The Mammalian Nervous System

Neurones

Motor neurone

  • Many short dendrites carry impulses from the CNS to the cell body
  • One large axon carries nerve impulses from the cell body to effector cells 
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Neurones

Sensory neurone

  • One long dendron carries nerve impulses from receptor cells to the cell body, which is located in the middle of the neurone.
  • One short axon carries nerve impulses from the cell body to the CNS.
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Neurones

Relay neurone

  • Many short dendrites carry nerve impulses from sensory neurones to the cell body.
  • An axon carries nerve impulses from the cell body to motor neurones.
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The resting potential

  • The resting potential is about -70mV
  • It is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone's membrane.
  • The sodium-potassium pump moves 3 sodium ions out of the neurone, but the membrane isnt permeable to sodium ions, so they cant diffuse back in. This creates a sodium ion electrochemical gradient becasue there are more positive sodium ions outside the cell than inside.
  • The sodium-potassium pump also moves 2 potasium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels. 
  • This makes the outside of the cell positively charged compared to the inside. So the membrane is polarised- there's a difference in charge.
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Action potential

1. Stimulus- this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the insdie of the neurone less negative.

2. Depolarisation- if the potential difference reaches the threshold (-55mV) more sodium ion channels open. More sodium ions diffuse into the neurone. 

3. Repolarisation- at a potential difference of around +30mV the sodium ion channels clase and potassium ion channels open. The membrane is more permeable to potassium, so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to the resting potential.

4. Hyperpolarisation- potassium ion channels are slow to close so there's a slight 'overshoot' where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (less than -70mV)

5. Resting potential- the ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane is excited by another stimulus.

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Action potential

Changes in potential difference during an action potential:

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Myelinated nerve fibres

The schwann cell membrane wraps itself repeatedly around the nerve fibre, forming a fatty layer know as the myelin sheath. There are gaps between the Schwann cells, known as nodes of Ranvier. The myelin sheath is important because it:

1. Protects the nerves from damage                                                                                                       2. Speeds up the transmission of the nerve impulses                                                                                            3. Provides insulation

With a myelinated axon, the sodium ion channels are only found at the nodes of Ranvier and so depolariastion can only occur at the nodes. This makes the impulse appear to jump from node to node. This is know as saltatory conduction.

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Synapses

Synapse diagram:

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Synapses

1. The action potential arrives at the synaptic knob. Depolarisation causes opening of sodium and calcium ion channels in the pre-synaptic neurone. Calcium ions diffuse into the synaptic knob, down their concentration gradient.

2. The influx of calcium ions into the synoptic knob causes the synaptic vesicles to move to the presynaptic membrane. They then fuse with the presynaptic membrane and the vesicles release the neurotransmitter into the synaptic cleft-exocytosis.

3. The neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. This causes sodium ion channels to open in the postsynaptic neurone.

4. The influx of sodium ions into the postsynaptic membrane causes depolarisation. An action potential on the postsynaptic membrane is generated if the threshold is reached.

5. Neurotransmitters are removed from the synpatic cleft so the repsonse doesnt keep happening. They may be broken down by enzymes or taken back into the presynaptic membrane.

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Effect of drugs on nervous system

Nicotine:

  • Nicotine mimics the effect of acetylcholine and binds to spcific acetylcholine receptors in post-synaptic membranes. It triggers an action potential in the post-synaptic neurone, but then the receptor remains unresponsive to more stimukation for some time.
  • It causes raised heart rate and blood pressure and also triggers the release of dopamine.

Lidocaine

  • Lidocaine molecules block voltage-gated sodium channels, preventing the production of an action potential in sensory nerves and so preventing you from feeling pain. 

Cobra venom

  • It binds irreversibly to acetylcholine receptors in post-synaptic membranes and neuromuscular junctions. It prevents the transmission of impulses across synapses and as a result muscles are not stimulated to contract and gradually the person affected becomes paralysed.
  • When the toxin reaches the muscles involved in breathing it causes death.
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Sensory systems

How the rods work in the light:

1. When a photon of light hits rhodopsin, it converts the cis-retinal into trans-retinal. This changes the shape of the retinal, and puts a strain on the bonding between the opsin and retinal. As a result, the rhodopsin breaks up into opsin and retinal- bleaching.

