Nerve cells and nerve impulses Flashcards
What is the nervous system?
The communication network and control centre of the body. Maintains a constant environment inside the body
What are neurons?
the basic structural and functional units of the nervous system
What is the myelin sheath
a layer of fatty material (in the CNS), cover the Schwann cells
Node of ranvier
gaps in the myelin sheath
Axon
carried nerve impulses away from the cell body
What are the functional types of neurons?
Afferent
Efferent
Interneurons
Afferent (sensory neurons)
carry messages from receptors in the sense organs, or in the skin, to the central nervous system
Efferent (motor neurons)
carry messages from the central nervous system to the muscles and glands (the effectors)
Inter-neurons (association or connector neurons)
- located in the central nervous system
- They are the link between the sensory and motor neurons
What are the structural types of neurons?
Multi-polar
Bipolar
Unipolar
Multipolar neurons
- Have one axon and multiple dendrites extending from the cell body
- The most common type
- Includes most of the interneurons in the brain and spinal cord and also the motor neurons
Bipolar neurons
- Have one axon and one dendrite
- Occur in the eye, ear and nose, where they take impulses from the receptor cells to other neurons
Unipolar neurons
- Have just one extension, an axon
- The cell body is to one side of the neuron
- Most sensory neurons
What is a neuron
A nerve cell
What is a nerve fibre
Any long extension of cytoplasm of a nerve cell body, although the term usually refers to an axon
What is a nerve
A bundle of nerve fibres held together by connective tissue
What is a synapse?
- The junction between the branches of adjacent neurons
- Usually occur between the axon of one neuron and a dendrite or cell body of another neuron
What is a nerve impulse?
- An electrochemical change that travels along a nerve fibre
- Is due to the changes in the concentration of ions inside and outside the cell membrane
Nerve impulses in un-myelinated fibres
Travel steady along the fibre
Nerve impulses in myelinated fibres
Impulse jumps from one node of Ranvier to the next. Known as salutatory conduction and allows the impulse to travel a lot faster
What is salutatory
when the impulse jumps form one node of Ranvier to the next
What does extracellular fluid contain?
A high concentration of sodium ions
What does intracellular fluid contain?
A high concentration of potassium ions
Why does the inside have a relatively negative charge?
Because there are less potassium ions than sodium ions
What is the membrane potential?
The potential difference in sodium and potassium ions
What is the resting membrane potential?
- The membrane potential of an unstimulated nerve cell
- Can be measured and is about -70mV
- This means that the potential of the inside is 70mV less than the outside
- The neuron is said to be polarised
What is the sub-threshold level?
the strength of an impulse needed for the cell membrane to become permeable to sodium ions (opening of the voltage gated sodium channels). Is usually about 15mV
What causes depolarisation
The opening of the sodium gates causing the Na+ to move into the cell
What causes the neuron to be repolarised?
Tthe closing of the sodium channels and the opening of the potassium channels
What is the role of the sodium potassium pump?
To return the membrane to its original state
What is the refractory period?
The brief time after the action potential where that part of the nerve fibre cannot be stimulated
What happens with nerve impulses across the synapse?
- When the action potential reaches the axon terminals, there is an influx of Ca2+
- This causes neurotransmitters to be released and diffuse across the synapse, attaching to receptors of the next neuron, thus stimulating the next action potential
Nerve impulses at the neuromuscular junction
- The action potential reaches the axon terminal
- This opens the calcium gates in the axon terminal membrane
- Extracellular Ca2+ move into the axon terminal
- This causes the neurotransmitter to be released (acetylcholine) into the synapse
- The neurotransmitter attaches to receptors on the muscle membrane
- This part of the muscle membrane is called the motor end plate
- The attachment of the neurotransmitter causes the opening of sodium gates
- Extracellular sodium ions move into the muscle fibre
- The sodium depolarises the muscle cell
- Depolarisation of the muscle cell causes the release of Ca2+ from the sarcoplasmic reticulum
- This calcium causes the actin and myosin myofilaments to slide over each other
- This makes the sarcomere shorter. The muscle is now contracted
- Acetylcholinesterase is an enzyme that breaks down acetylcholine
- The broken down acetylcholine is reabsorbed by the axon terminal and is recycled
