The Nervous System

The nervous system comprises of two parts. The CNS is composed of the brain and the spinal chord. The PNS consists of the nerves and ganglia on the outside of the cns. 

In mammals responses to many external and internal stimuli involve the reception of information and its transfer from a receptor to an effector via the nervous system or as hormones in the blood via the endocrine system. 

A stimulus is a detectable change in the external or internal environment of an organism. Thsi could be pressure on skin, chemicals in food or a light turning on. 

A receptor detects a stimulus and converts it into electrical energy to send the information as an electrical impules to the CNS via neurones. Some examples are photoreceptors, thermoreceptors, mechanoreceptors and osmoreceptors. 

An effector recieves electrical impulses from the CNS and brings about a response. Examples of receptors are muscles or glands. 

Neurones are highly specialised cells which generate and transmit nerve impulses. There are 3 types in mammals

Sensory - Carry impulses from the receptor to the CNS. 

Motor - Carry impulses from the CNS to the effector organs (Muscles and Glands)

Relay - found within the spinal cord, receive impulses from sensory neurones or other intermediate neurones and relay them to motor neurones or any intermediate neurones. 

Sensory neurones have the cell body in the middle of the neurone whereas the others have the cell body at the start of the neurone. 

Motor and sensory neurones have longer axons. 

Axon - Extension of the cytoplasm that transmits impulses away from the cell body to the axon endings. 

Dendrites - Thin extensions of cytoplasm that receive impulses from other nerve cells and transmit impulses towards the cell body. 

Cell Body - Contains the nucleus and groups of ribosomes in the cytoplasm that synthesise neurotransmitters. 

Synaptic end bulb - Swelling at the end of the axon where the neurotransmitter is stored. 

Axon Ending - Secretes neurotransmitter by exocytosis into the synaptic cleft. 

The following are only found in PNS neurones. 

Schwann Cells - Surround peripheral neurones in vertebrates and grow around the axon to form a multi-layered myelin sheath.  

Myelin Sheath - Formed as schwann cells grow around the axon in peripheral neurones. Provide electrical insulation to speed up nerve impulse transmission. 

Nodes of Ranvier - Intervals in the myelin sheath between schwann cells. 

• A long thin tubular bundle of nervous tissue and support cells that extends from the brain. 
• Protected by the spinal column. 
• Most of the peripheral nerves originate from the spinal cord. 
• The function of the spinal cord is the transmission of neural signals between the brain and the rest of the body. 
• It also contains neural circuits that can independently control numerous reflexes. 
• The spinal cord is made up of the central area of grey matter which contains mostly nerve cell bodies. 
• It is surrounded by white matter which consists of nerve fibres surrounded by myelin sheath. 
• The spinal cord is surrounded by membranes called meninges. 
• Sensory fibres from the PNS enter the spinal cord on the dorsal root and the cell bodies of the sensory fibres are found in the dorsal root ganglia. 
• Motor fibres leave via the ventral roots. 

• This is the simplest type of response which is an inborn response to a stimulus and is rapid automatic and beneficial. 
• It is involuntary so it does not involve the brain. 
• The receptor detects the stimulus. Sensory neurones send impulses to the relay neurone. The motor neurone sends impulses to the effector which produces a response. 
• Reflexes are usually protective so an example would be blinking or moving hands of something hot. 

• A neurone is an excitable cell which means it can change its resting potential which is the potential difference across the membrane when no nervous impulse is being conducted. 
• The potential difference across membranes can be measured in experiments involving inserting microelectrodes into axons and measuring the changes in electrical potential which can be read on a cathode ray oscilloscope.
•  When the two microelectrodes are both outside the neurone, no difference in electrical potential is recorded. 
• When two microelectrodes are placed on either side of the axon of the neurone, the difference between the outside and inside is measures, this is known as the potential difference. 

• Neurones transmit electrical impulses along the cell membrane u. They do this by changing the potential difference across the axon membrane of the neurone. 
• When no impulse is being sent, the inside of the cell has a negative electrical charge compared to the outside. This is known as resting potential and is approximately -70mV and we say that the membrane is polarised. This is achieved via the movement of Na+ and K+ ions. 
• 3 Na+ ions are actively transported out of the axon for every 2 K+ ions that are pumped in by the Sodium-Pottasium Pump. 
• Voltage-gated potassium ion channels, some of which are open, allow l+ to diffuse back out of the axon. 
• As the voltage-gated sodium ion channels are closed the membrane is more permeable to K+ than Na+. 
• The neurone has a resting potential of -70mV
• Some K+ channels allow K+ ions to leave the axon through leakage. 

• The energy of teh stimulus causes trhe volateb gated sodium channels to open. Thsi makes the voltage gated potassium channels close. m
• Tghis causes a permeability of the membrane to Na+ so they rapidly diffuse into the axon, depolarising the membrane. 
• The negative charge of -70mV inside the axon quickly becomes a positive charge of +40mV. 
• The membrane is said to be Depolarised. 

