Biological Bases of Behaviour
Neuroanatomy is the study of the parts and functions of neurons. Neurons are individual nerve cells. Each neuron is made of distinct parts.
Dendrites - Rootlike parts of the cell that stretch out from the cell body. Dendrites grow to make synaptic connections with other neurons. This is where a signal from another neuron is received.
Soma- The cell body, contains the nucleus and the cell’s organelles
Myelin Sheath- A fatty covering of the axon that allows neural impulses to speed up
Terminal Buttons- The branched end of the axon that contains neurotransmitters
Neurotransmitters- Chemicals contained in terminal buttons that enable neurons to communicate.
Synapse- The space between the terminal button of one neuron and the dendrites of another.
A resting neuron is slightly negative because mostly negative ions are within the cell and mostly positive ions are on the outside of it. The cell membrane is selectively permeable. When the terminal buttons are stimulated, neurotransmitters are released into the synapse. They fit into receptor sites on the dendrites of the other neuron. Once the threshold is reached (enough neurotransmitters are received) the cell membrane of the other neuron becomes permeable, and positive ions flood the cell, making the overall charge of the cell positive. This change in charge travels down the neuron- this is an action potential. Once the action potential reaches the button, neurotransmitters in the neuron are released into the synapse. The process then may begin again in another neuron. Either a neuron fires completely or doesn’t fire at all- this is the all-or-none principle. A neuron cannot fire a little or a lot. It’s the same every time.
Some neurotransmitters are excitatory, meaning that they excite the next cell into firing. Others are inhibitory meaning that they inhibit the next cell from firing.
Afferent Neurons- Take information from the senses to the brain
Interneurons- Take messages from afferent neurons and send them elsewhere in the brain or on to efferent neurons
Efferent Neurons- Take information from the brain to the rest of the body
The Organisation of the Nervous System
The Central Nervous System- The brain and the spinal cord
The Peripheral Nervous System- The other nerves in the body- it’s divided into two categories, the somatic nervous system (voluntary muscle movements) and the autonomic nervous system, which itself is broken up into two parts:
Sympathetic Nervous System- Mobilises the body in response to stress (fight or flight)
Parasympathetic Nervous System- Slows the body down in after a stress response (rest and digest)
Normal Peripheral Signal Transmission:
Sensory neurons are activated, transmitting a message up a neuron to the spine (afferent nerves) the message moves up through your spinal cord until it enters the brain through the brain stem to the sensory cortex. In response, the motor cortex sends impulses down the spinal cord to the muscles where the sensory neurons were activated, allowing a response to take place.
Reflexes are responses that take place outside of conscious control. Sensory information is taken to the spinal cord, and processed by the spinal cord, returning to the motor neurons.
Accidents: Accidents which damage the brain can serve to show what the damaged part of the brain does, as that section of the brain will no longer function properly. Phineas Gage’s famous railroad accident caused him to act without inhibition, showing the frontal lobe played a part in enforcing social norms. He was also highly emotional.
Lesions: The removal or destruction of part of the brain. Nowadays, this is never done purely for experiments. It’s often done to remedy certain illnesses. Back in the day, mental illness was often remedied by frontal lobotomies, as it would cause the patient to calm.
Electroencephalogram (EEG): Detects brain waves- allows scientists to study how active the brain is, especially in sleep research.
Computerised Axial Tomography (CAT) Scan: A sophisticated X-ray- cameras rotate the brain and combine the images into pictures which show a detailed 3D image of the brain’s structure.
Magnetic Resonance Imaging (MRI): Similar to a CAT scan, however, uses magnetic fields to determine the density and location of brain material to create a more detailed image of the brain.
Positron Emission Tomography (PET) Scan: Measures how much of a specific chemical (often glucose) the brain is using, showing which part of the brain is most active during certain tasks.
Functional MRI (fMRI): Combines the elements of an MRI scan and a PET scan. Shows the details of brain structure and information about blood flow in the brain.
How We Classify The Brain
There are 3 major sections of the brain; the hindbrain, the midbrain, and the forebrain.
Hindbrain: The life support system of the brain
The Medulla Oblongata: Involved in the control of blood pressure, heart rate, and breathing. Located above the spinal cord
The Pons: Above the medulla, connects the hindbrain with the mid and forebrain. Also involved in controlling facial muscles.
The Cerebellum: Located on the bottom rear of the brain, responsible for coordinating some habitual muscle movements
Midbrain: Just above the spinal cord. Very small in humans, but responsible for coordinating simple movement with sensory information. It integrates some types of sensory information and muscle movement. One important structure is the reticular formation which is a netlike collection of cells throughout the midbrain that controls general body arousal and focus. Without it, we would be in a coma.
Forebrain: The forebrain helps control what we consider consciousness. It is very large in humans, and what makes us, us.
Thalamus: Located on top of the brain stem. It receives sensory signals and relays them to the appropriate part of the brain.
