Biology

Human Nervous System

We have understood the basic model of the working of nervous system. The nervous system in humans and in other higher animals is composed of two major components i.e. central nervous system and peripheral nervous system. Central nervous system comprises coordinators i.e. brain and spinal cord while peripheral nervous system consists of nerves that arise from central nervous system and spread in different parts of body, All these components are made of neurons. Now we will first examine the structure and types of neuron and then we will go to the divisions of nervous system.

1. Nerve cell or Neuron

Nerve cell or neuron is the unit of the nervous system. The human nervous system consists of billions of neurons plus supporting (neuroglial) cells. Neurons are specialized cells that are able to conduct nerve impulses from receptors to coordinators and from coordinators to effectors. In this way they communicate with each other and with other types of body cells. The nucleus and most of the cytoplasm of a neuron is located in its cell body. Different processes extend out from cell body. These are called dendrites and axons. Dendrites conduct impulses toward cell body and axons conduct impulses away from cell body.
Neurons
Neurons
A nerve impulse is a wave of electrochemical changes that travels along the length) of neurons.
Schwann cells are special neuroglial cells  located at regular intervals along axons. In some neurons, Schwann cells secrete a fatty layer called myelin on an axon, there are non-myelinated myelin sheath, over axons. Between the areas of points, called the nodes of Ranvier.
Unlike ordinary cells, mature neurons never divide. But a protein called nerve-growth-factor promotes the regeneration of broken nerve cells. The degenerating brain cells could be repaired, by using embryonic stem cells.
Myelin sheath is an insulator so the membrane coated with this sheath does not conduct nerve impulse. In such a neuron, impulses ‘jump’ over the areas of myelin going from node to node. Such impulses are called saltatory (jumping’) impulses. This increases the speed of nerve impulse. On the basis of their functions, neurons are of three types:

1. Sensory neurons conduct sensory information (nerve impulse) from receptors towards the CNS. Sensory neurons have one dendrite and one axon.

2. Interneurons form brain and spinal cord. They receive information, interpret them and stimulate motor neurons. They have many dendrites and axons.

3. Motor neurons carry information from interneurons to muscle or glands (effectors). They have many dendrites but only one axon.

Nerve

A nerve means the union of several axons that are enveloped by a covering made of lipid. Based on the property of axons, the nerves are classified into three types.

1. Sensory nerves contain the axons of sensory neurons only.

2. Motor nerves contain the axons of motor neurons only.

3. Mixed nerves contain the axons of both i.e. sensory and motor neurons.

In certain parts of body, the cell  bodies of many neurons form a group enveloped by a membrane. This is called ganglion.

2. Divisions of the Nervous System

The details of the central and peripheral nervous systems are given below.

CENTRAL NERVOUS SYSTEM

The central nervous system consists of brain and spinal cord.

A-Brain

In animals, all life activities are under the control of brain. The structure of brain is suitable to perform this function. Brain is situated inside a bony cranium (part of skull). Inside cranium, brain is covered by three layers called meninges. Meninges protect brain and also provide nutrients and oxygen to brain tissue through their capillaries. The brain contains fluid-filled ventricles that are continuous with the central canal of spinal cord. Fluid within ventricles and central canal is called cerebrospinal fluid (CSF).
The Divisions of Brain
There are three major regions in the brain of human and other vertebrates. These are forebrain, midbrain and hindbrain. Important parts of each of these regions are described below:
Forebrain
Forebrain is the largest area of brain. It is most highly developed in humans. Following are the important parts of this region. (1) Thalamus lies just below cerebrum. It serves as a relay centre between various parts of brain and spinal cord. It also receives and modifies sensory impulses (except from nose) before they travel to cerebrum. Thalamus is also involved in pain perception and consciousness (sleep and awakening). (ii) Hypothalamus lies above midbrain and just below thalamus. In humans, it is roughly the size of an almond. One of the most important functions of hypothalamus is to link nervous system and endocrine system. It controls the secretions of pituitary gland. It also controls feelings such as rage, pain, pleasure and sorrow. (iii) Cerebrum is the largest part of forebrain. It controls skeletal muscles, thinking, intelligence and emotions. It is divided into two cerebral hemispheres. The anterior parts of cerebral hemispheres are called olfactory bulbs which receive impulses from olfactory nerves and create the sensation of smell. The upper layer of cerebral hemispheres i.e. cerebral cortex consists of grey matter.
Hippocampus is a structure that is deep in the cerebrum. It functions for the formation of new memories. People with a damaged hippocampus cannot remember things that occurred after the damage but can remember things that had occurred before damage.
The grey matter of nervous system consists of cell bodies and non-myelinated axons. Beneath this layer is present the white matter. The white matter of nervous system consists of myelinated axons. Cerebral cortex has a large surface area and is folded in order to fit in skull. It is divided into four lobes.
Lobe Function
Frontal Controls motor functions, permits conscious control of skeleton muscles and Coordinates movements involved in speech
Parietal Contains sensory areas that receive impulses from skin
Occipital Receives and analyzes visual information
Temporal Concerned with hearing and smell

