Nervous System

Question 1

Reflex

Answer 1

A rapid involuntary response to a change in the internal or external environment.

Question 2

What are the 4 key properties of a reflex?

Answer 2

1. stimulus - A stimulus is required to trigger a reflex.
2. involuntary - reflex occurs with no conscious thought.
3. Rapid - The reflex response is rapid with only a small number of neurons involved.
4. stereotyped - response occurs in the same way each time it occurs.

Question 3

Name the pathway of a reflex arc

Answer 3

1. A receptor at the end of a sensory neuron detects a change in the environment, and reacts to this stimulus by producing a nerve impulse.
2. Impulse travels from the receptor to the spinal cord through the DORSAL ROOT. ( sensory neuron is in the dorsal root)
3. Information is processed in CNS (spinal cord) The nerve impulse is passed across A SYNAPSE, either straight to a motor neuron or 1 or more interneurons to a motor neuron.
4. Motor neuron carries nerve impulse to an effector. through ventral root.
5. Effector (muscle or secretory cell) receives nerve impulse and carries out appropriate response.
6. summary - receptor> sensory neuron > interneuron > motor neuron > effector

Question 4

Label the diagram of a reflex arc

Question 5

Reflex arc

Answer 5

The pathway a nerve impulse follows in travelling from a receptor to an effector.

Question 6

Explain what Roots do

Answer 6

Roots link spinal nerve to spinal cord

Question 7

Explain what the dorsal root does

Answer 7

carries sensory neurons from receptors to the spinal cord. (back of body)

Question 8

Explain what the ventral root does

Answer 8

carries motor neurons from the spinal cord to the body. (front of body)

Question 9

What are learned reflexes

Answer 9

• reflexes that are established at birth
• Some motor patterns are learned through constant repetition and are called acquired reflexes, e.g. Maintaining balance on a bike.

Question 10

Spinal reflex

Answer 10

A reflex which is carried out by the spinal cord only, i.e. not carried up to the brain.

• eventually impulses may be sent to brain so that its aware of what occurred, person would become consciously aware after the response.

Question 11

State the location of thermoreceptors, what they detect and the two types.

Answer 11

• Thermoreceptors are located in the skin (peripheral) and hypothalamus(central)
• In the skin they detect the changes in temp of the external environment.
• In the hypothalamus they detect changes of the bodies internal temp. (37 degrees)
• Two types hot thermoreceptors which detect high temp and cold thermoreceptors which detect low temp.

Question 12

State the location of Osmoreceptors and what they detect.

Answer 12

• Osmoreceptors are located in the hypothalamus
• They detect changes in osmotic pressure ( thus they maintain water content)

Question 13

State the location of chemoreceptors and what they detect.

Answer 13

• chemoreceptors are located in the nose, mouth and blood vessels.
• They are stimulated by particular chemicals and are sensitive to PH of blood, and co2 and 02 conc. Chemoreceptors are also involved in regulating heartbeat and breathing

Question 14

State the location of touch receptors and what they detect.

Answer 14

• Touch receptors are located mainly in the skin
• They are sensitive to light touches, pressure and vibrations.

Question 15

state the location of Pain receptors (nociceptors) and what they detect.

Answer 15

• pain receptors are located in the skin and in mucous membranes (in the organs)
• They detect damaged to tissue or excessive stimulation from heat or chemicals.

Question 16

Define protective reflexes

Answer 16

Reflexes which protect the body from injury, e.g. Blinking, sneezing.

Question 17

Name the 5 major receptors

Answer 17

• thermoreceptor
• osmoreceptor
• chemoreceptor
• touch receptor
• pain receptor ( nociceptor )

Question 18

Define Neurons

Answer 18

A bundle of nerve fibres held together by connective tissue

Question 19

What do neurons consist of?

Answer 19

• A cell body - contains nucleus and cytoplasm with organelles
• dendrites- short extensions which receive messages and carry them toward the cell body.
• Axons - carries nerve impulses away from the cell body.

