Human Nervous System | Neurons | Nerve Impulse | Synapse

The nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events.

The neural system provides an organized network of point-to-point connections for quick coordination between various body functions.

It is composed of highly specialized nerve cells (or neurons), nerve fibers and receptors.

Parts of Nervous System

The Human Neural System is Divided into Two Main Parts:

1. Central Nervous System
2. Peripheral Nervous System

The central nervous system (CNS) includes the brain and the spinal cord and is the site of information processing and control.

The peripheral nervous system (PNS) is divided into three divisions called the somatic nervous system, autonomic nervous system, and enteric nervous system:

1. The somatic nervous system relays impulses from the CNS to skeletal muscles, its nerves mediate voluntary movement.
2. The autonomic nervous system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.
   • The autonomic neural system is further classified into the sympathetic nervous system and parasympathetic nervous system.
     1. The sympathetic nervous system is activated in cases of emergencies to mobilize energy
     2. The parasympathetic nervous system is activated when organisms are in a relaxed state.
3. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily.

The visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system.

Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

Neurons

Neurons are the structural and functional unit of the nervous system. A neuron structurally comprises three major parts:

1. Cell body: The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules.
2. Dendrites: Short fibres which branch repeatedly and project out of the cell body are called dendrites. These fibres transmit impulses towards the cell body.
3. Axon: The axon is a long fibre that arises from the cell body and its distal end is branched.

Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical waves travelling along thin fibres called axons, which cause chemicals called neurotransmitters to be released at junctions of two neurons called synapses.

• Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support.
• In the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. These include:
  • sensory neurons that transmute physical stimuli such as light and sound into neural signals, and
  • motor neurons that transmute neural signals into activation of muscles or glands.
• Nerve fibres: On the basis of structure, there are two types of nerve fibres, namely
  • myelinated and non-myelinated. The myelinated nerve fibres are enveloped with Schwann cells which form a myelin sheath around the axon.
  • The gaps between two adjacent myelinated sheaths are called nodes of Ranvier.

Nerve Impulse

Generation and conduction of nerve impulse:

When a neuron is not conducting an impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K⁺) and nearly impermeable to sodium ions (Na⁺).

• Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm.
• Consequently, the axoplasm inside the axon contains a high concentration of K⁺ and negatively charged proteins and a low concentration of Na⁺.
• In contrast, the fluid outside the axon contains a low concentration of K⁺, a high concentration of Na⁺ and thus forming a concentration gradient.
• The ionic gradients are maintained by the sodium-potassium pump which transports 3 Na⁺ outwards for 2 K⁺ into the cell.
• As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore, is polarized.
• The electrical potential difference across the resting plasma membrane is called the resting potential. The state of the resting membrane is called a polarized state.

When a stimulus is applied, the permeability of the membrane to Na⁺ ions is greatly increased at the point of stimulation.

• It is due to the fact that Na⁺ channels open and K⁺ channels remain closed. Thus sodium channels permit the influx of Na⁺ ions by diffusion.
• This results in a positive charge inside and negative charges outside. The change in polarity across the plasma membrane is known as action potential (nerve impulse) and the membrane is said to be depolarized.
• With the increase in Na⁺ ions inside the nerve fibre, the membrane becomes less permeable to Na⁺ ions and more permeable to K⁺ions.
• Na⁺ influx stops and K⁺ outflow begins until the original resting state of ionic concentration is achieved. Thus, resting potential is restored which is called repolarization of the membrane.

Salutatory conduction: The action potential jumps from node to node and passes along the myelinated axon faster than the series of smaller local currents in a non-myelinated axon. This type of conduction is called saltatory conduction.

Synapse

Conduction of nerve impulse through a chemical synapse:

The junction between two neurons, across which the impulse passes from one neuron to the next is called the synapse.

• A typical (generalized) synapse consists of a bulbous expansion of a nerve terminal called a pre-synaptic knob lying close to the membrane of a dendrite.
• The cytoplasm of the synaptic knob contains numerous synaptic vesicles. Each vesicle contains neurotransmitters (chemical substances).
• The membrane of the synaptic knob nearest to the synapse forms the presynaptic membrane.
• The membrane of the dendrite is called the postsynaptic membrane.
• These membranes are separated by a gap, the synaptic cleft.
• The postsynaptic membrane contains receptor sites for the neurotransmitter.
• The two main neurotransmitters in the vertebrate nervous system are acetylcholine (Ach) and noradrenaline although other neurotransmitters also exist.

When an impulse arrives at a presynaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the presynaptic knob.

• The calcium ions cause the movement of synaptic vesicles to the surface of the knob.
• The synaptic vesicles are fused with the presynaptic membrane and get ruptured to discharge their contents (neurotransmitter) into the synaptic cleft.
• The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the postsynaptic membrane.
• This binding action changes the membrane potential of the postsynaptic membrane, opening channels in the membrane and allowing sodium ions to enter the cell.
• This causes the depolarization and generation of the action potential in the postsynaptic membrane. Thus, the impulse is transferred to the next neuron.

Also read: Central Nervous System

Peripheral Nervous System

The nerves running outside the brain and spinal cord constitute the peripheral neural system. These nerves are of two types: cranial nerves (12pairs) (nerves originating from the brain) and spinal nerves (31pairs) (nerves originating from the spinal cord).

The cranial nerves are olfactory (sensory), optic (sensory), oculomotor (motor), trochlear (motor), trigeminal (mixed), abducens (motor), facial (mixed), auditory (sensory), glossopharyngeal (mixed), vagus (mixed), spinal accessory (motor) and hypoglossal (motor).

Spinal nerves are classified into 5 groups: 8 pairs of cervical nerves; 12 pairs of thoracic nerves; 5 pairs of lumbar nerves; 5 pairs of sacral nerves and 1 pair of coccygeal nerves.