Neurophysiology – CNS, PNS, Neuron Structure, Impulse Transmission, Neurotransmitters, Reflexes

Central Nervous System (CNS)

The Central Nervous System (CNS) is the command center of the body, responsible for processing information and coordinating responses. It comprises the brain and the spinal cord. The brain is the most complex organ, responsible for thought, memory, emotion, and movement. The spinal cord acts as a conduit for signals between the brain and the rest of the body, and also controls certain reflexes independently. The CNS is protected by the skull and vertebral column, and further cushioned by cerebrospinal fluid.

Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) consists of all the nerves that extend outside the CNS. Its primary role is to connect the CNS to the limbs and organs. The PNS is further divided into two main branches: the somatic nervous system, which controls voluntary movements, and the autonomic nervous system, which regulates involuntary bodily functions like heart rate, digestion, and breathing. The autonomic nervous system is then subdivided into the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.

Neuron Structure

Neurons, also known as nerve cells, are the fundamental units of the nervous system. They are specialized cells designed to transmit information throughout the body. A typical neuron consists of three main parts:

• Cell Body (Soma): This contains the nucleus and other essential organelles for the neuron's survival and function. It's the metabolic center of the neuron.
• Dendrites: These are branched, tree-like extensions that receive signals from other neurons and transmit them towards the cell body.
• Axon: This is a long, slender projection that transmits electrical impulses away from the cell body to other neurons, muscles, or glands. Axons are often covered by a myelin sheath, an insulating layer that speeds up nerve impulse transmission. The myelin sheath is produced by glial cells (Oligodendrocytes in the CNS and Schwann cells in the PNS).

The junction between two neurons is called a synapse, where information is transmitted from one neuron to another.

Impulse Transmission

Nerve impulse transmission, also known as action potential, is an electrochemical process. It begins with a stimulus that causes a change in the electrical potential across the neuron's membrane. Neurons maintain a resting potential when they are not transmitting a signal, which is typically around -70 millivolts (mV). This is due to an unequal distribution of ions (like sodium, Na+, and potassium, K+) across the cell membrane.

When a neuron is stimulated, ion channels in the membrane open, allowing Na+ ions to rush into the cell. This influx of positive charge causes depolarization, making the inside of the cell positive relative to the outside, reaching a peak of about +30 mV. This rapid change is the action potential.

Immediately after, the Na+ channels close, and K+ channels open, allowing K+ ions to flow out of the cell. This repolarization restores the negative charge inside the cell. The membrane may briefly hyperpolarize (become even more negative than the resting potential) before returning to its resting state.

This electrical signal then propagates down the axon as a wave of depolarization and repolarization. In myelinated axons, this transmission is much faster because the action potential "jumps" from one gap in the myelin sheath (Node of Ranvier) to the next, a process called saltatory conduction.

Synaptic Transmission

When an action potential reaches the axon terminal of a neuron (the presynaptic neuron), it triggers the release of chemical messengers called neurotransmitters into the synaptic cleft, the small gap between the presynaptic and postsynaptic neurons.

These neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the dendrites or cell body of the postsynaptic neuron. This binding causes a change in the electrical potential of the postsynaptic neuron's membrane.

If the binding causes depolarization (making the postsynaptic neuron more likely to fire an action potential), it's an excitatory postsynaptic potential (EPSP). If it causes hyperpolarization (making it less likely to fire), it's an inhibitory postsynaptic potential (IPSP). The postsynaptic neuron integrates all incoming signals (both EPSPs and IPSPs) and fires an action potential only if the sum of these signals reaches a certain threshold.

After their action, neurotransmitters are either broken down by enzymes or reabsorbed by the presynaptic neuron (reuptake) to terminate the signal and prepare for the next transmission.

Neurotransmitters

Neurotransmitters are chemical substances that transmit signals across a chemical synapse from one neuron to another, or from a neuron to a target effector cell. There are many types of neurotransmitters, each with specific functions and locations in the nervous system. Some of the major neurotransmitters include:

Reflexes

A reflex is an involuntary, rapid, and predictable response to a stimulus. Reflexes are mediated by the nervous system and are essential for protection and maintaining homeostasis. They allow the body to react quickly to potentially harmful situations without conscious thought, which can be life-saving.

The simplest neural pathway that mediates a reflex is called a reflex arc. A reflex arc typically involves the following components:

1. Receptor: Detects the stimulus (e.g., pain receptors in the skin).
2. Sensory Neuron: Transmits the nerve impulse from the receptor to the CNS.
3. Integration Center: This can be a single synapse (monosynaptic reflex) or multiple synapses (polysynaptic reflex) within the CNS (spinal cord or brainstem).
4. Motor Neuron: Transmits the nerve impulse from the CNS to the effector.
5. Effector: A muscle or gland that carries out the response (e.g., a muscle contracting to move the body away from the stimulus).

Types of Reflexes

Reflexes can be classified in several ways. A common classification is based on the effector:

• Somatic Reflexes: These involve the contraction of skeletal muscles. Examples include the withdrawal reflex (pulling your hand away from a hot object) and the stretch reflex (maintaining posture).
• Autonomic Reflexes: These involve the regulation of smooth muscle, cardiac muscle, or glands. Examples include changes in heart rate, digestion, and pupillary light reflex.

Another classification is based on whether the reflex is innate or learned:

• Innate (Unconditioned) Reflexes: These are genetically determined and present from birth. They are simple, involuntary responses. Examples include sucking reflex in infants and the knee-jerk reflex.
• Acquired (Conditioned) Reflexes: These are learned through experience and association. They are developed through a process of conditioning, as demonstrated by Pavlov's experiments with dogs.

Examples of Reflex Arcs

1. The Patellar Reflex (Knee-Jerk Reflex): This is a classic example of a monosynaptic somatic reflex.

• Stimulus: Tapping the patellar tendon (below the kneecap).
• Receptor: Muscle spindles in the quadriceps femoris muscle stretch.
• Sensory Neuron: Carries the signal to the spinal cord.
• Integration Center: The sensory neuron directly synapses with a motor neuron in the spinal cord (monosynaptic).
• Motor Neuron: Carries the signal back to the quadriceps femoris muscle.
• Effector: The quadriceps femoris muscle contracts, causing the lower leg to extend.

Simultaneously, the sensory neuron also excites an interneuron that inhibits the motor neurons controlling the opposing hamstring muscles, allowing the leg to extend smoothly. This is an example of reciprocal inhibition.

2. The Withdrawal Reflex (Flexor Reflex): This is a polysynaptic somatic reflex, designed to protect the body from painful stimuli.

• Stimulus: Touching a hot or sharp object.
• Receptor: Pain receptors (nociceptors) in the skin are activated.
• Sensory Neuron: Transmits the signal to the spinal cord.
• Integration Center: The sensory neuron synapses with one or more interneurons, which then synapse with motor neurons (polysynaptic).
• Motor Neurons: Carry signals to the flexor muscles in the limb.
• Effector: The flexor muscles contract, causing the limb to withdraw from the stimulus.

This reflex is often accompanied by the crossed-extensor reflex, where motor neurons on the opposite side of the spinal cord are activated to extend the other limb, providing support.