• The voltage-gated sodium ion channels close and potassium ion channels open. 
• This causes k+ ions to rapidly diffuse out of the axon reducing the potential difference across the membrane. 
• An overshoot causes the membrane to become hyperpolarised. 
• During the refractory period concentrations of K+ and Na+ are restored to that of the resting potential. 
• During this time this portion of the axon cannot transmit another action potential, this ensures that transmission is in one direction only. 

• If the intensity of a stimulus is below the -55mV threshold then no action potential is initiated. 
• If the intensity exceeds this threshold then an action potential is initiated. 
• This ensures that low-level stimuli are filtered out. 
• The action potential is always the same size when initiated, +40mV. 
• The all or nothing law states that an action potential is either initiated or it is not. and it is always the same size. 
• The size of the impulse is independent of the size of the stimuli. 
• the speed of the conduction of the stimuli is not altered by the intensity of the stimuli. 
• A large stimulus will simply produce a greater frequency of impulses.

• In a myelinated neurone, the axon is surrounded by Schwann cells that form a myelin sheath which acts as an electrical insulator. 
• The nerve impulse appears to jump from one node of Ranvier to the next as myelin is a lipid type substance that is impermeable to ions, therefore action potentials can only take place at nodes where there is an abundance of ion channels and Na+/K+ pumps. 
• The local currents caused by the diffusion of sodium ions are elongated and the speed of the transmission is increased, this is known as saltatory conduction. 

• Temperature - Ions move faster at higher temperatures as they will have more kinetic energy. Warm-blooded animals transmit impulses quickly and have faster responses. 
• Diameter of Axon - The greater the diameter, the greater its volume in relation to the area of the membrane. More an+ can travel along the axon so impulses travel faster. 
• Myeination - Thsi electrical insulation of the axon speeds up the rate of transmission as the action potential jumps from node to node in a process called saltatory propagation. 

• In nerve nets the sense receptors only respond to a limited number of stimuli and there is only a small number of effectors.

HYDRA VS HUMAN

• Hydra have a nerve net whereas humans have a central nervous system. 
• Hydra have no myelin sheath whereas humans do have myelin sheath. 
• Hydra conduction speed is slow whereas human speed is fast. 
• Hydra have the ability to regenerate neurones whereas humans do not. 

• Na+/K+ pumps are working. 3Na+ are actively transported out and 2 K+ are transported in. This process requires ATP as transport is against the concentration gradient. The membrane is polarised and the resting potential is -70mV. 
• Pressure receptor cells in your skin detect an external stimulus if the threshold is reached the voltage-gated Na+ channels in that area would open. 
• Na+ diffuse rapidly into the cell resulting in depolarisation
• An action potential has been generated and the membrane potential is now +40mV
• Voltage-gated K+ channels open, Na+ channels close and K+ rapidly diffuse out of the cell, this repolarises the membrane. 
• The amount of K+ ions that diffuse out is a slight overshoot so the membrane potential falls to approximately -75mV. This is hyperpolarisation. 
• The resting potential is restored during the refractory period which ensures that the nerve impulses continue in one direction. 
• Once an impulse is made, a local current is set up between the area where there is an action potential and the resting area next to it. 
• Na+ diffuse down their concentration gradient and depolarise the adjacent area to propagate the action potential along the axon. 
• In a myelinated neuron, the impulse is conducted more rapidly as the impulse jumps from one node of Ranvier to the next as myelin is an electrical insulator. 

• Neurons are separated by gaps known as synapses. There are 2 types of synapses
• Electrical Synapses - A tiny gap that is small enough for an electrical impulse to be transmitted directly across. 
• Chemical Synapses - Involves a gap of around 20nm. Branched Axons lie close to dendrites and the impulse is transmitted across the synaptic cleft as a neurotransmitter before being converted back into an electrical impulse. 
• An action potential arrives at the axon terminal. This causes voltage-gated calcium ion channels to open. Calcium ions diffuse into the presynaptic knob. 
• The influx of Ca2+ causes vesicles containing acetylcholine (neurotransmitter) to fuse with the presynaptic membrane. 
• The Acetylcholine is released into the synaptic cleft via exocytosis, diffuses over the cleft and binds to specific receptors on the postsynaptic membrane. 
• This causes sodium ion channels in the postsynaptic membrane to open and Na+ diffuses in, depolarising the postsynaptic membrane. If the depolarization reaches the threshold value an action potential is generated in the postsynaptic neurone. 
• Acetylcholine in the receptors is broken down by acetylcholinesterase into ethanoic acid and choline which diffuses back into the axon terminal through the presynaptic membrane. 
• ATP is then required to resynthesise and package the neurotransmitters into vesicles