Neural Control and Coordination
- 1. POOJA SINGH
- 2. NEURAL SYSTEM/NERVOUS SYSTEM • The human neural system is divided into two parts the central neural system (CNS) the peripheral neural system (PNS) • The CNS includes the brain and the spinal cord and is the site of information processing and control. • The PNS comprises of all the nerves of the body associated with the CNS (brain and spinal cord).
- 4. • The nerve fibres of the PNS are of two types afferent fibres efferent fibres • The afferent nerve fibres transmit impulses from tissues/organs to the CNS and • The efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs
- 5. • The PNS is divided into two divisions called somatic neural system autonomic neural system • The somatic neural system relays impulses from the CNS to skeletal muscles • while the autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. • The autonomic neural system is further classified into sympathetic neural system and parasympathetic neural system.
- 6. NEURON • Structural and functional unit of neural system • A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon • The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules. • Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites.
- 7. • These dendrite fibres transmit impulses towards the cell body. • The axon is a long fibre, the distal end of which is branched. • Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters. • The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction
- 8. • Based on the number of axon and dendrites, the neurons are divided into three types, • multipolar with one axon and two or more dendrites; found in the cerebral cortex • bipolar with one axon and one dendrite, found in the retina of eye • unipolar cell body with one axon only; found usually in the embryonic stage
- 9. • There are two types of axons, 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 myelin sheaths are called nodes of Ranvier. • Myelinated nerve fibres are found in spinal and cranial nerves. • Unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon, • It is commonly found in autonomous and the somatic neural systems
- 10. Generation and Conduction of Nerve Impulse • Neurons are excitable cells because their membranes are in a polarised state • Different types of ion channels are present on the neural membrane • These ion channels are selectively permeable to different ions. • When a neuron is not conducting any impulse, the axonal membrane is comparatively more permeable to potassium ions (K+ ) and nearly impermeable to sodium ions (Na+ )
- 12. • The membrane is impermeable to negatively charged proteins present in the axoplasm. • Consequently, the axoplasm inside the axon contains high concentration of K + and negatively charged proteins and low concentration of Na+ . • In contrast, the fluid outside the axon contains a low concentration of K + , a high concentration of Na+ and thus form a concentration gradient
- 14. • These ionic gradients across the resting membrane are maintained by the active transport of ions 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 polarised. • The electrical potential difference across the resting plasma membrane is called as the resting potential.
- 15. • When a stimulus is applied at a site on the polarised membrane, the membrane at the site A becomes freely permeable to Na+ . • This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. • The polarity of the membrane at the site A is thus reversed and hence depolarised. • The electrical potential difference across the plasma membrane at the site A is called the action potential, which is in fact termed as a nerve impulse
- 17. • Thus, the impulse (action potential) generated at site A arrives at site B. • The sequence is repeated along the length of the axon and consequently the impulse is conducted. • The rise in the stimulus-induced permeability to Na+ is extremely shortlived. • It is quickly followed by a rise in permeability to K+ . • Within a fraction of a second, K+ diffuses outside the membrane and restores the resting potential
- 18. Transmission of Impulses • A nerve impulse is transmitted from one neuron to another through junctions called synapses
- 19. • A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft. • There are two types of synapses, namely, electrical synapses and chemical synapses. • At electrical synapses, the membranes of pre- and post-synaptic neurons are in very close proximity. • Electrical current can flow directly from one neuron into the other across these synapses
- 20. • At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft • The axon terminals contain vesicles filled with these neurotransmitters. • When an impulse arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane
- 21. • synaptic vesicles fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. • The released neurotransmitters bind to their specific receptors, present on the post- synaptic membrane. • This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron.