Biological Bases of Behaviour. Lecture 3: Brain Cells

and Neural Communication

Learning Outcomes.

• At the end of this lecture you should be able to:

• 1. Describe the key elements of a nerve cell (neuron).

• 2. Describe the main support cells of the CNS. • 3. Explain what is meant by the term 'membrane

potential'. • 4. Explain how an action potential is initiated and

conducted down the axon

1. Neurons.

• According to Williams & Herrup (1988) the adult human brain contains around 100 billion neurons (nerve cells).

• These are specialised cells which receive and transmit information.

• They vary in size and shape but all consist of the same basic structures:

Structure of a Neuron.

• a) Cell body (soma). • Contains the nucleus,

which houses the chromosomes.

• The bulk of the cell consists of cytoplasm - a jelly like substance containing structures which carry out certain functions:

soma

nucleus

Structures in the Cytoplasm.

• Mitochondria: Extract energy from the breakdown of nutrients and provide energy in the form of adenosine triphosphate (ATP).

• Endoplasmic reticulum: Store and transport chemicals through the cytoplasm. 2 forms:

• i) Rough endoplasmic reticulum contain ribosomes which are involved in protein synthesis.

• ii) Smooth endoplasmic reticulum transport substances around the cytoplasm and produce lipid (fat).

• Golgi apparatus: A special type of endoplasmic reticulum breaks down substances no longer required by the cell.

• The plasma membrane separates the inside of the cell from the outside, it is selectively permeable with charged ions only able to pass through protein channels.

b) Dendrites.

• These are the information-receiving parts of a neuron.

• Dendrites receive chemical information across a tiny gap called a synapse.

• The surface of a dendrite is lined with synaptic receptors.

• Outgrowths called dendritic spines increase the surface area available for synaptic communication.

dendrites

Dendritic spines

c) Axon.

• This is the information-sending part of the neuron

• A neural impulse (action potential) flows along the axon.

• Many vertebrate axons are covered with an insulating substance called a myelin sheath.

• This consists of segments separated by unmyelinated regions

called nodes of Ranvier. Axon Node of Ranvier

myelin

The Information Flow.

• Action potentials flow along the axon to the presynaptic terminals.

• Axons that send information to the periphery are called efferent axons (e.g. motor neurons).

A motor neuron Muscle fibre

Presynaptic terminals

Flow of information

The Information Flow, continued.

• Axons that receive information from the periphery are called afferent axons (e.g. sensory endings in the skin).

• Thus, motor neurons act as efferents from the nervous system, sensory neurons act as afferents into the nervous system.

• So, efferent out, afferent in.

A sensory neuronCross section of skin

Sensory endings

Information flow

d) Presynaptic Terminals.

• At the end of an axon are the presynaptic terminals (or terminal buttons).

• When an action potential reaches the terminal buttons they secrete a transmitter substance which travels across the synapse to the next neuron in the chain.

• The neurotransmitter either excites or inhibits the postsynaptic receptors (dendrites) of another neuron.

• Thus an individual neuron receives information via its dendrites from the terminal buttons of axons from other neurons, the terminal buttons of its axon send information to other neurons.

Presynaptic Terminals.

Neuron A

Neuron BAxons from other nuerons influence neuron A

Message flows down axon of neuron A to influence neuron B

2. Support Cells.

• Neurons have a high metabolic rate and must be constantly supplied with oxygen and glucose or they will die.

• The various support cells are thus very important. • Glial cells hold neurons in place, control their supply

of chemicals, insulate them, and remove neurons that have died. There are several forms:

• i) Astrocytes (astroglia): Provide physical support to neurons and clear up debris (called phagocytosis).

• ii) Oligodendrocytes: Produce myelin in the CNS. In the PNS the same function is provided by Schwann Cells. These digest dying cells and then guide the axons to re-grow to a limited extent.

• This does not happen in the CNS so that nerve damage (e.g. in spinal neurons) is to be permanent.

Electrical Activity Within a Neuron.

• A microelectrode is placed in the axon of a giant squid.

• An electrode is placed in the surrounding medium.

• Both are connected to a voltmeter.

• The inside of an inactive axon is negatively charged with respect to the outside.

• This resting potential is • -70mV.

voltmeter

microelectrode

electrode

The Action Potential.

• A positives charge applied to the inside of the axon makes it more positive (depolarisation).

• If a sufficiently strong charge is applied then the threshold of excitation is reached, and the neuron produces an action potential.

• Here the membrane potential is rapidly reversed and becomes strongly positive (up to +40mV) with respect to the exterior.

• The membrane potential quickly returns to normal, but first it briefly overshoots its resting potential and drops to around -75mV (hyperpolarisation).

• This entire process takes about 2msec.

The Action Potential.

The Membrane Potential.

• The electrical charge within the axon results from the balance between two opposing forces:

• i) Diffusion: Molecules distribute themselves evenly throughout the medium in which they reside.

• ii). Electrostatic pressure: Particles with the same electrical charge repel one another while particles with the opposite charge attract one another.

• The environment inside the axon and in the fluid surrounding it contain different ions.

• Organic ions (A-) only found inside the axon.• Potassium (K+) found predominantly inside the axon.• Chloride ions (Cl-) found predominantly outside the

axon.• Sodium ions (Na+) found predominantly outside the

axon.

Resting Potential.

• The axonal membrane is selectively permeable.

• At rest, ion channels permit potassium and sodium to pass through slowly.

• Most of the sodium channels remain closed.

The sodium-potassium pump expels sodium ions and draws in potassium ions in the ratio of 3 sodium out to 2 potassium in.

During an action potential the sodium channels open and allow sodium ions to flood into the axon.

Events During the The Action Potential.

After the Action Potential.

• Neurons may have different thresholds of excitation but all obey the rule that once the threshold is reached, an action potential is triggered – this is called the ‘all-or-none rule’.

• Following the action potential, the sodium gates remain closed for around 1ms and so further action potentials cannot be triggered regardless of the stimulation.

• This is called the absolute refractory period. • The sodium gates then open but the potassium gates

remain open for a further 2-4ms ensuring the no action potentials can be generated.

• This is called the relative refractory period. • The axon cannot cope with repeated excitation as the

sodium-potassium pump cannot keep up, as a result sodium accumulates within the axon and no more action potentials are possible. Scorpion venom keeps open the sodium channels and causes paralysis.

Conduction of the Action Potential in Unmyelinated Axons.

• Each point along the axon membrane generates the action potential. The next area of membrane is depolarised, reaches its threshold and generates another action potential. In this manner the action potential passes down the axon like a wave.

Conduction of the Action Potential in Myelinated Axons.

• These axons are covered with an insulating layer of myelin, separated by small unmyelinated gaps (nodes of Ranvier).

• Action potentials travel down the axon reducing in strength until they reach the next node where another action potential is triggered.

Saltatory Conduction.

• The jumping of action potentials from one node to another in myelinated axons is referred to as saltatory conduction.

• There are two advantages to this• 1. Energy is saved as sodium-potassium pumps are

only required at specific points along the axon. • 2. Conduction of an action potential is much faster

within a myelinated axon (around 120 m/sec as opposed to around 35 m/sec) in unmyelinated ones.