Structure and Functions of the Neuron
A neuron is a cell that transmits nerve impulses. It consists of the following parts, shown in Figure 1:
- The cell body (soma or perikaryon) contains the nucleus and other cell organelles.
- There are clusters of rough endoplasmic reticulum (not shown in Figure 1) that are called Nissl bodies or are sometimes referred to as chromatophilic substances.
- The dendrite is typically a short, abundantly branched, slender process (extension) of the cell body that receives stimuli.
- The axon is typically a long, slender process of the cell body that sends nerve impulses. It emerges from the cell body at the cone‐shaped axon hillock. Nerve impulses arise in the trigger zone, generally located in the initial segment, an area just outside the axon hillock. The cytoplasm of the axon, the axoplasm, is surrounded by its plasma membrane, the axolemma. A few axons branch along their lengths to form axon collaterals, and these branches may return to merge with the main axon. At its end, each axon or axon collateral usually forms numerous branches ( telodendria), with most branches terminating in bulb‐shaped structures called synaptic knobs (synaptic end bulbs, also called terminal boutons). The synaptic knobs contain neurotransmitters, chemicals that transmit nerve impulses to a muscle or another neuron.
Classification of Neurons
Neurons can be classified by function or by structure. Functionally, they fall into three groups:
- Sensory neurons ( afferent neurons) transmit sensory impulses from the skin and other sensory organs or from various places within the body toward the central nervous system (CNS), which consists of the brain and spinal cord.
- Motor neurons ( efferent neurons) transmit nerve impulses from the CNS toward effectors, target cells that produce some kind of response. Effectors include muscles, sweat glands, and many other organs.
- Association neurons ( interneurons) are located in the CNS and transmit impulses from sensory neurons to motor neurons. More than 90 percent of the neurons of the body are association neurons.
Neurons are structurally classified into three groups, as shown in Figure 1:
- Multipolar neuronshave one axon and several to numerous dendrites. Most neurons are of this type.
- Bipolar neurons have one axon and one dendrite. They emerge from opposite sides of the cell body. Bipolar neurons are found only as specialized sensory neurons in the eye, ear, or olfactory organs.
- Unipolar neurons have one process of emerging from the cell body that branches, T‐fashion, into two processes. Both processes function together as a single axon. Dendrites emerge from one of the terminal ends of the axon. The trigger zone in a unipolar neuron is located at the junction of the axon and dendrites. Unipolar neurons are mostly sensory neurons.
The following terms apply to neurons and groups of neurons:
- A nerve fiber is an axon.
- A nerve is a bundle of nerve fibers in the peripheral nervous system (PNS). Most nerves contain both sensory and motor fibers.
- Cell bodies are usually grouped into separate bundles called ganglia.
- A peripheral nerve consists of three layers:
- The epineurium is the outer layer that surrounds the entire nerve.
- The perineurium surrounds bundles of axons. Bundles of axons are called fascicles. There could be 10 or more fascicles per nerve.
- Surrounding each individual axon is the endoneurium.
- A nerve tract is a bundle of nerve fibers in the CNS.
Receptors and effectors are connected to the central nervous system by neurones. A neurone’s function is to transmit electrical impulses across the nervous system quickly. A neurone is adapted for this function in the following ways:
- The cell body contains the cytoplasm and nucleus (the control centre of the cell).
- The axon is a long extension of the cytoplasm (can be up to 1m). This means nerve impulses can be transmitted to the extremities by one cell.
- The myelin sheath is a fatty layer that surrounds the axon. The sheath acts as an insulator and speeds up nerve impulses.
- The branched ends of the axon and the smaller branches coming from the cell body allow the neurone to make connections with many other neurones.
- Stimuli are changes in our environment that we respond to and are detected by receptors.
- Different receptors are sensitive to different stimuli (e.g. receptors in the eye are sensitive to light).
- A nerve cell/neurone carries information in the form of nerve impulses from the receptor to the coordinator.
- The coordinator – brain or spinal cord – determines whether or not to respond to the stimulus.
- A neurone carries information from the coordinator to an effector.
- An effector is a muscle or gland that can bring about a response.
Transmission of Impulses
Neurotransmission is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron), and bind to and react with the receptors on the dendrites of another neuron (the postsynaptic neuron) a short distance away.
When an impulse reaches the axon terminal of the axon, vesicles consisting of a chemical substance or neurotransmitter, such as acetylcholine, fuse with the plasma membrane. This chemical moves across the cleft called Synapse, which is a small gap that occurs between the axon terminal of the axon of one neuron and the dendrite of next neuron, and attaches to chemo-receptors present on the membrane of the dendrite of next neuron. This binding of chemical with chemo-receptors leads to the depolarization of membrane and generates a nerve impulse across nerve fibre. The chemical, acetylcholine, is inactivated by enzyme acetylcholinesterase. The enzyme is present in the postsynaptic membrane of the dendrite. It hydrolyses acetylcholine and this allows the membrane to repolarize.
