The Nervous System
The nervous system is a network of nerve cells within the brain, spinal cord and throughout the body. It coordinates movements and bodily functions. The nervous system is divided into various subdivisions starting with the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
CNS: is composed of the brain and spinal cord and receives the input from the PNS
PNS: is the nerves that run throughout the body- some of them as long as an entire limb
The system then contains an Afferent (sensory) division and Efferent (Motor) division.
Afferent: transmits information from periphery to the CNS and contains receptors, is divided into somatic sensory and visceral sensory
Efferent: transmits information from CNS to the rest of the body, sends motor information from efectors and can be divided into somatic motor and autonomic motor.
Nerve Tissue
Nerve tissue is composed of neurons and specialised neuroglia. Neurons send and receive transmissions throughout the nervous system and neuroglia support the neurons physically and functionally, they exchange materials with the extracellular fluid.
The Neuron
The neuron is a specialised structural and functional unit of the nervous system, they can occur in a wide variety of sizes and shapes and are the same as any other cell except they possess an axon and dendrites (to send and receive messages) and they lack a centrosome or centriole so they cannot undergo division (you are born with a lifetime supply of neurons-you can't get new ones if you lose them!) formation of neurons ceases in intrauterine life.
Classifications of Neurons
Neurons can be classified according to the following list:
3 Kinds of neurons connect the CNS to the body afferent, efferent and interneurons
Afferent Neurons:
Efferent Neurons
Interneurons
Nerve Types
Nerves types are based on velocity of conduction (different to classifications aforementioned) as well as on fibre diameter and have A, B and C types as well as Aa, Ab types etc. (alpha, beta, gamma and so on). The fastest conducting fibres are Aa (A alpha) types.
Myelination
Myelin sheaths occur around the axon of a neuron. In the PNS the myelin is made by schwann cells and in the CNS it is made by oligodendrocytes. The unmyelinated section between two schwann cells is called the Node of Ranvier. The myelin is a whitish, multilayered, refractile sheath made up of proteins and lipids and features a neurilemma which is the outermost covering on myelin or the sheath of schwann.
Resting Membrane Potential
A resting neuron has a charge difference between outside and inside the membrane. A resting membrane will have a negative charge (of about -70mv) inside the membrane whereas the outside of the membrane is positive therefore the membrane is polarized in a resting state. The chief intracellular ion is potassium and the chief extracellular ion is sodium and the membrane is relatively impermeable to both these ions.
Ion Channels
there are several types of ion channels including passive and gated and a sodium-potassium pump.
Changes in Membrane Potential
Depolarizing:
Repolarisation:
potassium ions rush out of the neuron after sodium ions rush in which repolarizes the membrane.
Repolarization involves restoring the inside of the membrane to a negative charge and the outer surface to a positive charge.
Graded Potential
A graded potential is a localised and short lived stimulus. As the stimulus becomes stronger the more voltages changes and the farther the current flow. Either depolarization or hyperpolarization can occu and these are triggered by opening of the gated ion channels. the current of a graded potential dissipates quickly and signals for short distance communication within an axon. Graded potentials lose strength as they move through the cell due to current leaks and cytoplasmic resistance. If the current is strong enough, graded potentials can reach the trigger zone in the axon hillock and initial segment.
Action Potential
Action potential is the primary means of neuronal communication and is used to send, receive and integrate information. An adequate stimulus will change te membrne potential to produce an action potential and change in the membrane potential is propagated along the membrane due to opening or closing of voltage gated ion channels. There are two main ionic changes that occur during an action potential and these include an increase in sodium permeability and sodium entry inside the cells through voltage gated sodium channels, delayed and longer increase in potassium premeability (potassium moving out of the cell.
Different Phases of an action potential
There are 3 different stages of an action potential.
1. Resting stage:
2. Depolarisation Stage
3. Repolarisation Stage
Threshold Stimulus
his is the minimal intensity of stimulating current that acting for a given duration, will just produce an action potential (this is about +15 mV depolarised from resting potential ). If depolarising currents reach threshold strength the cell fires an AP (this is an all or nothing response).
Conduction of Nerve Impulse
The propagation of action potential down the axon is the mechanism used by the nervous system to communicate over long distances and the action ptoential is transmitted along the nerve as nerve impulse. Action potentials propagate from the axon hillock and travel in one direction only. the nerve impulse is conducted in axon in both directions.
Conduction trough a myelinated nerve
Conduction trough an unmyelinated nerve
Speed of Action Potential
Characteristics of Action Potential
Learn More
Resting Membrane Potential Animation
Crash Course: The nervous system
Crash Course: Action! Potential!
Resting Membrane Potential Tutorial
Test Your Knowledge
Recall:
Conceptual Understanding:
The nervous system is a network of nerve cells within the brain, spinal cord and throughout the body. It coordinates movements and bodily functions. The nervous system is divided into various subdivisions starting with the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
CNS: is composed of the brain and spinal cord and receives the input from the PNS
PNS: is the nerves that run throughout the body- some of them as long as an entire limb
The system then contains an Afferent (sensory) division and Efferent (Motor) division.
