8.1 The nervous system and nerve impulses

Question 1

Reflex arc

Answer 1

1. RECEPTORS detect a stimulus and generate a nerve impulse.
2. SENSORY NEURONES conduct a nerve impulse to the CNS along a sensory pathway
3. Sensory neurones enter the SPINAL CORD through the dorsal route.
4. sensory neurone forms a synapse with a RELAY NEURONE
5. Relay neurone forms a synapse with a MOTOR NEURONE that leaves the spinal cord through the ventral route
6. Motor neurone carries impulses to an EFFECTOR which produces a RESPONSE.

Question 2

Impulse, Nerve impulse

Answer 2

A wave of electrical activity which passes along a nerve or over the surface of a muscle.

Question 3

organisation of the nervous system

Answer 3

1 Nervous system > 1.1 Central nervous system - 1.2 Peripheral nervous system. > 1.2.1 Somatic nervous system 1.2.2 Autonomic nervous system> 1.2.2a Sympathetic nervous system - 1.2.2b parasympathetic nervous system

Question 4

Central nervous system

Answer 4

The brain and spinal cord. The central nervous system is responsible for coordinating and controlling the nervous system.

Question 5

Peripheral nervous system

Answer 5

subsection of NS
consists of
-sensory nerves
- motor nerves

Question 6

Autonomic nervous system

Answer 6

The part of the nervous system that is not under conscious control. The nerves of the autonomic nervous system stimulate muscles and glands. The autonomic nervous system is made of up of two parts. The sympathetic system plays an important part in controlling the body’s reactions to stress. The parasympathetic is more important when the body is at rest. The somatic nervous system is under conscious control.

Question 7

Somatic nervous system

Answer 7

Part of the nervous system which is involved in transmitting impulses from sense organs to the central nervous system and from the central nervous system to the skeletal muscles which are under voluntary control.

Question 8

Parasympathetic nervous system, Parasympathetic nerve

Answer 8

The part of the autonomic nervous system (the nervous system which you have no control over) that is important when the body is at rest. Stimulation
of the parasympathetic nerves reduces heart and breathing rate but increases muscle action associated with digestion. The vagus nerve is a parasympathetic nerve. It leads from the cardiovascular control centre in the brain to the sinoatrial node in the heart. Nerve impulses passing along this nerve slow the heart beat.

Question 9

Sympathetic nerve, sympathetic nervous system

Answer 9

One of the nerves that goes from the brain to the organs of the body and which prepares the body for action, controlling the functions of organs during times of stress. For example, sympathetic nerves are involved in increasing heart rate and blood pressure.

Question 10

Neurone, Sensory neurone, Motor neurone, Relay neurone A nerve cell.

Answer 10

A neurone has a cell body containing a nucleus and long thin branches along which impulses pass. Neurones which carry impulses from receptors are referred to as sensory neurones. Those which transmit impulses to effectors are motor neurones. Relay neurones transmit impulses from one cell to another. Neurone cell bodies are located within the central nervous system. Neurones do not join directly to each other. They are separated by tiny gaps called synapses

Question 11

Cell body

Answer 11

Part of a neurone which contains the nucleus and many of the other cell organelles. Long processes extend from the cell body. These are the dendrites, which conduct impulses towards the cell body, and an axon, which conducts impulses away.

Question 12

Dendrite

Answer 12

One of the processes on a nerve cell or neurone which carries impulses towards the cell body.

Question 13

Axon

Answer 13

A long process extending from a neurone which carries impulses away from the cell body.

Question 14

motor neurone

Answer 14

cell body is always situated within the central nervous system(CNS) and the axon extends out, conducting impulses from the CNs to effectors.

Question 15

sensory neurone

Answer 15

these carry impulses from sensory cells to the CNS

Question 16

relay neurone

Answer 16

these are found mostly within the CNS. They can have a large number of connections with other nerve cells.

