Neural Control and Co-ordination

 Chapter-7

Neural Control and Co-ordination

Definition and classification of nervous system

The nervous system is a complicated, hiphly structured network of billions of cells called neurons and surrounding cells called neuroglia.

The nervous system is composed of the following parts:

• 1. The brain

• 2. Cranial nerves and their branches

• 3. The spinal cord, spinal nerves and their branches

• 4. Ganglia

• 5. Enteric plexuses

• 6. Sensory receptors

The nervous system can be classified into two main parts:

• 1. The Central Nervous System (CNS): It consists of the brain and spinal cord.
• 2. The Peripheral Nervous System (PNS): It consists of all nervous tissue outside the CNS.

Peripheral nervous system is further divided into three parts:

• (a) Somatic nervous system

• (b) Autonomic nervous system (ANS): It is further divided into two divisions- sympathetic and parasympathetic.

• (c) Enteric nervous system

Structure of a neuron

Nervous tissue consists of two types of cells as Neurons and Neuroglia. Neurons are responsible for all functions of nervous system like thinking, memory, sensing etc. Neuroglia surrounds neurons and are responsible for support and providing nutrition to neurons.

Neurons consists of three parts: cell body, dendrites and den Dixon.

1. Cell body

• It comprises of a nucleus surrounded by cytoplasm.

• Cytoplasm of cell body shows presence of cellular organelles such as golgi complex, lysosomes, mitochondria, free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies.

1. Dendrites

• These are the processes which emerge out from cell body.

• Dendrites are generally tapering, small and extremely branched (tree shaped).

• Dendrites play an important role in transfer of signals from one neuron to other.

• Dendrites are receiving part of a neuron.

• Their cytoplasm contains mitochondria, and other organelles.

1. Axon:

• These are long, thin, cylindrical projections emerging out from cell body of neuron.

• Axons also have presence of mitochondria, microtubules, and neurofibrils.

• Endoplasmic reticulum is absent in axon.

• The cytoplasm of an axon, called axoplasm.

• Axoplasm is surrounded by a plasma membrane known as the axolemma.

• The end part of axon ÏS divided into many fine processes called axon terminals.

• Based on presence of covering on outside, the axons can be classified into two types

1. (i) Myelinated axons: AxOns surrounded by a multi-layered lipid and protein covering, called the myelin sheath, are called as myelinated. The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction.
2. (ii) Unmyelinated: Axons without myelin sheath are called as unmyelinated.

• The single axon of a neuron plays an important role of transferring nerve impulses to another neuron, a muscle fibre, or a gland cell.

• The location of communication between two neurons or between a neuron and an effector cell is called a synapse.

Generation and conduction of nerve impulse

Nerve impulse-basic Concept:

• Neurons are excitable cells. Neurons are called excitable cells because they have capability to respond to stimuli by generating electrical signals such as action potentials, also called as nerve impulse i.e. electrical excitability.

• An action potential (nerve impulse) is an electrical signal which is generated by neuron due to stronp chanpe in the environment (internal or external) and which travels along the surface of the membrane of a neuron.

• The different concentrations of Na* and K* in cytosol and extracellular fluid are crucial for the ability of neurons to generate electrical signals such as action potentials.

• As we know, proteins are present in the plasma membrane, some proteins called as transmembrane proteins act as ion channels, which regulate when ions can move in or out.

• Neuron shows a membrane potential, an electrical potential difference (voltage) across the membrane called the resting membrane potential.

•  It exists because of accumulation of negative ions in the cytosol along the inside of the membrane, and an equal accumulation of positive ions in the extracellular fluid along the outside surface of the membrane. This potential is measured in millivolts.

• The resting membrane potential of a neuron typically is —70 mV.

Generation of Action Potential:

• Action potential is generated due to existence of potential called resting membrane potential and movement of ions (mainly Na+ and K+) across the membrane of neuronal cell.

• An action potential includes sequence of quickly occurring actions across neuronal membrane that decrease and reverse the membrane potential and then eventually restore it to the resting state.

• There are two phases in action potential:

• o Depolarizing phase: In this phase, the negative membrane potential becomes less negative, reaches zero, and then becomes positive. o Repolarizing phase: In this phase, the membrane potential is restored to the resting state i.e. at —70 mV.

• Due to depolarization, when potential across membrane reaches a level called threshold, an action potential is generated. Generally, the threshold potential in neuron is -55 mV.

Process of Generation of Action Potential:

• 1. The neuronal membrane is at resting state i.e. the potential across it is resting membrane potential. All ion channels are closed during this phase.