Hypothalamus: Controls several metabolic functions such as body temperature, sexual arousal, hunger, thirst, and the endocrine system.
Amygdala and Hippocampus: The arms surrounding the thalamus. The amygdala is vital to experiencing emotion, and the hippocampus is vital to the memory system. Memories are processed through this area and sent to other parts of the cerebral cortex for permanent storage
The grey wrinkled surface of the brain, densely packed with neurons. When we are born, the cerebral cortex is full of disconnected neurons. As we grow and learn, dendrites grow and connect with each other, forming a complex neural web. The surface of the cerebral cortex is wrinkled. Those wrinkles are called fissures and they increase the available surface area of the brain. The Cerebral Cortex is made of 8 different lobes- four on each hemisphere. Any part of the Cerebral Cortex not associated with receiving sensory information or dealing with motor function is called an association area
FRONTAL LOBES: Large areas of the cerebral cortex at the front of the brain behind the eyes. The anterior part of the frontal lobe is called the prefrontal cortex and is responsible for directing thought processes. It is the brains central executive and is essential for predicting consequences, pursuing goals, controlling emotions, and abstract thought. Most people’s left hemisphere frontal lobe contains one of the 2 language processing areas- Broca’s area (responsible for the muscles necessary for speech production)- some left-handed patients have it in the right hemisphere. There is a thin strip at the back of the frontal lobe called the motor cortex which sends signals to the muscles, controlling voluntary movement
PARIETAL LOBES: Behind the frontal lobe on top of the brain. Contains the Sensory/Somatosensory Cortex right behind the motor cortex. It receives oncoming sensations from the rest of the body. Like the motor cortex, the top of the sensory cortex receives signals from the bottom of the body, and so on.
OCCIPITAL LOBES: Located at the very back of the brain, farthest away from the eyes- and yet it interprets messages from the eyes in our visual cortex Impulses from the right half of each retina is sent to the right occipital lobe, and impulses from the left half of each retina is sent to the left occipital lobe
TEMPORAL LOBES: Process sounds sensed by our ears. Sound waves are processed by the ears, become neural impulses and are interpreted in the auditory cortices. The auditory cortex is not lateralised like the visual cortex. Sound received from the left ear is processed in the auditory cortices in both hemispheres. Wernicke’s Area, the second language area is also located here. It’s responsible for comprehending language.
The cerebral cortex is divided into two hemispheres, connected by a nerve bundle known as the corpus callosum. The hemispheres are like mirror images of each other, however, are slightly different. The left hemisphere receives sensory information and controls the motor function of the right half of the body. The right hemisphere receives sensory messages and controls the motor function of the left half of the body. This is a concept known as contralateral control. The left hemisphere may be active during the logic and sequential tasks and the right during spatial and creative tasks, but not enough information is known to fully make that conclusion. The specialisation of function in each hemisphere is known as Brain Lateralisation/Hemispheric Specialisation. A lot of research into this comes from split-brain patients, whose corpus callosum has been severed. This is a treatment used to deal with severe epilepsy and was pioneered by Roger Sperry and Michael Gazzaniga.
The brain is somewhat flexible. If necessary, other parts of the brain can adapt to perform other functions- dendrites can grow to replace damaged sections of the brain, especially in younger children whose dendrites are growing at their fastest.
The nervous system sends rapid signals that go away quickly. The endocrine system sends hormonal signals which travel throughout the body and last for a long time. The hypothalamus controls this system.
Adrenal Glands: Produce adrenaline, signals the rest of the body to prepare for fight or flight
Ovaries and Testes: Produce sex hormones (estrogen, progesterone, and testosterone) which allow for sexual development.
Genetics is another important part of psychological research that is just now becoming realised. Most human traits like body shape, introversion or extroversion are the results of nature (genetics) and nurture (environment). Part of psychology is attempting to understand how much each of these concepts contributes to the development of traits.
Twins are an essential part of nature vs nurture experiments, as monozygotic twins (identical twins) contain the exact same DNA, studying them in separate environments can reveal the connection between nature and nurture. Studies like these have found that IQ is strongly connected to nature, however, there are biases in these studies. Since the twins share physical similarities, others may treat them in similar ways, creating the same effective psychological environment.
Chromosomal Abnormalities
Sometimes, errors occur during cell development, or as a fetus develops. There is more information on this in my post on molecular genetics. Here are some examples of disorders that occur as a result.
Turner’s Syndrome: Babies born with only one X chromosome, causing shortness, webbed necks, and differences in physical development.
Klinefelter’s Syndrome: Babies born with two X chromosomes and a Y chromosome, causing minimal sexual development and extremely introverted traits.
Down’s Syndrome: One of many disorders which inhibit mental development. It’s caused by an extra 21st chromosome and can cause a rounded face, shorter fingers and toes, slanted far set eyes, and poor mental development.
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