Midbrain

Midbrain lies between hindbrain and forebrain and connects the two. It receives sensory information and sends it to the appropriate part of forebrain. Midbrain also controls some auditory reflexes and posture.
Structure of Human brain
Structure of Human brain

Hindbrain

Hindbrain consists of three major parts. (1) Medulla oblongata lies on the top of spinal cord. It controls breathing, heart rate and blood pressure.  It also controls many reflexes such as vomiting, coughing, sneezing etc. Information that passes between spinal cord and the rest of brain pass through medulla.
The medulla oblongata, pons, and midbrain connect the rest of brain to spinal cord. They are collectively referred to as brain stem

(i) Cerebellum is behind medulla. It coordinates muscle movements.

(ii) Pons is present on top of medulla. It assists medulla in controlling breathing. It also serves as a connection between cerebellum and spinal cord.

B-Spinal Cord

The spinal cord is in fact a tubular bundle of nerves. It starts from brain stem and extends to lower back. Like brain, spinal cord is also covered by meninges. The vertebral column surrounds and protects spinal cord.
Spinal cord is the continuation of medulla oblongata.
The outer region of spinal cord is made of white matter (containing myelinated axons). The central region is butterfly shaped that surrounds the central canal. It is made of grey matter (containing neuron cell bodies). 31 pairs of spinal nerves arise along spinal cord.  These are “mixed” nerves because each contains axons of both sensory and motor neurons. At the point where a spinal nerve arises from spinal cord, there are two roots of spinal nerve. Both roots unite and form one mixed spinal nerve (Fig. 12.4).
Spinal cord is roughly 40cm long and about as wide as your thumb for most of its length.
    • The dorsal root contains sensory axons and a ganglion where cell bodies are located.
    • The ventral root contains axons of motor neurons.
Spinal cord and Spinal Nerves
Spinal cord and Spinal Nerves
Spinal cord performs two main functions:

1. It serves as a link between body parts and brain. Spinal cord transmits nerve impulses from body parts to brain and from brain to body parts.

2. Spinal cord also acts as a coordinator, responsible for some simple reflexes.

PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system (PNS) is composed of nerves and ganglia. Ganglia are the clusters of neuron cell bodies outside CNS. Nerves arise or lead to brain and spinal cord. So they are named as cranial and spinal nerves. Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves. Some cranial nerves are sensory, some are motor and some are mixed. On the other hand, all spinal nerves are mixed nerves. The cranial and spinal nerves make two pathways i.e. sensory pathway (conducting impulses from receptors to CNS) and motor pathway (conducting impulses from CNS to effectors). Motor pathway makes two systems.

Somatic Nervous System

It is responsible for the conscious and voluntary actions. It includes all of the motor neurons that conduct impulses from CNS to skeletal muscles.
Autonomic Nervous System
It is responsible for the activities, which are not under conscious control. It consists of motor neurons’ that send impulses to cardiac muscles, smooth muscle and glands. Autonomic nervous system comprises of sympathetic system and parasympathetic system. Sympathetic nervous system prepares body to deal with emergency situations. This is often called the “fight or flight” response. During an emergency situation, this system takes necessary actions. For example; it dilates pupils, accelerates heartbeat, Increases breathing rate and inhibits digestion. When stress ends, the parasympathetic nervous system takes action and normalizes all the functions. It causes pupils to contract, promotes digestion, and slows the rate of heartbeat and breathing rate.
Division of the Nervous system
Division of the Nervous system

3. Reflex Action

When central nervous system sends impulses to muscles and glands, two types of actions (responses) result.

1. The higher centres of brain control conscious actions or voluntary actions.

2. When impulses are not passed to the higher centres of brain, it results in responses which are not under conscious control. Such responses are called involuntary actions. Sometimes, the involuntary response produced by the CNS is very quick. Such a response is called reflex action. The pathway followed by the nerve impulses for producing a reflex action, is called reflex arc.

The most common example of reflex action is the withdrawal of hand after touching a hot object. In this reflex action, spinal cord acts as coordinator. Heat stimulates temperature and pain receptors in skin.
It doesn’t matter how clever we are we will always pull our hand away from a flame without thinking about it.
A nerve impulse is generated which is carried by sensory neurons to the interneurons of spinal cord. From interneurons, the impulse is passed to motor neurons, which carry it to the muscles of arm. As a result, the muscles contract to withdraw hand. During it, other interneurons transmit nerve impulses up to brain so that the person becomes aware of pain and what happened.
Reflex arc in a reflex action
Reflex arc in a reflex action

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