• 2. A stimulus is responsible for change in membrane potential. If due to any stimulus, when membrane potential of axon reaches threshold, Na* channels open and Na* ions flow into neuron from extracellular fluid. This flow of Na* ions, increases charge inside the neuron which results in increase in membrane potential. This is depolarization phase.

• 3. After depolarization, Na* channels close and K” channels open. K* ions starts moving outside the neuron to decrease the positivist caused due to excess entry of Na* ions during depolarization phase. This is called as repolarization phase. Outflow of K‘ ions restore resting membrane potential.

• All ion channels are closed.

Propagation of Action Potentials:

• Action potential spreads along the membrane. This mode of conduction is called propagation.

• The action potential travels down the axon as voltage gated ion channels are opened by the spreading depolarization.

• There are two types of conductions: continuous and salutatory.

• In unmyelinated axons, this happens in a continuous fashion because there are voltage gated channels throughout the membrane.

• In myelinated axons, propagation is described as saltatory because voltage gated channels are only found at the nodes of Ranvier and the electrical events seem to ”jump” from one node to the next. Saltatory conduction is faster than continuous conduction.

• Thus, myelinated axons propagate their signals faster as compared to unmyelinated axons

Structure of brain and spinal cord

1. Brain

Brain is an important organ of central nervous system. Brain is located in skull. The brain is composed of four major parts:

• 1. Cerebrum

• 2. Cerebellum

• 3. Diencephalon

• 4. Brain stem

Functions of cerebrum,

Cerebrum

• The cerebrum is the most important, largest and uppermost part of brain.

• The cerebrum consists of an outer cerebral cortex, an internal region of cerebral white matter, and gray matter nuclei deep within the white matter.

Cerebral Cortex

• The cerebral cortex is a repoint of gray matter that forms the outer border of the cerebrum.

• The folds present in cortex are called as gyri.

• The deepest grooves between folds are known as fissures; the shallower grooves between folds are termed sulci.

• Cerebrum consists of two hemispheres. The most prominent fissure, the longitudinal fissure, separates the cerebrum into right and left halves called cerebral hemispheres.

• The cerebral hemispheres are connected internally by the corpus callosum.

• Each cerebral hemisphere can be further subdivided into four lobes. The lobes are named after the bones that cover them: frontal, parietal, temporal, and occipital lobes.

• Functions

• Cerebrum is responsible for multiple activities like consciousness, thinking, memory, sensations, emotions, and willed movements.

Brain Stem

It is a slightly elongated posterior region of brain which connects it to spinal cord. Brain stem is divided into three parts: Medulla oblongata, Pons and Midbrain.

Medulla Oblongata

It forms the inferior part of the brain stem. It is continuous with the superior part of the spinal cord.

cerebellum

It is also called as little brain is located posterior to the brain stem.

• The cerebellum is posterior to the medulla and pons and inferior to the posterior portion of the cerebrum.

• The shape of the cerebellum look like a butterfly.

• The central constricted area is called as vermis, and the lateral “wings” or lobes are the cerebellar hemispheres.

• Each hemisphere consists of lobes separated by deep and distinct fissures.

• Cerebellum has a highly folded surface that greatly increases the surface area of its outer gray matter cortex.

• The superficial layer of the cerebellum is called as cerebellar cortex.

• Cerebellum is separated from cerebrum by a deep groove known as the transverse fissure, alonp with the tentorium cerebelli.

Functions of Cerebellum:

• (i) The main function of cerebellum is to regulate posture and balance of body.

• (ii) It is also responsible for coordinating contractions of skeletal muscles. It produces smooth co-ordinated movements, maintain equilibrium, and sustain normal postures. Cerebellum may have a role in cognition and language processing.

Diencephalon:

• It is a small region superior to the brain stem. It consists of three parts: Thalamus,

• Hypothalamus, Epithalamus.

hypothalamus and medulla oblongata

Hypothalamus: The hypothalamus is a small part of the diencephalon located inferior to the thalamus.

Functions of the Hypothalamus:

• (i) It control of activities of autonomic nervous system.

• (ii) Hypothalamus is also called as body’s thermostat. It regulates body temperature.

• (iii) Hypothalamus is involved in regulation of food intake. It comprises of a feeding center, which promotes eating, and a satiety center, which causes a sensation of fullness and cessation of eating. It also contains a thirst center. Hypothalamus is connected to pituitary gland, which is located inferior to it. Hypothalamus produces various hormones which have effect on pituitary gland

• (i) Hypothalamus is the body’s internal biological clock.

• (ii) Hypothalamus is involved in regulation of circadian rhythms.

• (vi) Hypothalamus regulates emotional and behavioral patterns.