Brain and Behaviour

Neurons

specialized cells that make up the nervous system that send electrical and chemical signals through the brain and body. 

Glial Cells

non-neural cells which support and nourish neurons → produce myelin

White matter

axons

Grey matter

Soma and dendrites

Sensory Neurons

receive signals from sense organs

Motor Neurons

send signals to muscle cells

Interneurons

connect neurons to other neurons

Electrical signalling

Electrical activity sends signals along the axon. This involves the movement of ions, which are electrically-charged atoms or molecules (ones which have gained or lost electrons and so have a net positive or negative charge).

Electrical Signalling p1 - Resting Potential

• The neural membrane has ion channels which selectively allow positively/negatively charged ions through. 

• The salty fluid outside cell has positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl^-) 

• Inside the cell has positively charged potassium ions (K⁺) and negatively charged protein molecules (A^-). 

• Sodium-potassium pumps actively transport Na⁺ ions out of the cell and K⁺ ions into the cell, such that the inside of the cell is more negative than the outside. 

• The resting potential (voltage) is about -70 millivolts (mV). 

Electrical Signalling p2 - Action Potential

• Electrical stimulation of the axon membrane can cause sodium channels to open. 

• Na⁺ ions flow into the cell 

• The interior becomes more positive than the outside: the voltage shifts from -70 mV to +40 mV in about 1 millisecond. 

Electrical signalling p3 - resting potential is restored

• Sodium channels close 

• Potassium channels open 

• K+ ions flow out of the cell, repolarizing and then slightly hyperpolarizing the membrane 

• The K+ channels close and the sodium-potassium pump gradually restores the resting potential 

Myelination

Myelin is a white layer of fatty insulation that surrounds many axons. 

• Nodes of Ranvier are points where the myelin is very thin or absent. 

• Depolarization at one node can activate the next; the signal “jumps” from node to node  

• This allows more rapid transmission than when the signal has to pass continuously along the membrane. 

• Myelin is not fully formed at birth; its development may partly account for improvement in co-ordination etc. 

• Loss of myelin can cause health problems (e.g., Multiple Sclerosis, in which movement problems result from destruction of myelin by the immune system). 

Somatic Nervous system

Part of peripheral nervous system

consists of Afferent sensory neurons, which transmit signals from the sense organs (e.g., eyes, ears, nose) 

• Efferent motor neurons, which carry signals from the CNS to muscles in order to produce voluntary movements. 

Autonomic nervous system

part of peripheral nervous system

senses and influences the body’s internal state by regulating the activities of glands and involuntary muscles comprising the heart, blood vessels, and lining of the digestive system. It is divided into sympathetic and parasympathetic

Sympathetic Nervous System

typically involved in activation/arousal and acts as a unit.  

• E.g., in response to stress, the sympathetic system speeds up the heart, dilates the pupils, constricts peripheral arteries and reduces digestive activity so more blood flows to the muscles and brain, and triggers the release of adrenaline in order to raise blood sugar. 

Parasympathetic Nervous System

more selective, affecting one or a few organs at a time. It typically produces effects opposite to those of the sympathetic nervous system to promote a state of rest. 

• E.g., The parasympathetic nervous system slows the heart and dilates the blood vessels supplying the digestive system. 

MRI

• Spinning protons have tiny magnetic fields 

• Apply strong magnetic field and they align 

• Beam radio waves to perturb them 

• Turn off radio waves and they relax back to alignment… 

• …releasing radio waves, with a time course that depends on the environment (e.g., the density of the tissue). 

DTI

• Graded magnetic fields establish freedom of water molecules to diffuse 

• Axons are like straws, so DTI identifies white matter tracts 

MRI and DTI can be used to link structural properties of the brain to changes and differences in mental functioning. 

Microelectrode recording

In animals, researchers can use electrodes to record the activity of individual neurons or groups of neurons in the brain. Occasionally, electrical recording is possible for humans undergoing brain surgery.  

• Electrode array records activity while the patient performs tasks 

• Can record single cell activity 

• Limited opportunities for implementation! 

