelihood.

The overall effect, known as summation, depends on the balance of excitatory and inhibitory inputs. If the combined input reaches a certain threshold, an action potential is triggered.

 

The Endocrine System

Glands and Hormones

The endocrine system consists of glands that produce hormones, which are released into the bloodstream to regulate bodily functions. Hormonal responses are generally slower than nervous responses but often work alongside the nervous system.

The pituitary gland is known as the master gland because it controls the release of hormones from other glands. The hypothalamus regulates the pituitary gland.

The pineal gland releases melatonin to regulate sleep-wake cycles. The thyroid gland releases thyroxine to control metabolism. The adrenal glands release adrenaline and noradrenaline during the fight or flight response and cortisol during prolonged stress.

The ovaries release oestrogen to regulate the menstrual cycle and pregnancy, while the testes produce testosterone, which influences male characteristics and muscle development.

 

The Fight or Flight Response

Fight or Flight

The fight or flight response occurs when a stressor is perceived in the environment. The amygdala sends a signal to the hypothalamus, which activates the pituitary gland. This then triggers the sympathetic nervous system, which activates the adrenal medulla.

The adrenal medulla releases adrenaline and noradrenaline, which prepare the body for action. After the threat has passed, the parasympathetic nervous system restores the body to its resting state.

The Role of Adrenaline

During the sympathetic state, breathing rate and heart rate increase, pupils dilate, digestion is inhibited, and saliva production is reduced.

During the parasympathetic state, these changes are reversed. Breathing and heart rate decrease, pupils constrict, digestion resumes, and saliva production increases.

Evaluation of the Fight or Flight Response

One limitation is that it does not include the “freeze” response, which suggests that individuals may initially pause to assess danger.

There is also evidence of gender bias, as females may adopt a “tend and befriend” response rather than fight or flight.

Additionally, the response may be maladaptive in modern environments, as repeated activation due to everyday stressors can lead to health problems such as heart disease.

 

Ways of Studying the Brain

Post-Mortem Examinations

Post-mortem examinations involve studying the brain after death to identify abnormalities that may explain behaviour. This method was particularly useful before modern technology. However, it is difficult to establish cause and effect, and issues of consent can arise.

Functional Magnetic Resonance Imaging (fMRI)

fMRI measures brain activity by detecting changes in blood flow. It produces detailed, three-dimensional images of brain activity. It is non-invasive and has high spatial resolution, but it has poor temporal resolution due to delays between neural activity and blood flow changes.

Electroencephalogram (EEG)

EEGs measure electrical activity in the brain using electrodes placed on the scalp. They have high temporal resolution but cannot accurately locate the exact source of activity.

Event-Related Potentials (ERP)

ERPs are derived from EEG recordings and involve averaging responses to specific stimuli. They provide detailed information about timing but can be affected by lack of standardisation and interference.

 

Localisation of Function

Functions of Different Areas

The motor cortex controls voluntary movements on the opposite side of the body. The somatosensory cortex processes sensory information from the skin.

The visual cortex processes visual information, with each hemisphere processing opposite visual fields. The auditory cortex processes sound and speech information.

Broca’s area, usually located in the left hemisphere, is responsible for speech production, and damage can result in difficulty producing speech. Wernicke’s area is responsible for language comprehension, and damage can result in speech that lacks meaning.

Evaluation of Localisation of Function

Brain scan studies support localisation by showing that different areas are active during different tasks. However, research has shown that damage to Broca’s area may involve other regions as well.

Some psychologists argue that functions are more distributed across the brain rather than strictly localised. Research with animals has also suggested that some functions, such as learning, are not localised but are processed more holistically.

 

Hemispheric Lateralisation

Left and Right Hemispheres

Some functions, such as language, are lateralised, meaning they are primarily processed in one hemisphere, usually the left. However, many functions are distributed across both hemispheres.

The brain is contralateral, meaning the left hemisphere controls the right side of the body and the right hemisphere controls the left side.

Split Brain Research

Split-brain research involves patients whose corpus callosum has been severed, usually to treat epilepsy. Studies have shown that information presented to the right visual field can be verbally described, but information presented to the left visual field cannot, as it is processed by the right hemisphere, which lacks language ability.

However, patients can still demonstrate understanding through non-verbal responses, such as selecting an object with the left hand.

Evaluation of Hemispheric Lateralisation

There are individual differences between patients, and some brains may be more or less affected by surgery.

The findings may be oversimplified, as plasticity shows that functions can shift between hemispheres.

Research may lack ecological validity due to artificial testing conditions, and findings may not generalise to people without epilepsy.

 

Plasticity and Functional Recovery

Plasticity

Plasticity refers to the brain’s ability to change and adapt as a result of experience and learning. During early childhood, the brain has more neural connections than in adulthood. Synaptic pruning strengthens frequently used connections and removes unused ones.

Research has shown that experience can change brain structure, such as increased grey matter in areas associated with specific skills.

Functional Recovery

Functional recovery refers to the brain’s ability to recover after injury by reorganising itself. This can involve the growth of new neural connections, known as axon sprouting, or nearby areas taking over functions, known as recruitment of homologous areas.

Other processes include denervation supersensitivity, where neurons become more responsive, and neuronal unmasking, where inactive pathways are activated.

Evaluation of Plasticity

Research supports plasticity, showing changes in brain structure following learning or experience. The use of control groups strengthens the validity of these findings.

However, factors such as age can affect plasticity, as recovery tends to be more effective in younger individuals.

Evaluation of Functional Recovery

Research has shown that recovery can occur following brain injury, and this has led to the development of rehabilitation therapies.

However, recovery is influenced by individual differences, such as age, and may not be equally effective in all individuals.