Neurons in The NS Flashcards
Basic functions of the nervous system
- Sensation
- Integration
- Reaction/response
Sensation
Monitors changes/events occurring in and outside the body. Such changes are known as stimuli and the cells that monitor them are receptors.
Integration
The processing and interpretation of sensory information to determine the appropriate response.
Reaction/response
Motor output - the activation of muscles on glands to carry out a response.
Neurons
Nerve cells. They receive stimuli and transmit information (electrochemical).
Neuroglia cells
Support and protect neurons
Neuron structure
- Cell body/soma
- Dendrites
- Axon
- Myelin sheath
- Neurilemma
- Synaptic knob
Cell body/soma
Cytoplasm mas that contains the nucleus .
- Round central structure
- Contains DNA
- Directs metabolism
- No role in neutral signalling
Dendrites
Projections that carry impulses towards the cell body. Information collectors that receive inputs from neighbouring neurons.
Axon
Projections that carry impulses away from the cell body. One per cell 2 distinct parts.
- Tube like structure - axis cylinder
- Axon terminals
Axon terminals
Branches at end of axon that connect to dendrites of other cells.
Myelin sheath
White fatty casing on axon produced by Schwann cells. Acts as an electrical insulator. It isn’t present on all cells, but when present, it increases the speed of neural signals down the axon.
Neurilemma
A membrane that covers the myelin sheath and axons of neurons. Assists in the repair of neurons.
Synaptic knob
At the end of axon terminals the area is enlarges to form a synapse. This is where the electrical signal is connected into a chemical signal to pass to the dendrite of another neuron or to a muscle or gland.
Neuroglia of PNS
- Schwann cells
- Satellite cells
Schwann cells
Wrap around portion of only one axon to form myelin sheath.
Satellite cells
Surround neuron cell bodies in ganglia, provide support and nutrients.
Myelinated axons
Myelin protects and insulates axons from one another. Not continuous
Nodes of Ranvier
The gap in the myelin sheath. Speed up nerve transmission
Unmyelinated axons
Slower transmission
Afferent/sensory/receptor neurons
Take nerve impulses from receptor to the central nervous system.
- Receptor ends to dendrites
- Long dendrites and short axons
- Cell body not or the main line of transmission
- Myelinated axons and dendrites
- Cell bodies usually grouped to form ganglion near axon terminals
Interneurons/association/connector neurons
These are the neurons of the central nervous system. Impulse from sensory to motor neuron in CNS.
- Cell body central
- Axons and dendrites of similar size
- Often many connections to other neurons
Efferent/motor neurons
Take nerve impulses from the central nervous system to effect or structures.
- Short dendrites
- Cell body on main axis
- Long axons
- Axon terminals may be enlarged to form a synaptic knob
- Can be myelinated or unmyelinated
Types of neurons
- Multipolar
- Unipolar
- Pseudo unipolar
- Bipolar
Multipolar neurons
Have a single axon and several dendritic fibres. All somatic motor neurons are multipolar (carry message to skeletal muscles.)
E.g. most interneurons of the brain and spinal cord
“Multiple poles into the cell body”
Unipolar neurons
The axon and dendritic fibre are continuous and the cell body lies off to one site. Most sensory neurons are unipolar.
E.g. True unipolar only in insects
“One pole into the cell body”
Pseudo unipolar neurons
Properties of uni and bipolar. Single axon which separates into two extensions. One connects to dendrites. One connects to dendrites, one ends in axon terminals. Cell body off to side of main axon.
E.g. most sensory neurons in humans
“One pole into the cell body”
Bipolar neurons
They have a distinct axon and a dendritic fibre separated by a cell body.
E.g. eye, ears, nose (take impulses from regular cells to other neurons)
“Two poles into the cell body”
Nerve
A bundle of nerve fibres
Nerve fibre
An axon and it’s associated coverings
Nerve impulse
The electrochemical change resulting in a message carried by a nerve (AKA action potential)
Propagation
Movement of impulses along a neuron.