Question 17

Myelin, Myelin sheath, Myelination

Answer 17

Fatty material which forms a layer round the axons in many mammal neurones. It acts as an insulator and only allows the electric currents which are set up by an action potential to flow at the gaps where no myelin is present. Nerve impulses therefore travel along myelinated axons in a series of jumps. This allows them to transmit impulses much faster than non-myelinated neurones.

Question 18

Schwann cell, Schwann cells

Answer 18

A cell that produces myelin, the fatty material that surrounds many axons in mammalian nerve cells. Each Schwann cell wraps round and round the axon forming a layer of myelin. The small gaps between neighbouring Schwann cells, called nodes of Ranvier, allow a nerve impulse to travel along a myelinated axon in a series of jumps.

Question 19

Reflex

Answer 19

A simple pattern of behaviour which involves a rapid response to a stimulus. For example, if the hand touches a hot object, it is rapidly withdrawn. Most nerve pathways involve many neurones. A reflex, however, usually only involves two or three. This explains why reflexes are very rapid.

Question 20

Reflex arc

Answer 20

The nerve pathway associated with a reflex. Some reflexes, such as the knee-jerk reflex, only involve two neurones: a sensory neurone and a motor neurone. Others, such as that involving the withdrawal of the hand after touching a hot object, involve three neurones. These are a sensory neurone, a relay neurone and a motor neurone.

Question 21

Receptor, Receptor cells

Answer 21

Receptor cells that detect stimuli and send impulses to the central nervous system. Many receptors are spread throughout the body, but some types of receptor cells are grouped together into sense organs. Rods and cones are the receptor cells in the eye that detect light.

Question 22

Effectors, Effector cells

Answer 22

A tissue or organ that responds to a nerve impulse or hormone. An effector brings about a response or a change. In a mammal, muscles and glands are examples of effectors

Question 23

Radial muscle (eye)

Answer 23

• Controlled by autonomic nervous system
• controlled by a sympathetic reflex
• contract to dilate pupil

Question 24

Circular muscle (eye)

Answer 24

• controlled by the autonomic nervous system
• controlled by a parasympathetic reflex
• contract to constrict the pupil

Question 25

Photoreceptor

Answer 25

A receptor cell that is stimulated by light and enables us to detects changes in our environment. In the human eye photoreceptors are contained in the retina. There are two distinct types, rods and cones.

Question 26

Nerve

Answer 26

contains A bundle of axons of many neurones surrounded by a protective covering

Question 27

Purpose of pupil reflex

Answer 27

Prevent damage to the retina from high intensity light. In dim light it ensures maximum light reaches the retina

Question 28

Resting potential

Answer 28

The difference in electrical charge across the membrane of a resting neurone. When the inside of the axon is more negative than the outside, the membrane is said to be polarised. The potential difference across the membrane is about –70 mV; this is known as the resting potential. This is due to the difference in permeability of the cell surface membrane to different ions which results in their uneven distribution. A nerve impulse involves the temporary reversal of this negative potential difference, changing it to a positive value.

Question 29

How the resting potential is achieved

Answer 29

1. Na+/K+ pump creates concentration gradients across membrane. High K+ inside cell. High Na+ outside cell
2. K+ diffuse out of cell down the K+ conc grad through channel, making outside if membrane +ve and inside -ve creating a pd.
3. The pd will pull K+ back into the cell.
4. At -70mV pd, the 2 gradients counteract each other and there is no net movement of K+.

Question 30

Depolarisation, Depolarise

Answer 30

The reversal of the
potential difference across the cell surface
membrane of a nerve cell as a nerve impulse
passes along it. At rest, the inside of the cell
surface membrane of an axon is negative and the
outside is positive. During the passage of an
impulse, the potential difference is locally
reversed, making the inside positive compared to
the outside. There is also depolarisation of the
heart muscle cells during contraction of the
heart.

