neurons aims: must be able to describe and label neuron. should be able to relate the structure of...

Neurons Aims:

Must be able to describe and label neuron.

Should be able to relate the structure of neurons to the functions they perform.

Could be able to outline the process by which a nerve impulse is passed along a neuron.

Nerve cell (neuron)

Nerve cells are the basic unit of the nervous system.

Axons and dendrites may vary in length.

A number of neurons bound together is called a nerve.

Types of Neurons1. Sensory (Affector) neurons

PNS to CNS

2. Motor (Effector) neurons CNS to PNS

3. Inter/connecting neurons Found in CNS

Activity Answer the questions from pages 95 and

96 in Biozone book.

Communication between neurons

The cells (receptors) of a sensory nerve detect an environmental disturbance.

Sensory neurons generate a signal that is passed as an electrical message between neurons to particular effector cells.

Adjacent neurons do not actually touch each other.

There is a gap between them called the synaptic cleft.

The connection between the axon of one neuron and the dendrite of another is called a synapse.

The Synapse As the impulse reaches the

axon terminal the terminal produces chemicals known as neurotransmitters.

Neurotransmitters diffuse across the synaptic space.

Neurotransmitters bind to the dendrites of the next neuron.

Neuron then generates and carries an electrical impulse to it’s axon terminals.

Neurotransmitters Neurotransmitters are specialised chemical messengers:

produced by nerve cells transported in the axon released at the synapse

cause chemical and electrical changes in adjacent cells.

Most neurotransmitters play different roles throughout the body, many of which are not yet known.

Activity Answer the questions from page 101 in

Biozone book.

Neurohormones A neurohormone is any hormone secreted

by or acting on a part of the nervous system.

Neurohormonal activity is distinguished from that of classical neurotransmitters as it

can have effects on cells distant from the source of the hormone.

Neurohormone action:

Neurohormone action: Neurons in the hypothalamus

in the brain secrete neurohormones into blood capillaries that become associated with the pituitary.

Signals received by cells of the pituitary are transduced and the pituitary responds by producing a hormone as instructed.

This hormone enters the bloodstream and is transported to its target cells.

The Receptors Sensory neurons serve as the body’s receptors

as they are able to detect disturbances in the environment: Chemoreceptors:

chemicals Mechanoreceptors:

pressure Photoreceptors:

light Thermoreceptors:

temperature Pain receptors:

pain

The Nerve Impulse Cell membrane of an axon is

polarised:

(difference in charge between the inside and outside of the cell).

An axon at rest (not transmitting an impulse) negatively charged on the inside relative to the outside.

This is called a resting potential.

Na

Na

K

K

The Nerve Impulse To become polarised: sodium

– potassium pumps actively pump sodium ions out of the cell and potassium ions into the cell.

Three sodium ions are expelled for every two potassium ions pumped in.

Na

Na

K

K

The Nerve Impulse Na+ accumulates outside

and K+ inside.

They are both positive ions - but more ++ outside – so: negative inside and positive outside.

Na

Na

K

K

The Nerve Impulse As the impulse moves

along the axon, the permeability of the membrane changes…

… positive ions – Na+ move into the cell –

This results in the outside being more negative than the inside…

Na

Na

K

K

The Nerve Impulse A nerve impulse involves a

change in the charge across the axon membrane.

A nerve impulse is a wave of electrical change (an action potential) that passes rapidly along an axon.

After the nerve impulse has been transmitted – the distribution of ions across the cell membrane is restored.

Na

Na

K

K

Action Potential

Activity Video:

Answer the questions from pages 100 in Biozone book.

After Nerve Impulses

After the nerve impulse has been transmitted along the axon of one neuron it is transmitted further to: Another neuron An effector - Muscle An effector - Gland

Neuron pathways A neuron cell body

may have hundreds of axon terminals synapsing with it.

Two kinds of pathways:

1. Diverging pathway

2. Converging pathway

Diverging Pathway Single axon links to

several postsynaptic neurons

One cell can influence many others.

Converging Pathway Several axons link to

few postsynaptic neurons

One cell can be influenced by many others.

Neurotoxins. Homeostatic mechanisms, hormonal and

nervous, can be brought to a halt by adverse event.

Toxins often act on the nervous system,

causing paralysis and pain, even

death, unless treatment is obtained immediately…

Reflex Arc Happens in situations with

extreme stimulus.

Not enough time to involve the brain without serious injury.

Relay neurones in the spinal cord pass message from sensory neuron straight to motor neuron, they also transmit a pain signal to the brain.

React to the stimulus – Then feel pain after.

Comparing ResponsesReflex Arc Voluntary

ResponseInvoluntary

Response

Receptors Involved

General Pain Any Any

Transmission to CNS

Sensory (afferent) nerves

Sensory (afferent) nerves

Sensory (afferent) nerves

CNS Involved Spinal Cord Brain Brain - Hypothalamus

Transmission from CNS

Motor Neurons Motor Neurons Autonomic Nerves

Activity Answer the questions from page 99 in

Biozone book.