Nervous System: Receptors, Neurons, and Synapses
Receptors
Receptors are sensory cells that detect internal or external stimuli and convert one form of energy into electrochemical energy.
Coordinators
Coordinators are part of the Central Nervous System (CNS), which includes the brain and spinal cord. They process the information received from receptors and initiate an appropriate response by communicating with effectors.
Effectors
Effectors are muscles or glands that bring about a response.
Peripheral Nervous System
The Peripheral Nervous System consists of individual neurons and ganglia that connect the receptors and effectors to the CNS.
- Sensory Neurons: Transmit impulses from receptors to the CNS.
- Motor Neurons: Transmit impulses from the CNS to effectors.
- Relay Neurons: Link sensory and motor neurons within the CNS.
Motor neurons contain a nucleus and other organelles, such as ribosomes, that synthesize neurotransmitters and mitochondria (which provide ATP to Na+/K+ pumps).
Dendrites transmit impulses towards the cell body, while the axon transmits impulses away from the cell body.
The myelin sheath, formed by Schwann cells, extends and wraps around the axon during myelination. It insulates the neuron and prevents the movement of Na+/K+ in and out of the axon. Small gaps called nodes of Ranvier allow this movement of ions in and out.
Resting Potential
Na+/K+ pumps actively pump out 3 Na+ ions for every 2 K+ ions into the neuron. An ion concentration gradient establishes a potential difference across the axon membrane. The membrane is relatively impermeable to Na+ ions, so they diffuse back in slowly. The inside of the axon becomes negatively charged and is polarized at -70mV.
Action Potential
When a neuron is stimulated, voltage-gated Na+ channels open, increasing permeability to Na+. Na+ ions diffuse in rapidly and depolarize the membrane. The potential difference across the membrane is reversed and becomes +40mV, and an action potential is created. Voltage-gated Na+ channels then close, and the membrane begins to repolarize. Voltage-gated K+ channels open and allow K+ ions to diffuse out. Due to the increased permeability to K+, the potential difference ‘overshoots’ to -80mV, and the neuron is hyperpolarized. Voltage-gated K+ channels now close, and Na+/K+ pumps restore the resting potential.
Reaching the Threshold
An action potential is only created if a threshold value is reached (above -70mV). A stimulus must be strong enough to create an action potential within the neuron.
All-or-Nothing Law: The size or strength of the action potential is always the same, irrespective of the strength of the stimulus. The greater the intensity of a stimulus, the greater the frequency of action potentials.
Refractory Period
Briefly after an action potential, voltage-gated Na+ channels are inactive, preventing the inward movement of any Na+. Another action potential can therefore not be generated. An action potential can only propagate in one direction. A second action potential is separate, limiting their frequency.
Propagation and Saltatory Conduction
Propagation or saltatory conduction is where an action potential jumps from one node of Ranvier to the next. When the neuron is stimulated, Na+ ions rush in, depolarizing the axon membrane and creating an action potential.
This creates a local electrical current, causing voltage-gated Na+ channels to open at the adjacent node of Ranvier (at resting potential), which then causes depolarization in that region. A wave of depolarization travels across the membrane. Behind the impulse, K+ ions begin to leave, allowing the neuron to slowly repolarize. Na+ channels are also inactivated behind the impulse during the refractory period, and by the time they are reactivated, the action potential is too far down the axon to affect the original region, ensuring a unidirectional impulse.
Conduction Speed
Conduction speed ranges from 0.5ms-1 to 120ms-1.
- Diameter: The larger the diameter, the lower the resistance of the axoplasm on the local current (ion flow), increasing transmission speed. This is in non-myelinated axons.
- Temperature: An increase in temperature up to about 40°C increases the speed of transmission, as nervous impulses require ATP and are thus directly related to the rate of respiration (enzymes).
- A Larger Myelin Sheath: Increases the speed of transmission as it increases the distance over which local currents can bring about depolarization, causing the action potential to jump from node to node (due to more nodes).
Synapses
Synapses are junctions between two neurons where the transmission of an impulse from one neuron to the next is produced via neurotransmitters, such as acetylcholine, produced by motor neurons targeting muscle cells (noradrenaline, dopamine, serotonin).
A nerve impulse reaches the synaptic knob, depolarizing the presynaptic membrane, causing voltage-gated Ca2+ channels to open. Ca2+ ions diffuse into the presynaptic neuron, causing synaptic vesicles to move and fuse with the presynaptic membrane, where they release acetylcholine (ACh) into the synaptic cleft via exocytosis. ACh diffuses across the cleft and binds with receptor proteins on the postsynaptic membrane. Receptor proteins are attached to Na+ channels and cause them to open, allowing Na+ ions to diffuse into the postsynaptic neuron. If the threshold is reached, an action potential is generated within the postsynaptic neuron.