Healthy nerve cells contain a balance of ions, ions being positively or negatively charged atoms. Because of these ions, the movement of nerve impulses through individual nerve cells involves both chemical and electrical changes. Before a neuron transmits an impulse, its outside layer is composed of electrically positive ions, its inside layer is composed of electrically negative ions. When the neuron is at rest, there is no movement of chemicals into or out of the cell. When the neuron is stimulated, electrical and chemical changes occur. At the stimulated point, the outside of the nerve cell becomes negative and the inside becomes positive. The ions change places. As soon as the impulse passes, the stimulated point returns to its original electrical and chemical state. The passage of the nerve impulse along the cell causes a similar pattern of changes throughout the neuron. Before individual parts of the cell can pass another impulse, they must rest. In some neurons, this recovery period is as brief as a thousandth of a second
Nerve impulses have a domino effect. Each neuron receives an impulse and must pass it on to the next neuron and make sure the correct impulse continues on its path. Through a chain of chemical events, the dendrites (part of a neuron) pick up an impulse that’s shuttled through the axon and transmitted to the next neuron. The entire impulse passes through a neuron in about seven milliseconds — faster than a lightning strike. Here’s what happens in just six easy steps:
- Polarization of the neuron’s membrane: Sodium ions are on the outside, and potassium ions are on the inside.
Cell membranes surround neurons just as any other cell in the body has a membrane. When a neuron is not stimulated — it’s just sitting with no impulse to carry or transmit — its membrane is polarized. Being polarized means that the electrical charge on the outside of the membrane is positive while the electrical charge on the inside of the membrane is negative. The outside of the cell contains excess sodium ions (Na+); the inside of the cell contains excess potassium ions (K+).
- Resting potential gives the neuron a break.
When the neuron is inactive and polarized, it is said to be at its resting potential. It remains this way until a stimulus comes along.
- Action potential: Sodium ions move inside the membrane.
When a stimulus reaches a resting neuron, the gated ion channels on the resting neuron’s membrane open suddenly and allow the Na+ that was on the outside of the membrane to go rushing into the cell. As this happens, the neuron goes from being polarized to being depolarized. (Remember that when the neuron was polarized, the outside of the membrane was positive, and the inside of the membrane was negative. Well, after more positive ions go charging inside the membrane, the inside becomes positive, as well; polarization is removed and the threshold is reached.)
Each neuron has a threshold level — the point at which there is no holding back. After the stimulus goes above the threshold level, more gated ion channels open and allow more Na+ inside the cell. This causes complete depolarization of the neuron and an action potential is created. In this state, the neuron continues to open Na+ channels all along the membrane. When this occurs, it is an all-or-none phenomenon. “All-or-none” means that if a stimulus doesn’t exceed the threshold level and cause all the gates to open, no action potential results; however, after the threshold is crossed, there’s no turning back: Complete depolarization occurs and the stimulus will be transmitted.
When an impulse travels down an axon covered by a Myelin Sheath, the impulse must move between the uninsulated gaps called nodes of Ranvier that exist between each Schwann cell.
- Repolarization: Potassium ions move outside, and sodium ions stay inside the membrane.
After the inside of the cell becomes flooded with Na+, the gated ion channels on the inside of the membrane open to allow the K+ to move to the outside of the membrane. With K+ moving to the outside, the membrane’s repolarization restores electrical balance, although it’s opposite of the initial polarized membrane that had Na+ on the outside and K+ on the inside. Just after the K+ gates open, the Na+ gates close; otherwise, the membrane could not repolarize.
- Hyperpolarization: More potassium ions are on the outside than there are sodium ions on the inside.
When the K+ gates finally close, the neuron has slightly more K+ on the outside than it has Na+ on the inside. This causes the membrane potential to drop slightly lower than the resting potential, and the membrane is said to be hyperpolarized because it has a greater potential. (Because the membrane’s potential is lower, it has more room to “grow.”). This period doesn’t last long, though (well, none of these steps take long!). After the impulse has traveled through the neuron, the action potential is over, and the cell membrane returns to normal (that is, the resting potential).
- Refractory period puts everything back to normal: Potassium returns inside, sodium returns outside.
The refractory period is when the Na+ and K+ are returned to their original sides: Na+ on the outside and K+ on the inside. While the neuron is busy returning everything to normal, it doesn’t respond to any incoming stimuli. After the Na+/K+ pumps return the ions to their rightful side of the neuron’s cell membrane, the neuron is back to its normal polarized state and stays in the resting potential until another impulse comes along.
The following figure shows transmission of an impulse.
Transmission of a nerve impulse: Resting potential and action potential.
Like the gaps between the Schwann cells on an insulated axon, a gap called a synapse or synaptic cleft separates the axon of one neuron and the dendrites of the next neuron. Neurons don’t touch. The signal must traverse the synapse to continue on its path through the nervous system. Electrical conduction carries an impulse across synapses in the brain, but in other parts of the body, impulses are carried across synapses as the following chemical changes occur:
- Calcium gates open.
At the end of the axon from which the impulse is coming, the membrane depolarizes, gated ion channels open, and calcium ions (Ca2+) are allowed to enter the cell.