Afferent: transmits information from periphery to the CNS and contains receptors, is divided into somatic sensory and visceral sensory
- somatic: receives sensory information from skin, fascia, joints skeletal muscles and special senses
- visceral sensory: receives sensory information from viscera
Efferent: transmits information from CNS to the rest of the body, sends motor information from efectors and can be divided into somatic motor and autonomic motor.
- somatic: 'voluntary' nervous system (innervates skeletal muscles)
- Autonomic: 'involuntary' nervous system (innervates cardiac muscle, smooth muscle and glands) and can be further divided into sympathetic and parasympathetic components.
- sympathetic: gets the body into action (Fight or flight response)
- parasympathetic: relaxes the body
Nerve Tissue
Nerve tissue is composed of neurons and specialised neuroglia. Neurons send and receive transmissions throughout the nervous system and neuroglia support the neurons physically and functionally, they exchange materials with the extracellular fluid.
The Neuron
The neuron is a specialised structural and functional unit of the nervous system, they can occur in a wide variety of sizes and shapes and are the same as any other cell except they possess an axon and dendrites (to send and receive messages) and they lack a centrosome or centriole so they cannot undergo division (you are born with a lifetime supply of neurons-you can't get new ones if you lose them!) formation of neurons ceases in intrauterine life.
Classifications of Neurons
Neurons can be classified according to the following list:
- unipolar, bipolar, multipolar or pseudunipolar (according to the number of processes extending from the soma/nerve body)
- myelinated and unmyelinated
- somatic and autonomic
- cranial and spinal
- afferent and efferent
- adrenergic (neurotransmitter is adrenalin, noradrenaline or dopamine) and cholinergic (neurotransmitter is acetylcholine)
3 Kinds of neurons connect the CNS to the body afferent, efferent and interneurons
Afferent Neurons:
- Cell body lacks dendrites and presynaptic imputs and is adjacent to spinal cord
- has long peripheral axon which is commonly called afferent nerve fibre
- peripheral ending of axon is modified as sensory receptor
Efferent Neurons
- the cell bodies of tese neurons are in the CNS and receive many inputs from other neuron which influences their output to effector organs.
- Axons of efferent neurons leave the CNS to find their way to the muscles or glands
- thus efferent neurons convey integrated output for the effector organs to put into effect.
Interneurons
- these neurons lie entirely within the CNS (between afferent and efferent neurons)
- most neurons (about 99%!) lie within this category and their main role is regulation of peripheral responses to peripheral information- the more complex the required action, the greater number of interneurons interposed between afferent and efferent neurons.
Nerve Types
Nerves types are based on velocity of conduction (different to classifications aforementioned) as well as on fibre diameter and have A, B and C types as well as Aa, Ab types etc. (alpha, beta, gamma and so on). The fastest conducting fibres are Aa (A alpha) types.
Myelination
Myelin sheaths occur around the axon of a neuron. In the PNS the myelin is made by schwann cells and in the CNS it is made by oligodendrocytes. The unmyelinated section between two schwann cells is called the Node of Ranvier. The myelin is a whitish, multilayered, refractile sheath made up of proteins and lipids and features a neurilemma which is the outermost covering on myelin or the sheath of schwann.
Resting Membrane Potential
A resting neuron has a charge difference between outside and inside the membrane. A resting membrane will have a negative charge (of about -70mv) inside the membrane whereas the outside of the membrane is positive therefore the membrane is polarized in a resting state. The chief intracellular ion is potassium and the chief extracellular ion is sodium and the membrane is relatively impermeable to both these ions.
Ion Channels
there are several types of ion channels including passive and gated and a sodium-potassium pump.
- Passive: these channels leak, they are non-gated and so are alway open
- ligand gated: these channels are gated and open by binding of specific transmitters
- voltage gated: these channels open or close in response to changes in membrane potential (changes in voltage)
- mechanically gated: these channels open or close in response to physical deformation of receptors.
- Sodium-potassium pump: this is an active pump that moves 3 sodium ions out and 2 potassium ions in and is present in all cells (it is responsible for maintaining sodium and potassium concentration gradients) and membrane potential also depends on the sodium-potassium pump (increased sodium potassium pump activity= increased interior negativity)
Changes in Membrane Potential
Depolarizing:
- A stimulus initiates local depolarisation
- a stimulus changes the permeability of a local 'patch' of the membrane
- the membrane is now permeable to sodium as sodium channels open
- sodium ions diffuse rapidly into the cell
- this changes the polarity of the membrane (the inside become more positive, the outside becomes more negative) at that site.
Repolarisation:
potassium ions rush out of the neuron after sodium ions rush in which repolarizes the membrane.
Repolarization involves restoring the inside of the membrane to a negative charge and the outer surface to a positive charge.