Question 31

Repolarisation

Answer 31

The return to a resting potential
after the membrane of a nerve cell has been
depolarised during an action potential. At rest, the potential difference of the inside of an axon is negative with respect to the outside. During an
action potential, this potential difference is
reversed making the inside positive compared to the outside. This reversal of potential difference is known as depolarisation. During
repolarisation the potential difference across the membrane changes back from approximately +40 mV to the resting potential of 70 mV.

Question 32

Action potential

Answer 32

The changes which take place
across the cell surface membrane of a nerve
cell during the passage of an impulse. These
changes are brought about by the opening and closing of voltage-dependent Na+ and K+ gated channels in the membrane.

Question 33

Voltage-dependent gated channels

Answer 33

Protein channels that span membranes which are opened or closed by a change in the potential difference (voltage) across the membrane.

Question 34

What causes an action potential?

Answer 34

1. Depolarisation - Voltage dependent Na+ channels open. Na+ flow into axon, depolarising the membrane.
2. Repolarisation - voltage dependent Na+ channels close. voltage dependent K+ channels open. K+ leave the axon, repolarising the membrane.
3. Restoring resting potential - The membrane is now hyperpolarised. Voltage dependent K+ channels close. K+ diffuse back into the axon to recreate the resting potential

Question 35

Positive feedback

Answer 35

Positive feedback is the
sequence of events where a change in a system sets in motion processes that cause the system to change even further. For example, water vapour in the atmosphere is one of the greenhouse gases which result in global warming. As a consequence of higher temperatures caused by global warming, more water will evaporate from the Earth’s surface. The amount of water vapour
in the atmosphere will increase further. This is an example of positive feedback.

Question 36

All-or-nothing

Answer 36

An expression used to describe an
action potential. When an action potential is
produced in a nerve cell, it is always the same size. It does not matter how big the initial stimulus is, the action potential will always involve the same change in potential difference across the cell surface membrane. Because of this, the only way that information about the strength of a stimulus can be carried is by varying the number of nerve impulses in a given
time.

Question 37

Hyperpolarisation, Hyperpolarised

Answer 37

Making the
potential difference across the cell surface
membrane of a nerve cell more negative. At
rest, the membrane is polarised and the inside of
the axon is more negative than outside: the
potential difference across the membrane is
about 40 mV. If the inside becomes more
negative with respect to the outside, the
membrane is said to be hyperpolarised.

Question 38

Refractory period

Answer 38

The short period of time after an impulse has passed along a neurone when a new action potential cannot be generated. It lasts until all the sodium ion and potassium ion
channels have closed and the resting potential has been restored.

Question 39

Nodes of Ranvier

Answer 39

The small gaps between
neighbouring Schwann cells. Each Schwann cell wraps round and round the axon forming a layer of myelin that insulates the axon of a neurone. The axon membrane can only depolarise at the nodes of Ranvier so nerve impulses travel rapidly along a myelinated axon in a series of jumps.

Question 40

Saltatory conduction

Answer 40

The way in which a nerve
impulse travels along a myelinated axon in a
series of jumps. Myelin is fatty material which
forms a layer round the axons in many mammal neurones. It acts as an insulator and only allows the electric currents which are set up by an action potential to flow at the gaps where no myelin is present. This allows myelinated axons to transmit impulses much faster than non myelinated ones.

Question 41

Synapse, Synapses

Answer 41

A small gap between two
neurones. A nerve impulse travels along the
presynaptic neurone. When it arrives at the
synapse, it causes small amounts of a chemical
neurotransmitter to be released. The
neurotransmitter diffuses across the synapse and
produces a nerve impulse in the postsynaptic
neurone.

Question 42

Synaptic cleft

Answer 42

The gap between a synaptic knob
and post-synaptic membrane at a synapse. When a nerve impulse arrives at a synapse, it causes small amounts of a chemical neurotransmitter to be released. The neurotransmitter diffuses
across the synaptic cleft and produces a nerve impulse in the second neurone.