The resting state of a neuron
The neuron is maintained by a potential voltage difference between extracellular fluid (ECF) and intracellular fluid (ICF)
Concentration of Ions
- The ECF (outside the cell) has a higher concentration of Na+ and Cl- ions
- The ICF (inside the cell) has a higher concentration of K+ ions and large negatively charged ions/proteins
- This difference in charge is quite large in neurons, making a resulting membrane potential of -70mV (inside the neuron)
Permeability of cell membrane to ions
- The cell membrane is highly permeable to K+ and Cl- ions, slightly permeable to Na+ ions and impermeable to large negatively charged organic ions/proteins.
How is the -70mV resulting membrane potential maintained
Due to the nature of the cell membrane, it maintains the -70mV resulting membrane potential by
- Sodium potassium pumps
- Large negative ions/proteins
Sodium potassium pumps
These actively pump Na+ out of the cell and K+ in (at a 2K+ : 3Na+ ratio)
Large negative ions/proteins
There aren’t enough K+ ions to counteract these negative ions/proteins which can’t diffuse across.
Steps to nerve impulse
- Stage 1: Arrival of impulse
- Stage 2: Depolarisation
- Stage 3: Repolarisation
- Stage 4: Re-establishing original resting rate
Stage 1 - Arrival of impulse
- At rest - neuron is polarised - 70mV
- When a sufficient stimulant is applied OR an impulse arrives, the cell membrane increases it’s permeability to sodium ions.
- This allows an influx of Na+ ions into the cell
- This movement is too great for the outward movement of K+ ions
- This reduces the potential differences between the ECF and ICF (inside becomes more positive)
Stage 2 - Polarisation
- If the change in potential difference exceeds 15 mV (or membrane potential reaches -55 mV) - the threshold - then an action potential will occur
- ‘All or none response’
- Once the threshold is reached, voltage gated sodium channels open, allowing an even greater influx of Na+ ions into the cell
- This stimulates channels in the next section of the axon to open
- The Na+ ion movement makes the inside become positively charged (+30 mV)
- This process is knows as depolarisation - the less of the charge difference between the ECF and ICF.
All or none response
The size of response is not proportional to the strength of stimulus, it just has to reach the threshold
Stage 3 - Repolarisation
- Once the membrane potential reaches +30 mV, the sodium voltage gated channels open
- Once the membrane potential reaches +30 mV, the sodium voltage gated channels open
- K+ ions move out of the cell
- This reaches a potential difference in voltage approaches and then reaches -70mV
- There is a state where the potential difference gets below -70mV and is known as hyperpolarisation
- This is due to channels being sloe to close, more K+ leaves cell, even more negative inside (voltage gated potassium walls delay in closing)
Stage 4 - Re-establishing original resting rate
- Axon returns to original resting rate (-70mV)
- Movement of ions generates the impulse
- Releases small amount of energy into
- Adjustment nerves which create a chain reaction
- Adjacent parts of the neuron to spread the length of the axon
Refractory period
No new impulse can occur in any part that is not at rest,. This allows unidirectional propagation (prevents AP’s moving backwards)
Factors affecting the speed of nerve transmission
- Axon diameter
- Presence of myelin
- Chemicals of transmission
Axon diameter
Greater diameter = faster transmission
Presence of myelin
- Myelin = Saltatory conduction (action potential jumps from node to node) = faster
- Unmyelinated = Continuous conduction = slower
Chemicals and transmission
Many chemicals affect transmission along neuron or at synapses
- Stimulants
- Depressants
- Lethal neurotoxins
- Lethal nerve agents/gases
Synapses
A synapse is the junction between two neurons, or between a neuron and a muscle (neuromuscular junction) or gland
Synaptic cleft
A very narrow gap of about 20nm between neurons.
Why do we need neurotransmitters
An electrical impulse cannot cross the synaptic cleft, so more impulses are carried by neurotransmitters
Pre-synaptic neuron
Neuron sending impulse
Post-synaptic neuron
Neuron receiving impulse
Transmission across a synapse
In one direction only. Only one side of the synapse is capable of releasing the neurotransmitter and the other having receptor sites to receive the transmitter.
Steps to transmission across a synapse
Neurotransmitters
- Acetylcholine
- Adrenaline and noradrenaline
- Dopamine
Acetylcholine
Released throughout the peripheral nervous system
Adrenaline and noradrenaline
Released within the sympathetic nervous system only
Dopamine
Brain transmitter