Electroencephalography (EEG)

A net of electrodes is placed on the scalp to detect the voltages associated with the electrical activity of large groups of neurons.  

• Signal reflects graded postsynaptic potentials resulting from neurotransmitters binding to postsynaptic membranes (not the very short-lived action potentials that send signals along the axon). 

• Primarily reflects activity near brain surface 

• High temporal precision 

• Spatial origin of the signals is unclear 

• Can detect regular oscillations (“brain waves”)… 

• …and event-related potentials (ERPs) evoked by specific stimuli 

Magnetoencephalography (MEG)

Because electricity and magnetism are two sides of the same coin, we can use magnetometers to detect tiny changes in the magnetic field around the brain, induced by the electrical activity of groups of neurons. 

MEG has: 

• High temporal resolution. 

• Better spatial resolution than EEG because there is less distortion by skull and skin. 

Functional magnetic resonance imaging (fMRI)

fMRI is a variant of MRI that is based on the Blood Oxygenation Level Dependent (BOLD) contrast. 

• Active neurons need oxygen. 

• There is a temporary rise in oxyhaemoglobin at active sites. 

• Oxyhaemoglobin and deoxyhaemoglobin respond differently to magnetic fields. 

• fMRI detects which brain regions have increased oxyhaemoglobin levels, indicating greater neural activity. 

• Poorer temporal resolution than EEG/MEG because changes in blood flow take time.  

• Provides more information about spatial patterns of activation than EEG/MEG. 

• Can measure activity in subcortical structures.  

Functional near-infrared spectroscopy (fNIRS)

fNIRS involves beaming infrared light through the skull and measuring the light which is reflected back. 

• Oxygenated and deoxygenated blood absorb the infrared light to different extents, so it is possible to infer which areas of the cortex are currently using energy. 

• The light can only be used to probe a short distance below the scalp, so only cortical activity is measured. 

• Poor temporal resolution 

• Worse spatial resolution than fMRI 

• However, fNIRS is cheaper, easier to set up, and the participant can move whereas fMRI requires the person to be completely still. 

Positron-emission tomography (PET)

PET involves introducing radioactive versions of chemicals used by the body and then detecting the emission of radiation. (The radioactive decay involves emission of positrons which collide with electrons leading to emission of gamma rays which can be detected by the scanner). 

• Radiolabelled glucose and oxygen are used to detect regions using energy (e.g., where neurons are active). 

• Radiolabelled neurotransmitters can be used to gauge where particular types of chemical signal are being sent. 

Double Dissociation

occurs when damage to one region affects Task A but not Task B, and damage to another region affects Task B but not Task A. This provides stronger evidence for localization than does a single dissociation. 

Broca’s aphasia

preserved speech comprehensions but impaired speech production

Wernicke’s aphasia

impaired speech comprehension but preserved speech production (although words are often jumbled)

Chemical Stimulation

the introduction of a cannula (tube) into a specific part of the brain; neurotransmitters or other chemicals are introduced to affect the activity of the neurons. 

Electrical stimulatiom

• inserting electrodes, which can be small enough to simulate individual cells. 

• Electrical stimulation is occasionally possible when humans are undergoing brain surgery. 

Transcranial electrical stimulation

Selected brain regions can also be stimulated non-invasively: 

• An electrical current is applied to electrodes on the scalp. 

• The current affects the activity of underlying neurons. 

• There is some evidence that this affects task performance. 

Transcranial magnetic stimulation

non-invasive technique. 

• An electrical current is passed through a coil to create a magnetic field. 

• The magnetic field disrupts the electrical activity of neurons under the coil. 

• Disrupting specific regions provides a way to study their functions, akin to the role of lesions in neuropsychology. 

• The effects are temporary and the procedure is regarded as safe. 

• TMS is also used as a treatment for depression. 

Hindbrain

lowest and most “primitive” part of the brain. It is formed where the spinal cord enters the brain. Together with the midbrain it forms the brain stem. The hindbrain consists of the: 

• Medulla oblongata 

• Cerebellum

• Pons

Medulla Oblongata

• First structure above spinal cord. 

