3.8 The Nervous System

• The Nervous system enables us to gather, transmit and coordinate information. The system is comprised mainly of neurons.

• Unlike hormones, this happens very quickly (milliseconds).

• Skin- Pain, Pressure, Temperature.

• We have receptors in our Eyes, Tongue, Nose & Ears.

• Motor Organs → Muscles and Glands.

• Receptors receive external stimuli and transfer information to effector organs via the nervous system or hormones (via blood)/.

• Usually works on the principles of negative feedback.

• Rod cells detect dark and light.

• Cone Cells detect colour.

• Sensory receptors also act as transducers.

• CNS- Brain and spinal cord.

• Peripheral nervous system (PNS) - Nerves that come off the spinal cord.

Reflex Arc

• A reflex action is a rapid, automatic response to a stimulus.

• A simple reflex involves neurons in the PNS and the spinal cord and often occurs before the conscious brain is aware of it.

Pin Prick → Receptors →Impulse neuron →Spinal Cord →Impulse jumps the synapse →Motor Neurone →Muscle Contracts → Finger Pulls away.

• Monosynaptic reflex - only one synapse between a sensory neuron and motor neurone e.g knee jerk reflex.

• Synaptic transmission slows down reactions.

• White matter → has a myelin sheath around the neurons.

• Synapses are in the grey matter.

• Polysynaptic Reflex - These involve more than one synapse and connecting relay interneurons between sensory and motor neurons e.g hand-withdrawal reflex.

• Afferent Neurone → Goes into the spinal cord.

• Efferent Neurone → Goes out of the spinal cord.

Purpose of the Reflex Arc

• Protect the body eg blinking, coughing, and hand withdrawal.

• Make rapid adjustments to stimuli e.g salivation, pupils, and balances.

• Make Haemostatic changes e.g adjust breathing, circulation, and body temperature.

Spinal Cord

• Protected by cerebrospinal fluid.

• Co-ordinates reflexes.

• Contains grey matter ( neurons) and white matter (axons) which transmit impulses up and down the spinal cord.

• Between each vertebra are partial spinal nerves.

• Cerebrospinal fluid acts as a cushion

Structure of the spinal cord.

• 3 Membrane layers - The meninges are 3 membranes that envelop the brain and spinal cord.

• Dura mater (tough layer)

• Arachnoid mater (cavity)

• Pia Mater.

• Cerebrospinal fluid is located in the space between the arachnoid mater and pia mater.

• Their functions are to protect the Central Nervous System.

• Meningitis affect the meningies.

Nerve Nets

• Other organisms such as hydra have nerve nets.

• Nerve nets are the simplest type of nervous system.

• They are a diffused network of cells that group into ganglia but do not form a brain.

• Ganglion Cells provide connections in several directions.

• Ganglion cells are sensory cells that can detect stimuli e.g light, sound and touch.

• Its nerve net is in its outer layers allowing them to sense light, contact and chemicals.

• They can contract, perform locomotion, hunt and feed without a brain.

Nerves

• Nerves are bundles of neurons ( which contain axons) enclosed in a sheath of connective tissue.

• Some nerves contain both sensory and motor neurons.

• They need vast amounts of blood as they require ATP.

• There are 4 types of neuron; Bipolar (Interneuron), Unipolar (Sensory neuron), Multipolar ( Motor Neuron) and Pyramidal.

• Neurons- ELongated cells consisting of cell body and long, thin axons.

• Connectors are called dendrites.

• Surrounded by a myelin sheath for insulation. Stops charges escaping.

• Nerves are unique and can only be made once.

Schwann Cells → Glial cells that surround and support nerve fibres, In embryos they surround the developing axons and withdraw the cytoplasm leaving a multilayered myelin sheath.

Nodes of Ranvier → 1um gaps in myelin sheath where adjacent schwann cells meet and where the axon membrane is exposed. Allow impulses to transmit rapidly.

Nissl Granules → Cytoplasmic granules comprising ribosomes grouped on the Rough Endoplasmic Reticulum. (Channel Protein)

Dendrite → Thin fibre carrying impulses towards the cell. A cell body may have several dendrites.

Sensory → Transmit messages from sense receptors to the brain or spinal cord. All sensory neurons have their cell bodies sitting outside the spinal cord dorsal root ganglions,

• Cell bodies of autonomic motor neurons (heart glands) are also found in dorsal root ganglions.

