Nerve cells and nerve impulses Flashcards
1
Q
How has the nervous system evolved?
A
- Nervous tissue is found in most species of multicellular animals, however some single-celled organisms have electrical information transmissions
- E.g 600 mill years ago, sponges had no neurons but synaptic junctions
- E.g Jellyfish diffuse nerve nets but there’s not central nervous system
2
Q
What is cephalisation?
A
- Cephalisation is an evolutionary process where nervous tissue is concentrated towards one end of an organism, often resulting in the formation of a brain
3
Q
What does the nervous system consist of?
A
- The central nervous system (CNS) which is the brain and the spinal cord
- The peripheral nervous system (PNS) which is the nervous system other than the brain and spinal cord e.g nerves from sense organs to the CNS
4
Q
What is the microstructure of the nervous system?
A
- There are nerve cells and glia cells
- Santigago Ramon y Cajal and Camillo Gogli, where awarded a Nobel prize for their work on structure of nervous system
- They established that neurons are separable meaning that there is a small gap between the tips of one neuron’s and the next neuron
- So the final conclusion was that the nervous system consists of individual neurons
5
Q
How many neurons are in the nervous system?
A
- There are 70 billion neurons in the cerebellum
- In the cerebral cortex there are at least 12-15 billion neurons
- In the spinal cord there are 1 billion neurons
6
Q
What are glia / glial cells / neuroglia ?
A
- They are non-neuronal cells that support and protect neurons
- They are smaller than neurons but exceed neurons in number (by x 1.2)
- The most common glia are oligodendrocytes (76%), astrocytes (17%) and microglia (6%)
7
Q
What is the function of glia?
A
- Glia are essential support cells in the nervous sytem
- They provide structure i.e surround neurons and hold them in place (astrocytes)
- They insulate nerve cells with myelin sheath - produce myelin sheath (e.g oligodendrocytes in the CNS , schwann cells in the PNS)
- They supply nutrients and oxygen to neurons (astrocytes)
- They remove dead neuronal tissue and support immune defence of the CNS (microglia - phagocytes)
- During development, glial cells provide scaffolds for neurons to migrate to their final destinations (radial glia)
- They modulate neurotransmissions in the synapses
8
Q
What is multiple sclerosis?
A
- Multiple sclerosis is a neurological condition which affects around 100,000 people in the UK
- Common symptoms are : vision problems, fatigue, difficulties with walking
- The cause of multiple sclerosis is the demyelination of axons in the brain and spinal cord
9
Q
What are neurons?
A
- They are cells in the nervous system that specialise in performing information-processing tasks
- Each neuron consists of a body cell (soma), and fibres (dendrites ands axons)
10
Q
What is the structure of the soma of a neuron?
A
- A soma contains the cells nucleus and ‘machinery’
- The cell nucleus contains the cell’s genetic material organised as DNA molecules
- The cell machinery consists of the :
1) mitochondria - which performs metabolic activities, and extracts energy from nutrients
2) ribosomes - which produces protein
3) enoplasmic reticulum - which transports protein to other locations - also includes gogli apparatus and nissl bodies etc
11
Q
What is the structure of the dendrites?
A
- Dendrites are branching fibres (out of cell body) receiving information from other neurons
- The greater the surface area of the dendrite, the more information it can receive
- There are synapses locates on the surface of the dendrite
- Some dendrites have spines which enlarge the surface on which synapses may be places
12
Q
What is the structure of the axon?
A
- An axon is a thin fibre which transmits information to the other neurons (long straight line)
- It can be a few micro (μm) or 1m long
- In its distal part, an axon has many branches (similar to dendrites) - each which swells at the tip, forming a presynaptic terminal (end bulb or bouton)
- Some axons have boutons along the fibre, and not just at the end
13
Q
What is the myelin sheath?
A
- It is the insulating layer surrounding the axon
- It helps speed up electrical transmissions
- An ancient virus helped to form a myelin sheath around nerve fibres
14
Q
What are the three major types of neruons?
A
1) Afferent neurons
2) Efferent neurons
3) Interneurons
15
Q
What do Afferent neurons do?
A
- They are sensory neurons which ARRIVE
- So they carry information from receptors
16
Q
What do Efferent neurons do?
A
- They EXIT
- So they carry signals away to the effector muscles or glands
17
Q
What do interneurons do?
A
- They connect other neurons
18
Q
What is resting polarization?
A
- Is the state when the membrane of a neuron maintains an electrical gradient (the difference in electrical charge) between the inside and outside of a cell
19
Q
What is the resting potential of a neuron?
A
- It is when the electrical potential inside the neuron is slightly lower than the outside (-70mV)
20
Q
What is the excitation of a neuron?
A
- It is a process when information is transmitted through the neuron by the change of its potential
- There is a mechanism of analysis deciding to pass or block a message
- It includes 2 stages :
- transmitted from exterior to cell body
- transmitted from cell body to out of neuron
21
Q
What are the two stages of the excitation of a neuron?
A
- Stage 1 - It is through the dendrite from exterior to cell body and there are many postsynaptic potentials. There are changes in electrical potential and all of the potentials from all dendrites sum up
- Stage 2- If the sum of potentials is strong enoug, the neuron ‘fires’ and action potential , same strength of the added up potentials. and information leaves the cell body and out of neuron through axon
22
Q
What are excitatory synapses?
A
- Excitatory synapses include EXCITATORY postsynaptic potential.
- Excitatory postsynaptic potential is a change in polarisation moving along dendrites toward the cell’s body.
- Excitatory postsynaptic potential provokes depolarisation (decreases polarization)
23
Q
What are inhibitory synapses?
A
- Inhibitory synapses include INHIBITORY postsynaptic potential
- Inhibitory postsynaptic potential is a change in polarisation moving along dendrites towards the cell’s body
- Inhibitory postsynaptic potential is negative so it provokes hyperpolarisation (increases polarization)
24
Q
What happens to postsynaptic potentials when they meet?
A
- Postsynaptic potentials (excitatory and inhibitory) move along the dendrite towards the body cell and sum up when they meet other postsynaptic potentials or if followed by others
So there are two types of summation:
1) Over space - so from different dendrites
2) Over time - so from the same dendrites
25
What is the 'free potential'?
- In the cell's body, 'free potential' is formed and moves towards the proximal part of the axon
- If the 'free potential' is low (does not reach threshold levels) it dies
- If the 'free potential' is high (reaches threshold level) it provokes a sudden electrical excitation (the action potential) at the proximal part of the axon
26
What happens to the action potential after being released at axon?
- The action potential continues moving along the axon without any loss in its parameters to reach the presynaptic membrane where it produces the release of chemical substance
27
What is the all-or-none principle of the action potential?
- The amplitude of an action potential is independent of the amount of current which produced it
- The amplitude of an action potential is constant for a given axon
- action potentials are all-or-none so they either occur fully or do not occur at all
28
Describe the propagation of the action potential
- The first action potential is on the axon hillock (where the cell body transitions into axon)
- Action potentials move down an axon towards another cell using saltatory conduction
- Saltatory conduction is where action potentials hop along the axon recurring at successive nodes of ranvier (the gap between different myelin sheaths on an axon) - so it means fast propagation
- The myelin prevents any charge leakage throughout the axon