• Helps regulate vital functions (e.g., heart rate and breathing). 

• So, damage usually => death 

• Sensory and motor neurons cross over as they pass through the medulla on their way to/from higher brain structures – so the left side of brain processes information for the right side of body, and vice-versa. 

Cerebellum

• Co-ordinates precise motor neurons. 

• Has more neurons than the rest of the brain. 

Pons

• Just above the medulla 

• Helps regulate basic functions and sleep 

• Relays sensory information between cerebellum and cerebral cortex.  

Midbrain

lies just above hind brain and includes part of the:

• reticular foemation

Reticular formation

• Extends from hindbrain to lower portions of forebrain. 

• Alerts higher brain regions that sensory signals are coming. 

• Admits or blocks the passage of these signals. 

• Is therefore important in sleep, consciousness, and attention. 

Forebrain

most recently-evolved part of the brain. It wraps around the brain stem (hindbrain and midbrain). It includes the diencephalon and the cerebrum (aka, the telencephalon). 

Diencephalon

lies between mid brain and cerebellum. Structures include:

• Thalamus

• Hypothalamus

Thalamus

• A “switchboard” for organizing and routing sensory information to other parts of the brain. 

• Disruption is implicated in schizophrenia. 

hypothalamus

• Important in basic emotions and motivations – e.g., sleeping, eating, aggression. 

• E.g., damage to one part can reduce sex drive. 

• Affects pituitary gland which in turn shapes hormone secretions by other parts of the endocrine system

Cerebrum

(aka telencephalon) includes an outer layer of cerebral cortex, and subcortical structures below this.  

Subcortical structures include: 

• Hippocampus

• Amygdala

Hippocampus

• involved in forming and retrieving memories

• damage can prevent formation of new memories

Amygdala

involved in emotion, especially fear and aggression

Cerebral Cortex

the most recently-evolved part of the forebrain. It is about 6.3mm thick and made of grey matter. 

The cortex is wrinkled so it has sulci (fissures) which constitute about 75% of its area, and gyri (raised structures). 

Prominent sulci anatomically divide the cortex into broad regions: 

• The medial longitudinal fissure runs along the centre at the top of the brain. 

• The central fissure runs roughly from the top down the middle of each side. 

• The sylvian fissure runs from front to back along the side.  

These fissures divide each hemisphere into 4 regions: 

• The temporal lobe. 

• The occipital lobe. 

• The parietal lobe. 

• The frontal lobe. 

Motor Cortex

situated near the central fissure, this controls voluntary muscle movements. Body areas are represented as an upside-down version of their layout in the body. The area devoted to each part of the body is proportional to the complexity of the movements it makes – e.g., there is a large region for the fingers. The neurons sending messages from the motor cortex cross at the medulla, so damage to one hemisphere impairs movement of the other side of the body. 

Sensory Cortices

distinct regions of cortex receiving information from each sense

The somatosensory cortex

just behind the motor cortex and has a similar roughly-upside-down mapping of the body, with more sensitive parts of the body being represented by larger areas of cortex. The somatosensory cortex processes information about touch, temperature, balance, and movement. 

The primary auditory cortex

lies on the temporal lobe; each side receives information from both ears.

the primary visual cortex

• lies at the back of the occipital lobe. 

Some cells in the sensory cortices are selective for particular types of sensory signal – e.g., some visual neurons fire only in response to lines of a particular orientation. 

Association Cortex

the 75% (approximately) of the cortex that is not sensory or motor cortex.  

• Stimulation does not generate sensation or movement. 

• Damage disrupts complex functions like problem-solving and speech. 

  • E.g., visual agnosia leads to an inability to connect information sent to visual cortex with information about the nature/names of objects that is stored in other cortical areas. 

Prefrontal cortex

a region of association cortex at the front of the frontal lobe and is argued to be especially important in “executive functions”. These include holding and manipulating information to solve problems; controlling one’s impulses (e.g., delaying pleasure or suppressing aggression); and expressing appropriate social behaviours.