• Cell bodies of somatic motor neurons are found within the brain or spinal cord and connected to voluntary muscles.

• Neurotransmitters (chemical messengers) jump over synapses.

Myelination

• Insulation of the cell.

• Prevents the ion flow across the membrane.

• Gives impulses shorter distances to travel.

• Thicker myelination speeds up the transmission of impulses from stimulus to response.

• Larger diameter, faster conduction speed.

Nerve Transmission

• When a neurone is not conducting an impulse there is a difference in the electrical charge across the plasma membrane.

• There is a higher concentration of positively charged ions outside the cell than inside.

• This causes a potential difference (voltage) in electrical charges in the resting nerve cell.

• This is called the resting potential of a neurone.

• Impulses are generated when Na+ and K+ change the balance of ions.

Maintaining the resting potential.

• Sodium-Potassium pips actively transport sodium ions out of the neurone and potassium ions into the neurone. (3Na+ for each 2K+ in, per ATP)

• Voltage-gated sodium channels are closed but some potassium channels allow K+ to leak out of the axon.

• Large protein anions and organis phosphates remain in the cytoplasm, producing a negative potential difference across the membrane at around -70mV relative to the exterior of the axon.

• Membranes are hyperpolarised when more K+ ions move out than Na+ diffuse in.

• Absolute refractory period lasts around 1ms, Hyperpolarisation lasts around 3-4ms.

‘All or Nothing’ Law

→ A nervous impulse is either initiated or not and it is always the same size.

→ It allows the action potential to act as a filter, preventing minor stimuu from setting up nervous impulses, so the brain is not overloaded with information.

Saltatory Conduction

→ Transmission of a nervous impulse along a myelinated axon in which the action potential jumps from one node of Ranvier to the adjacent node.

Synapse→ A synapse is the junction between 2 nerve cells. There is a gap (synaptic cleft) of around 20nm between cells, and chemicals called neurotransmitters diffuse across it.

Factors affecting speed of conduction of nerve impulses.

1. Temperature → Ions move faster at higher temperatures than at lower temperature as they have more kinetic energy. Warm-blooded animals transmit impulses more quickly and have faster responses.

2. Diameter of Axon→ The greater the diameter of the axon, the greater the volume so more sodium ions can flow through the axon so impulses travel faster. Animals in cold habitats have big axons to compensate for the temperature.

3. Myelanation → Speeds up the rate of transmission by insulating the axon.

Synaptic Transmission

1. Action potential reaches axon terminals.

2. Ca2+ channels open and calcium floods in.

3. Ca2+ causes vesicles to move to the synaptic membrane.

4. Neurotransmitters are released.

5. They bind to neuroreceptors.

6. Ion channels open and action potential is released.

Acetylcholine Resynthesis

In the synaptic cleft;

1. Acetylcholine + acetylcholinesterase → ethanoic acid + choline.

In the pre-synaptic neurone;

1. Acetyl CoA + Choline → CoA + Acetylcholine

Spatial summation

→ Several presynaptic neurones synapse with the same postsynaptic neurones and all contribute to the growing depolarisation which generates an action potential when large enough.

Temporal Summation

→ depolarisation builds up over time to reach the threshold at which action potential is initiated.

Agonist Drugs (Stimulants) → Increase the usual effect of a neurotransmitter at a synapse

→ increase inhibitory or excitatory action.

→ e.g Nicotine increases the excitatory effect.

Antagonist Drugs (Sedatives) →Decrease the usual effect of a neurotransmitter at a synapse. E.g Poisons.

Cocaine

• Non-competitive inhibitor of dopamine.

• Stops dopamine from being broken down and recycled.

• Dopamine keeps firing, sending feel-good messages to the brain.

• Dopamine floods the brain and gives a euphoric sensation.

Cannabis

• Slows down movement.

• Turns off the release of inhibitory neurotransmitters, so dopamine is released.

• THC acts as the body's own anandamide, which ‘squirts’ dopamine.

• Unlike anandamide, THC doesn't break down as quickly so the euphoric effects last.

LSD

• LSD resembles serotonin and elicits its effects by binding to serotonin receptors.

• Sometimes LSD can excite receptors and sometimes it can inhibit the receptors.

Ecstasy

• Mimics serotonin so is readily taken up by serotonin receptors.

• Transporter gets confused and takes serotonin out of the cell.