2. This triggers a cascade reaction that results in the closing of sodium ion channels, so the rod cell membrane becomes much less permeable to sodium ions and fewer sodium ions diffuse into the cell. 

3. The sodium pump continues to work at the same rate, pumping sodium ions out of the rod cell, so the interior becomes more negative than usual. This hyperpolarisation is known as the generator potential. 

4. If this is large enough to reach the threshold, neurotransmitter substances are released into the synapses on both the photoreceptor and the nerve fibre. An action potential is then set up in the bipolar cell that passes across the synapse to cause an action potential in the sensory neurone. All the ensory neurones leave the eye at the same point to form the optic nerve. 

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Sensory systems

Change in light intensity:

  • Once the visual pigment has been bleached, the rod cannot be stimulated again until the rhodpsin is resynthesised.. It takes ATP, produced by the mitochondria, to convert the trans-retinal back to cis-retinal. 
  • In normal daylight the rods are almost entirely bleached and can no longer respond to dim light- they are light adapted.
  • After about 30 minutes in complete darkness the rhodposin will be almost fully reformed- the eye is dark adapted.

Cones and colour vision:

  • The cones visual pigment is iodopsin. Iodopsin needs to be hit with more light energy than rhodpsin in order to break down, and so it is not sensitive to low light intensities. 
  • There appears to be three types of iodpsin, each sensitive to one of the primary colours of light. 
  • The cones provide colour vision because the brain interprets the numbers of different types of cones stimulated as different colours.
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The CNS

The brain:

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The CNS

Regions of the brain:

  • Cerebrum- controls voluntary behaviour including movement, site of intelligence, learning, memory, personality and the ability to reason.
  • Cerebral cortex- the outer layer of the cerebrum, made up entirely of grey matter (nerve cell bodies, dendrites and synapses), deeply folded to give a large surface area
  • Corpus callosum- made of white matter (band of axons), connects left and right hemispheres
  • Cerebellum- controls balance and coordination of movement
  • Medulla oblongata- reflex control of breathing, heart rate, blood pressure
  • Hypothalamus- thermoregulation and osmoregulation, coordinates autonomic nervous system, involved in thirst, hunger, aggression.
  • Ventricles- fluid filled cavities inside the brain
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The CNS

The spinal cord:

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The CNS

Structure and functions of the spinal cord:

  • Dorsal root- carries only sensory nerve fibres from the spinal nerve into the spinal cord
  • Ventral root- carries only motor nerve fibres from the spinal cord into the spinal nerve
  • The function of the reflex arc is to bring about an appropriate response to a particular stimulus as rapidly as possible without the time delay that occurs when the conscious centres become involved. 
  • There are two main types of reflexes:

1. Spinal reflexes (eg. hand withdrawing from heat or sharp object)                                    

2. Cranial reflexes (eg. blinking, pupil reflexes)

  • Grey matter consists of the cell bodies of neurones in the CNS
  • White matter consists of the nerve fibres of neurones in the CNS
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The peripheral nervous system

The peripheral nerves are divided into two systems: the sensory or affarent nerves and the motor or efferent nerves. The motor nerves can be sub-divided into two main types:

1. The voluntary nervous system- involves motor neurones that are under voluntary or  conscious control involving the cerebrum.                                                                                     2. The autonomic nervous system- involves motor neurones that are not under the control ofthe conscious areas of brain. They control bodily functions that are normally involuntary. 

The autonomic nervous system itself can be sub-divided into the sympathetic and parasympathetic nervous system.

  • The sympathetic nervous system produces noradrenaline at the synapses and usually produces a rapid response in the target organ system. It is sometimes referred to as the 'fight or flight' system. In the sympathetic system, the ganglia are very close to the CNS, so the preganglionic fibres are short and the postganglionic fibres are long.
  • The parasympathetic nervous system produces acetylcholine at the synapses and often has a slower, damping down or inhibitory effect on organ systems. In the parasympathetic system the ganglia are near to or in the effector organ, so the preganglionic fibres are very long and the postganglionic fibres and very short.
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