Biological and Evolutionary Basis of Behaviour

Peripheral nervous system and motor neuron structure

The nervous system is divided anatomically into the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which consists of all neural tissue outside them. The PNS is itself divided functionally into the somatic nervous system, which controls voluntary skeletal muscle and carries sensory information, and the autonomic nervous system (ANS), which regulates involuntary functions of smooth muscle, cardiac muscle and glands. In short, PNS = somatic + autonomic. The term 'automatic nervous system' is not a recognised term in psychology or neuroscience; the correct name is autonomic nervous system.

Within the nerve pathways, the afferent (sensory) pathway carries messages from receptors to the CNS, while the efferent (motor) pathway carries commands from the CNS to muscles and glands. The autonomic system has two branches. The sympathetic division is the 'fight-or-flight' system, which mobilises the body for emergencies. The parasympathetic division maintains body functions under normal, resting conditions.

A vertebrate motor neuron has three main structural parts. The soma (cell body) contains the nucleus and integrates incoming signals. The axon is the long process that conducts the action potential away from the soma toward the muscle fibre. In peripheral nerves, the axon is wrapped by a myelin sheath that insulates it and speeds conduction. The myelin sheath is the white, fatty insulating layer around the axon. It is formed by Schwann cells in the PNS and by oligodendrocytes in the CNS. Gaps between myelin segments, called nodes of Ranvier, allow saltatory conduction, in which the impulse jumps from node to node and travels much faster than along an unmyelinated axon. Note that cortisol is a steroid hormone released by the adrenal cortex; it is not a structural part of any neuron.

Brain structures and functions

Each brain region has a characteristic role, and exam questions often test the pairing of structure and function.

• Cerebellum — the hindbrain structure that coordinates movement and controls fine motor skills.
• Thalamus — the relay station that receives sensory information, filters it, and transmits it to the cerebral cortex.
• Tectum — the midbrain roof, made up of the superior and inferior colliculi, which orients the organism toward stimuli in the environment.
• Medulla oblongata — the lowest part of the brainstem, which coordinates heart rate, circulation and respiration.

The cerebral cortex contains association areas, which are not dedicated to one sensory or motor function. They lie in both hemispheres, integrate information from primary sensory and motor areas, and support coordination, interpretation of information and higher mental processing such as reasoning and memory. Broca's area and Wernicke's area, by contrast, are specialised language regions, usually localised to the left hemisphere, so they do not describe a general integrative function.

The cortex is also described by its tissue and surface features. Gray matter consists of neuronal cell bodies and forms the folded sheet of neurons in each hemisphere. White matter consists of myelinated axons, which form the fibre tracts lying beneath the cortex. A fissure is a deep groove in the cerebral cortex, while a sulcus is a shallower groove. Spikes are electrical nerve impulses.

The hypothalamus regulates hunger. Lesions of the lateral hypothalamus abolish feeding (aphagia), which identifies it as the hunger-exciting region. Lesions of the ventromedial hypothalamus cause overeating and obesity, which identifies it as the region involved in stopping eating (satiety).

Neurotransmitter functions

Neurotransmitters are chemical messengers released at synapses, and each is linked to a characteristic set of functions.

• Acetylcholine — involved in skeletal muscle action, learning and memory. It also plays a role in attention and memory processes.
• Norepinephrine — primarily associated with alertness, arousal and wakefulness.
• Serotonin — regulates mood, eating, sleeping, anxiety and arousal.
• Dopamine — associated with movement control and with the sensation of pleasure and reward. Parkinson's disease is linked to the loss of dopamine-producing neurons, not to serotonin or GABA.
• GABA (gamma-aminobutyric acid) — the major inhibitory neurotransmitter in the brain.

Endorphins are endogenous opioid neuropeptides that the body itself synthesises and releases, mainly in the pituitary gland, the hypothalamus and other neural tissue. They bind to opioid receptors and inhibit the transmission of pain signals, so their principal effect is to reduce the perception of pain. They are naturally occurring rather than synthetic, although synthetic opioid drugs can mimic their action. An endorphin therefore does not increase the feeling of pain.

Behaviour genetics methods

Behaviour genetics seeks to separate the influence of genes from the influence of environment. In humans, this is done with naturally occurring variation in relatedness and rearing conditions, not with controlled breeding. The three principal designs are family studies, twin studies and adoption studies.