Question 20

What is a myelin sheath and its functions

Answer 20

• The myelin sheath is a layer of lipid material which covers the axon

Function
- Speeds up nerve impulses
- insulation
- protects axon from damage

if have sheath = myelinated
if don’t have sheath = unmyelinated

• Sheaths formed from Schwan cells

Question 21

What are the nodes of Ranvier?

Answer 21

Gaps in the myelin sheath

Question 22

What is neurilemma ?

Answer 22

• outermost coil of Schwan cell
• helps repair injured fibres

Question 23

Synapse and why it’s important

Answer 23

• A synapse is the junction between two neurons, or between a neuron and a muscle or gland.
• Nerve impulse transmission occurs because special neurotransmitter chemicals are released into the tiny gap (the synaptic cleft), which separates the two nerve cells.

Question 24

Name and explain the structural ways and functional ways a neuron can be classified

Answer 24

Structure:

• Unipolar – just one extension, an axon. These are not found in humans, just insects.
• Bipolar – they have one distinct axon and one dendritic fiber separated by a cell body. Occur in the eye, ear and nose.
• Multipolar – have a single axon and several dendritic fibers extending from cell body. Most interneurons in the brain and spinal cord are this type.

Function:

• Sensory (afferent)- take nerve impulses from receptors to the central nervous system.
• Motor (efferent)- take nerve impulses from the central nervous system to effector structures.
• Interneurones – these are the neurones of the central nervous system.

Question 25

Transmission across a synapse steps

Answer 25

1. Nerve impulse reaches the axon terminals & activates calcium ion (Ca2+) channels
2. Ca2+ floods into the cell and causes the synaptic vesicle which contain neurotransmitter to migrate to the end of the presynaptic neuron
3. Neurotransmitter is released from the presynaptic neuron (by exocytosis) into the synapse
4. Neurotransmitter diffuses across the synapse
5. Neurotransmitter combines with receptors at the postsynaptic neuron
6. This stimulates protein channels to open, allowing an increase of sodium and the action potential to continue on the post synaptic neuron.

Question 26

Define refractory period

Answer 26

• The short period after an action potential where the nerve fibre cannot be stimulated again.
• This ensures that the impulse travels in one direction.

Question 27

How does a cell become depolarised

Answer 27

Depolarisation:
- A sudden increase in membrane potenial. This occurs if the level of stimulation exceeds about 15mV (threshold)
- When a neuron is stimulated, some ligand-gated sodium channels are opened. Once these channels are opened, more sodium ions move into the cell. This makes the intracellular fluid less negative, increasing the potential difference
- If the stimulus is strong enough to increase the potential from resting (-70 mV) to -55mV, then voltage-gated channels opened. This produces a movement of sodium ions into the cell that proceeds independently of the stimulus. This is known as the ALL OR NONE RESPONSE.
- This inward movement of sodium ions can’t be matched by the outward movement of potassium ions and so when the cell’s membrane potential reaches about 40mV, the cell is said to be DEPOLARISED.

Question 28

How does repolarisation occur?

Answer 28

• After a short period, repolarisation occurs. The sodium channels close, stopping the influx of sodium ions.
• At the same time, voltage gated potassium channels open increasing the flow of potassium ions into the cell, making the inside of the membrane more negative than the outside and decreases the membrane potential
• The potassium channels remain open longer than needed, resulting in membrane potential dropping below resting potential. This is known as HYPERPOLARISATION

Question 29

Action potential steps

Answer 29

1. Sufficiently strong stimulus causes membrane to depolarise until the threshold of 15mV ( - 55mV ) is reached (if the stimulus isn’t there nothing occurs)
2. Sodium channels open, Na+ flows into cell, membrane becomes positive. At 15 mV Na+ channels close, stopping the amount of movement of Na+
3. At 15mV potassium ion (k+) channels open , causing K+ to flood out of cell. Causes membrane to repolarise. At -70mV K+ channels close, and K+ ions stop moving out.
4. Sodium potassium pumps move 3 Na+ molecules ( which are now inside the cell) back outside the cell and 2 K+ molecules ( which are now outside the cell) back inside the cell.
   This returns the cell to resting membrane potential.