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C3 - Nervous System (Nerve net (A nerve net is a diffuse network of cells…
C3 - Nervous System
Nerve net
A nerve net is a diffuse network of cells that group into ganglia but don't form a brain. It's the simplest type of nervous system
Functions of a hydra's nerve net
- Sense light, physical contact and chemicals.
- It can contract, move, hunt and feed
- It can't detect the direction of a stimulus but a larger stimulus has a larger response
Types of cell in a nerve net
- Ganglion cells provide connections in several directions
- Sensory cells detect stimuli
Structure of a hydra's nerve net
- Simple pattern of nerve cells with short extensions joined to each other and branching in different directions (like a net)
- Not many sensory receptors so can only respond to a small number of stimuli
Differences between a hydra nerve net and a human central nervous system
- Number of cell types: 2 | Many
- Regeneration: Rapid | Very slow/none
- Myelin sheath: Absent | present
- Conduction speed: slow (5m s-1) | fast (120 m s-1)
- Regenerate neurones: Yes | No
Components
Functions
- Detects stimuli inside the body and in the environment
- Processes and stores information
- Initiates responses
Sensory receptors
- Give an organism its senses
- E.g. pressure sensors in the skin
- They're transducers because they detect energy and convert it into electrical energy
Stimulus and response mechanism
- A change (stimulus) is detected by sensory receptors
- The receptors create nervous impulses that travel along neurones
- They initiate a response in an effector e.g. a muscle or gland
The nervous system is made up of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
Central Nervous System (CNS)
- Involves the brain and spinal cord
- Processes info from a stimulus
- Surrounded by protective membranes called meninges
- Spinal cord has white matter containing nerve fibres surrounded by myelin (fatty and white)
- Grey matter of spinal cord has less myelin and is mostly the nerve fibres of relay neurones and the cell bodies of relay and motor neurones
Peripheral Nervous System (PNS)
- Involves the somatic nervous system i.e. pairs of nerves coming from the CNS and their branches, contain sensory and motor neurones carrying impulses to and from the CNS and effectors
- Involves the autonomic nervous system - provides unconscious control of the functions of internal organs e.g. heartbeat and digestion
Nervous Impulse
The resting potential
- The potential difference across the cell membrane when no nervous impulse is being conducted.
- It's around -70mV
- Polarised
How is the resting potential established?
- More K+ ions and less Na+ in the cell than outside
- K+ diffuses out faster than Na+ diffuses in as some K+ channels are open while most Na+ channels are closed
- Sodium-potassium pumps actively transport sodium ions out and potassium ions into the neurone (2Na+ out, 2K+ in per ATP hydrolysed). Maintain the concentration gradient
- Na+ ions are pumped out faster than K+ ions are taken in so the inside of the membrane is more negative than the outside
The action potential is the rapid rise and fall of the electrical potential across a nerve cell membrane as a nervous impulse passes
1. Depolarisation
- Energy from the stimulus opens some voltage-gated sodium channels.
- Membrane becomes more permeable Na+ ions so they diffuse rapidly into the axon
- -70mV jumps to +40mV.
- Cell membrane is depolarised and sodium ion channels close
Depolarisation is a temporary reversal of potential across a neurone membrane so the inside is less negative than the outside as the action potential is transmitted
2. Repolarisation
- Potassium channels open and K+ ions diffuse out
- Cell becomes less positive and the membrane is repolarised
3. Hyperpolarisation
More K+ ions diffuse out than Na+ ions diffuse in so the potential difference is more negative than the resting potential
4. Resting potential
Sodium-potassium pump pumps K+ ions back in and Na+ ions back out, restoring the ion balance and resting potential
How does the action potential travel along an axon?
- Na+ ions move along the membrane
- Wave of depolarisation spreads along the axon
- Resting potential needed for action potential
- At the origin of the AP, sodium channels are inactivated until resting potential is re-established - absolute refractory period
- During hyperpolarisation and the restoration of RP, a strong impulse may cause a new AP to pass - relative refractory period
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Synapse
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Properties of synapses
- Transmit info between neurones
- Act as junctions
- Protect the response system from overstimulation as the impulse is always the same size
- Filter out stimuli below -55mV
Effects of chemicals on synapses
Disrupt the functions of neurotransmitters, producing abnormal patterns of nervous impulses
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Psychoactive drugs
- Act on the CNS by affecting different neurotransmitters or their receptors.
- Alters brain function, mood, behaviour, etc.
Nicotine
- Same shape as neurotransmitters
- E.g. nicotine and acetylcholine bind to the same receptor, increasing AP frequency.
- Nicotine not hydrolysed so keeps making impulses
- Body then only functions normally with nicotine
- If no nicotine, abnormal impulses and withdrawal
Organophosphates
- Prevent the hydrolysis of neurotransmitters by inhibiting acetylcholinesterase
- Acetylcholine keeps initiating impulses
- Muscles can keep contracting
The Reflex Arc
What is a reflex arc?
- The simplest type of nervous response to a stimulus.
- It's the neural pathway taken by the nervous impulses of a reflex action
- E.g. a withdrawal reflex - pulling your hand away from a hot object
A reflex action is a rapid, automatic response to a stimulus, usually as protection. The brain isn't involved - involuntary
Reflex arc:
- Stimulus e.g. heat
- Sensory receptor e.g. temperature and pain receptors in the skin
- Sensory neurone sends an impulse to the spinal cord
- Relay neurone in spinal cord (CNS) transmits the impulse
- Motor neurone sends the impulse to an effector e.g. muscle
- Response e.g. arm muscles contract and the hand pulls away from the heat source
Neurones
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Types of neurones
- Sensory neurones carry impulses from the sense receptors/organs into the CNS
- Motor neurones carry impulses from the CNS to the effector organs i.e. muscles/glands
- Relay neurones receive impulses from sensory neurones or other relay neurones and transmit them to motor neurones or other relay neurones
Components of a motor neurone
- Cell body (centron) - contains nucleus and cytoplasm
- Cytoplasm - has many ribosomes (granular)
- Nucleus - holds DNA
- Nissl granules - cytoplasmic granules made of ribosomes grouped on RER
- Dendrite - thin fibre carrying impulses towards the centron. More than one
- Axon - thin fibres carrying impulses away from the centron. Only one in the centron
- Schwann cells - surround and support nerve fibres
- Myelin sheath - multi-layered and phospholipid. It's an electrical insulator and speeds up impulse transmission
- Nodes of Ranvier - 1um gaps in the myelin sheath. Allow rapid impulse transmission
- Synaptic end bulb - Swelling at the end of the axon in which the neurotransmitter is made
- Axon terminal - secretes a neurotransmitter, which transmits an impulse to the adjacent neurone