Graded Potential
A graded potential is a localised and short lived stimulus. As the stimulus becomes stronger the more voltages changes and the farther the current flow. Either depolarization or hyperpolarization can occu and these are triggered by opening of the gated ion channels. the current of a graded potential dissipates quickly and signals for short distance communication within an axon. Graded potentials lose strength as they move through the cell due to current leaks and cytoplasmic resistance. If the current is strong enough, graded potentials can reach the trigger zone in the axon hillock and initial segment.
Action Potential
Action potential is the primary means of neuronal communication and is used to send, receive and integrate information. An adequate stimulus will change te membrne potential to produce an action potential and change in the membrane potential is propagated along the membrane due to opening or closing of voltage gated ion channels. There are two main ionic changes that occur during an action potential and these include an increase in sodium permeability and sodium entry inside the cells through voltage gated sodium channels, delayed and longer increase in potassium premeability (potassium moving out of the cell.
Different Phases of an action potential
There are 3 different stages of an action potential.
1. Resting stage:
- this is the RMP (Resting Membrane Potential) before the AP (Action Potential) begins.
- The resting membrane is said to be polarized with negative inside and positive outside.
2. Depolarisation Stage
- This is the excitation of membranes of excitable tissues open voltage gated channels
- in the beginning normally few voltage gated sodium ion channels open
- sodium ions move into the cell
- Once depolarization reaches the threshold potential (increases by 15 mv) the rate of depolarisation increases rapidly because of opening of more voltage gated sodium channels once membrane reaches threshold level.
- opening of voltage gated sodium ion channels allows mroe sodium to move into the cell
- membrane potential then overshoots so the cell becomes positive on the inside and negative on the outside.
- membrane potential may become +35 mv
3. Repolarisation Stage
- At the peak of AP, voltage gated sodium channels abruptly close and hence sodium permeability decreases
- at the same time voltage gated potassium channels open and potassium ion goes out of the cell and the membrane potential begins to recover back to resting stage.
Threshold Stimulus
his is the minimal intensity of stimulating current that acting for a given duration, will just produce an action potential (this is about +15 mV depolarised from resting potential ). If depolarising currents reach threshold strength the cell fires an AP (this is an all or nothing response).
Conduction of Nerve Impulse
The propagation of action potential down the axon is the mechanism used by the nervous system to communicate over long distances and the action ptoential is transmitted along the nerve as nerve impulse. Action potentials propagate from the axon hillock and travel in one direction only. the nerve impulse is conducted in axon in both directions.
Conduction trough a myelinated nerve
- depolarization is travelling along the nerve membrane and jumps from node (of ranvier) to node (saltatory conduction).
- Meylinated nerve conducts 50x faster than an unmeylinated nerve
Conduction trough an unmyelinated nerve
- Every patch of the membrane contains sodium and potassium channels to produce action potentials along the entire length of the axon.
- Depolarization initiated by sodium influx helps to depolarize the adjacent regions of membrane
- depolarization will be initiated on each side of the stimulated region
- thus action potential 'wave' travels along the neuron.
Speed of Action Potential
- the speed of action potential in a neuron is influenced by the diameter of the axon (larger axons are faster)
- resistance of axon membrane to ion leakage out of the cell (myelinated axons are faster-saltatory conduction)
Characteristics of Action Potential
- stereotypical size and shape: each normal action potential for a give cell type looks identical, depolarizes to the same potential, and repolarizes back to the same resting potential.
- propagation: an action potential at one site causes depolarisation at adjacent sites, bringing those adjacent sites to threshold (propagation of action potentials from one site to the next is nondecremental)
- All-or-None response: an action potential either occurs or does not occur (if an excitable cell is depolarized to threshold in a normal manner, the occurrence of an action potential is inevitable. If the membrane is not depolarized to threshold, no action potential can occur).
Learn More
Resting Membrane Potential Animation
Crash Course: The nervous system
Crash Course: Action! Potential!
Resting Membrane Potential Tutorial
Test Your Knowledge
Recall:
- describe what a myelin sheath is and how it is formed
- what is the relationship between the size f a nerve and conduction velocity?
- what is the difference between efferent and afferent nerves?
- what are the phases of an action potential?
- what are the different types of channels in a membrane and how do they differ?
- what are interneurons and what is their function?
- what are the 3 main neurons in the CNS?
- draw a mind map explaining the different divisions of the nervous system.
- list the different classifications of nerves.
Conceptual Understanding:
- If the permeability of the membrane to potassium increased, what affect, if any, do you think there would be on the resting membrane potential? do you think the membrane potential would increase (become more negative) or decrease ( become less negative)
- if the permeability of the membrane to sodium increased, what affect, if any, do you think there would be on the resting membrane potential? do you think the membrane potential would increase (become more negative) or decrease ( become less negative)
- During the depolarisation phase of the action potential, the magnitude of the membrane potential becomes less negative and rapidly reverses in polarity. What might be occurring in terms of ion movement across the membrane and changes in membrane and changes in membrane permeability that would result in depolarization o the membrane potential?
- what might happen to RMP if the sodium-potassium pump stopped working?
- what is the functional importance of a myelin sheath?