- Releasing a neurotransmitter.
When the calcium ions rush in, a chemical called a neurotransmitter is released into the synapse.
- The neurotransmitter binds with receptors on the neuron.
The chemical that serves as the neurotransmitter moves across the synapse and binds to proteins on the neuron membrane that’s about to receive the impulse. The proteins serve as the receptors, and different proteins serve as receptors for different neurotransmitters — that is, neurotransmitters have specific receptors.
- Excitation or inhibition of the membrane occurs.
Whether excitation or inhibition occurs depends on what chemical served as the neurotransmitter and the result that it had. For example, if the neurotransmitter causes the Na+ channels to open, the neuron membrane becomes depolarized, and the impulse is carried through that neuron. If the K+ channels open, the neuron membrane becomes hyperpolarized, and inhibition occurs. The impulse is stopped dead if an action potential cannot be generated.
After the neurotransmitter produces its effect, whether it’s excitation or inhibition, the receptor releases it and the neurotransmitter goes back into the synapse. In the synapse, the cell “recycles” the degraded neurotransmitter. The chemicals go back into the membrane so that during the next impulse, when the synaptic vesicles bind to the membrane, the complete neurotransmitter can again be released.
Watch these videos at Youtube for visual explanations https://www.youtube.com/watch?v=QNpPad_Ecvw
In response to a threshold action potential or graded electrical potential, a neurotransmitter is released at the presynaptic terminal. The released neurotransmitter may then move across the synapse to be detected by and bind with receptors in the postsynaptic neuron. Binding of neurotransmitters may influence the postsynaptic neuron in either an inhibitory or excitatory way. The binding of neurotransmitters to receptors in the postsynaptic neuron can trigger either short term changes, such as changes in the membrane potential called postsynaptic potentials, or longer term changes by the activation of signaling cascades.
Neurons form complex biological neural networks through which nerve impulses (action potentials) travel. Neurons do not touch each other (except in the case of an electrical synapse through a gap junction); instead, neurons interact at close contact points called synapses. A neuron transports its information by way of an action potential. When the nerve impulse arrives at the synapse, it may cause the release of neurotransmitters, which influence another (postsynaptic) neuron. The postsynaptic neuron may receive inputs from many additional neurons, both excitatory and inhibitory. The excitatory and inhibitory influences are summed, and if the net effect is inhibitory, the neuron will be less likely to “fire” (i.e., generate an action potential), and if the net effect is excitatory, the neuron will be more likely to fire. How likely a neuron is to fire depends on how far its membrane potential is from the threshold potential, the voltage at which an action potential is triggered because enough voltage-dependent sodium channels are activated so that the net inward sodium current exceeds all outward currents. Excitatory inputs bring a neuron closer to threshold, while inhibitory inputs bring the neuron farther from threshold. An action potential is an “all-or-none” event; neurons whose membranes have not reached threshold will not fire, while those that do must fire. Once the action potential is initiated (traditionally at the axon hillock), it will propagate along the axon, leading to release of neurotransmitters at the synaptic bouton to pass along information to yet another adjacent neuron.
Stages in neurotransmission at the synapse
- Synthesis of the neurotransmitter. This can take place in the cell body, in the axon, or in
the axon terminal.
- Storage of the neurotransmitter in storage granules or vesicles in the axon terminal.
- Calcium enters the axon terminal during an action potential, causing release of the neurotransmitter into the synaptic cleft.
- After its release, the transmitter binds to and activates a receptor in the postsynaptic membrane.
- Deactivation of the neurotransmitter. The neurotransmitter is either destroyed enzymatically, or taken back into the terminal from which it came, where it can be reused, or degraded and removed
Neurotransmission at a Chemical Synapse: A signal propagating down an axon to the cell body and dendrites of the next cell
Types of Nervous Actions: Reflex and Voluntary Actions
- Reflex Action:
Reflex action or reflex is an involuntary action in response to a stimulus. This is a spontaneous action without thinking. For example, we adjust our eyes when exposed to bright light. The peripheral nervous system (PNS) is a system of nerves which connect the central nervous system (CNS) (includes the brain and spinal cord) with other parts of the body. Reflex action is the result of the coordination of the spinal cord and peripheral nervous system. This action does not involve the brain. The pathway in which impulses travel during the reflex action is called a reflex arc.
Illustration of Reflex Actions e.g. knee jerk
Knee-jerk reflex, also called patellar reflex, is sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap. One of the several positions that a subject may take for the test is to sit with knees bent and with one leg crossed over the other so that the upper foot hangs clear of the floor. The sharp tap on the tendon slightly stretches the quadriceps, the complex of muscles at the front of the upper leg. In reaction these muscles contract, and the contraction tends to straighten the leg in a kicking motion.
- Voluntary action:
When an action is produced with the involvement of thoughts, they are called voluntary action. It involves actions like walking, eating, jumping and running. These actions are produced consciously. Both spinal cord and brain are involved and these coordinate with PNS to generate necessary movements.