Nervous System in Humans

NERVOUS SYSTEM IN HUMANS

• The neural system comprises specialized cells known as neurons, which detect, receive, and transmit various stimuli.
• Neural organization varies across different animal species, from simple networks in lower invertebrates to more complex structures in vertebrates.
• In organisms like Hydra, the neural system consists of a basic network of neurons.
• Neurons in these organisms primarily facilitate basic sensory and motor functions.
• Insects exhibit a more advanced neural organization compared to lower invertebrates.
• They possess a brain along with multiple ganglia and neural tissues, allowing for more sophisticated sensory processing and motor control.
• The human neural system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
• Each part serves distinct functions in information processing and control.

Central Nervous System (CNS):

• The CNS consists of the brain and spinal cord.
• It is the primary site for processing sensory information, coordinating motor responses, and regulating bodily functions.

Peripheral Nervous System (PNS):

• The PNS includes all nerves outside the CNS, connecting it to the rest of the body.
• Nerve fibers in the PNS are categorized into afferent fibers and efferent fibers.
• Afferent fibers transmit sensory impulses from tissues and organs to the CNS.
• Efferent fibers convey regulatory impulses from the CNS to peripheral tissues and organs.

Divisions of the PNS:

• Somatic Neural System:

Relays impulses from the CNS to skeletal muscles, enabling voluntary movement and sensory perception.

• Autonomic Neural System:

Transmits impulses from the CNS to involuntary organs and smooth muscles, regulating automatic bodily functions.

Further divided into:

Sympathetic Neural System: Activates "fight or flight" responses, preparing the body for action during stress or emergencies.

Parasympathetic Neural System: Promotes "rest and digest" activities, conserving energy and regulating bodily functions during restful states.

• Visceral Nervous System:

Part of the PNS that encompasses a network of nerves, fibers, ganglia, and plexuses.

Facilitates communication between the CNS and visceral organs, regulating physiological processes such as digestion, circulation, and metabolism.

Neuron

• A neuron is a microscopic structure consisting of three main parts: the cell body, dendrites, and axon.
• Cell Body: Contains cytoplasm with typical cell organelles and Nissl's granules, which are involved in protein synthesis.
• Dendrites: Short fibers branching from the cell body that transmit impulses towards the cell body. They also contain Nissl's granules.
• Axon: A long fiber extending from the cell body, with branched endings called synaptic knobs. These knobs contain synaptic vesicles filled with neurotransmitters and transmit nerve impulses away from the cell body to synapses or neuro-muscular junctions.

Types of Neurons:

• Multipolar Neurons: Have one axon and two or more dendrites. Found in the cerebral cortex.
• Bipolar Neurons: Have one axon and one dendrite. Commonly found in the retina of the eye.
• Unipolar Neurons: Have a single axon emerging from the cell body. Typically found in the embryonic stage.

Types of Axons:

• Myelinated Axons: Surrounded by Schwann cells that form a myelin sheath around the axon. Nodes of Ranvier are gaps between adjacent myelin sheaths. These axons are found in spinal and cranial nerves.
• Unmyelinated Axons: Enclosed by Schwann cells but lack a myelin sheath. Commonly found in both the autonomic and somatic neural systems.

Generation and conduction of nerve impulse

• Neurons are excitable cells due to their polarized membranes. This polarization arises from selective permeability to ions through various ion channels present on the neural membrane.
• Resting Membrane Potential: The electrical potential difference across the plasma membrane of a neuron at rest.

- At rest, the axonal membrane is more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+), maintaining a concentration gradient across the membrane.

- The sodium-potassium pump actively transports 3 Na+ outwards for 2 K+ into the cell, maintaining the ionic gradients and resulting in a polarized state known as the resting potential.

Mechanism of Nerve Impulse Generation:

• When a stimulus is applied to a polarized membrane, such as at point A, the membrane becomes permeable to Na+ ions, leading to rapid influx of Na+ and reversal of polarity at that site (depolarization).
• This reversal of polarity generates an electrical potential difference known as the action potential or nerve impulse.
• Current flows along the inner surface of the axon from site A to site B, while on the outer surface, it flows from site B to site A, completing the circuit of current flow.
• Consequently, the impulse generated at site A arrives at site B, and the sequence repeats along the length of the axon, facilitating impulse conduction.

Conduction of Nerve Impulse:

• The rise in Na+ permeability is short-lived and followed by a rise in K+ permeability.
• K+ diffuses outside the membrane, restoring the resting potential and making the fiber responsive to further stimulation.

Transmission of impulses

• Nerve impulses are transmitted from one neuron to another through specialized junctions called synapses, formed by the membranes of pre-synaptic and post-synaptic neurons.

Types of Synapses:

• Electrical Synapses: Found when the membranes of pre- and post-synaptic neurons are in close proximity, allowing direct flow of electrical current between them. These synapses facilitate rapid impulse transmission.
• Chemical Synapses: Characterized by a fluid-filled space called the synaptic cleft separating pre- and post-synaptic neurons. Transmission of impulses across chemical synapses involves the release of neurotransmitters. Neurotransmitters are chemicals released by axon terminals into the synaptic cleft, facilitating communication between neurons.

Mechanism at Chemical Synapses:

• When an impulse reaches the axon terminal of the pre-synaptic neuron, it triggers the movement of synaptic vesicles filled with neurotransmitters towards the membrane.
• These vesicles fuse with the plasma membrane and release neurotransmitters into the synaptic cleft.
• Neurotransmitters bind to specific receptors on the post-synaptic membrane, leading to the opening of ion channels.
• Entry of ions through these channels generates a new potential in the post-synaptic neuron, which can be either excitatory, promoting the generation of a nerve impulse, or inhibitory, preventing it.