ACh is hydrolyzed by acetylcholinesterase into choline and ethanoic acid, located on the postsynaptic membrane, and prevents successive impulses from merging at the synapse. The resulting molecules diffuse back across the synaptic cleft and are actively transported back to the presynaptic knob. Energy from ATP is needed to reform ACh, which is stored in synaptic vesicles.
Functions of Synapses
- Ensures the unidirectional flow of an impulse.
- Filters out low-level stimuli due to a low-level frequency of impulses, resulting in a low amount of neurotransmitter released, causing a small number of Na+ channels to open on the postsynaptic membrane, which may be insufficient to exceed the threshold, and no action potential is generated.
Summation
Summation is where several impulses arrive at a synapse and cause a large quantity of neurotransmitter to be released, causing many Na+ channels to open and an action potential to be generated.
- Temporal Summation: A high-frequency impulse arrives in quick succession from a single presynaptic neuron and leads to an accumulation of neurotransmitter in the synapse.
- Spatial Summation: Several impulses arrive at the same time from different presynaptic neurons.
Inhibitory Synapses
Inhibitory synapses cause hyperpolarization to occur in the postsynaptic membrane and prevent an impulse from being generated in the postsynaptic neuron.
Excitatory Drugs (Agonists)
Excitatory drugs (agonists), such as organophosphorus insecticides, have a similar shape to the neurotransmitter and inhibit cholinesterase, resulting in ACh remaining attached to the postsynaptic membrane. This brings about the continuous stimulation of the postsynaptic neuron and muscles, which can result in paralysis or death due to the continuous contraction of cardiac and intercostal muscles.
Inhibitory Drugs (Antagonists)
Inhibitory drugs (antagonists), such as beta-blockers, bind to and block receptors on the postsynaptic membrane, preventing neurotransmitters from binding and preventing the impulse from being transmitted. This can cause paralysis or death due to the inability of muscle contraction.
Psychoactive Drugs
Psychoactive drugs alter brain function, resulting in temporary changes in perception, mood, consciousness, and behavior. Examples include tobacco, cannabis, ecstasy, cocaine, heroin, and amphetamines.
Cocaine
Cocaine is excitatory and prevents the normal uptake of dopamine, which accumulates in synapses, causing repeated action potentials in postsynaptic neurons. Dopamine stimulates pleasure centers of the brain (well-being and happiness), which cocaine overstimulates, leading to a euphoric feeling.
THC
THC, the active ingredient in marijuana, inhibits receptors on the presynaptic membrane, inhibiting the release of excitatory neurotransmitters and preventing stimulation of the postsynaptic membranes, resulting in a relaxed and calm feeling.
Reflexes
Reflexes are rapid, involuntary, and are not under the conscious control of the brain. They have a protective function to increase the chances of survival.
Reflex Arc
A reflex arc consists of the neurons that form the pathway taken by nerve impulses in a reflex action.
Spinal Cord
The spinal cord is within the vertebral column. The central grey matter contains cell bodies and relay neurons. The outer white matter contains myelinated axons that run up and down the spinal cord to and from the brain.
Sensory neurons transmit impulses from receptors to the CNS and enter via the dorsal root. Their cell bodies are on the dorsal root ganglion (a swelling). Motor neurons transmit impulses from the CNS to effectors and leave via the ventral root.
For example, when heat is detected by a heat receptor in the finger, an action potential is generated, and an impulse is transmitted along the sensory neuron to the spinal cord via the dorsal root. The sensory neuron releases neurotransmitters, generating an action potential in the grey matter. This is repeated at the synapse between the relay and motor neurons. The action potential is transmitted along the motor neuron via the ventral root to the bicep muscles. The release of ACh causes the bicep muscles to contract, resulting in the automatic withdrawal of the hand.
Reflex arcs are not separate, and impulses can travel up and down the spinal cord. In the grey matter, sensory neurons can synapse with other neurons, transmitting impulses to the brain. The information received can be stored in memory. The brain can relate the information received with other sensory inputs, such as the eyes, and modify a response. The brain can make a conscious decision and transmit impulses down the spinal cord via neurons that terminate in inhibitory synapses to prevent an action.
Nerve Nets
Hydra do not have a brain or true muscles. Photoreceptors and touch-sensitive nerve cells in the body wall and tentacles respond to light intensity and touch. The number of effectors is small. The nerve net consists of simple nerve cells with short extensions joined and branched in different directions, resulting in the slow transmission of nerve impulses.