Question 43

Presynaptic

Answer 43

A cell or membrane which comes
before a synapse is described as presynaptic.
Nerve cells or neurones do not join directly to each other. They are separated by tiny gaps called synapses. A nerve impulse travels along the first neurone, across the synapse and then along the second neurone. The first neurone is
the presynaptic neurone because it comes before the synapse.

Question 44

Postsynaptic

Answer 44

A cell or membrane which comes
after a synapse is described as postsynaptic.
Neurones do not join directly to each other.
They are separated by tiny gaps called synapses. A nerve impulse travels along the first neurone, across the synapse and then along the second neurone. The second neurone is the postsynaptic neurone because it comes after the synapse.

Question 45

Synaptic vesicle

Answer 45

A small membrane-surrounded
sac in the presynaptic neurone containing
neurotransmitter. When an action potential
arrives at the synapse, calcium ions enter
through channels that open in the presynaptic
membrane. They cause the synaptic vesicles to
fuse with the membrane and empty
neurotransmitter into the synaptic cleft.

Question 46

Synaptic knob

Answer 46

A swelling at the end of a presynaptic
neurone. Synaptic knobs release
neurotransmitter into the synaptic cleft.

Question 47

Neurotransmitter

Answer 47

Chemical that is responsible

for carrying a nerve impulse across a synapse.

Question 48

Acetylcholine

Answer 48

Acetylcholine is a
neurotransmitter. It is involved in the
transmission of impulses across synapses.
When an impulse arrives at the presynaptic
membrane, acetylcholine is released into the
synaptic cleft. It diffuses across the cleft and
attaches to receptors on the postsynaptic
membrane. This triggers an impulse in the
postsynaptic neurone.

Question 49

Acetylcholinesterase

Answer 49

An enzyme found in
synapses that have acetylcholine as a
neurotransmitter. Acetylcholinesterase breaks
down acetylcholine once it has triggered an
impulse in the postsynaptic neurone. This is
important as unless acetylcholine is removed
rapidly from a synapse there would be
continuous transmission along the postsynaptic
neurone.

Question 50

Excitatory synapse

Answer 50

Neurotransmitters released
from an excitatory synapse make the
postsynaptic membrane more permeable to
sodium ions, increasing the likelihood that an
action potential will be triggered in the
postsynaptic neurone. Several excitatory
impulses added together produce sufficient
depolarisation to produce an action potential in the postsynaptic cell.

Question 51

Summation

Answer 51

When several impulses added
together produce an action potential in the
postsynaptic cell. Spatial summation is when
these impulses come from synapses with
different neurones. Temporal summation is
when several impulses arrive along a single
neurone.

Question 52

Spatial summation

Answer 52

Spatial summation is when

impulses from different synapses add together to produce an action potential in the postsynaptic cell.

Question 53

Temporal summation

Answer 53

Temporal summation is
when several impulses arrive along a single
neurone and add together to produce an action potential in the postsynaptic cell.

Question 54

Inhibitory synapse

Answer 54

A synapse which will make
it less likely that an action potential will occur
in the postsynaptic nerve cell. The
neurotransmitter in these synapses opens
chloride and potassium ion channels so that the potential across the postsynaptic membrane
becomes more negative than usual.

Question 55

Hormone

Answer 55

A substance that acts as a chemical
messenger. Hormones are secreted into the
blood by endocrine glands. The blood transports
them to particular organs where they help to
regulate body processes. Via altering gene expression. The hormone insulin, for example, is secreted by the pancreas. It
travels in the blood to various organs including
the liver. It is important in helping the cells of
the liver to control the concentration of glucose
in the blood.

Question 56

Plant growth substance

Answer 56

A molecule which is
produced in small amounts by plant cells and
which controls growth and development. Plant growth substances have many commercial uses. They are used, for example, in micropropagation to produce new plants from genetically modified plant cells.

Question 57

Indoleacetic acid, IAA

Answer 57

The most commonly
occurring plant hormone of the class auxins.
IAA has many effects in the control of growth
and development through cell elongation and
division. Synthetic auxins have been widely
used as weed killers.