The Peripheral Nervous System and the Neurons

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Welcome to class! 

In today’s class, we will be talking about the peripheral nervous system and the neurons. Enjoy the class!

The Peripheral Nervous System and the Neurons

CONTENT

• Peripheral nervous system
• Somatic and autonomic nervous systems
• The neurones (Structure and function)
• Classification of neurones
• Transmission of nerve impulses

The peripheral nervous system

These include the sensory system (receptor and nerves leading from all parts of the body to the CNS) and the motor system (nerves running from the CNS to the effectors). The PNS consists of twelve cranial nerves (connecting the brain to the head and neck region) and thirty-one spinal nerves (connecting the spinal cord to the thorax, abdomen and limbs).

The motor system is subdivided into the somatic and autonomic nervous system.

Evaluation

Differentiate between the cranial and spinal nerves

Somatic and autonomic nervous systems

The SNS consists of motor (efferent) neurones that connect the CNS to each skeletal muscle. It serves the parts of the body which take part in responses to external stimuli and all voluntary actions.

The ANS consists of motor (efferent) neurones that connect the CNS to glands, smooth muscles and cardiac muscles. It regulates all the body’s involuntary activities such as heartbeat, respiratory movement e. t. c. The ANS is subdivided into the sympathetic and parasympathetic nervous system which works in opposition to one another.

1. The sympathetic nervous system is made up of nerves which connect internal organs to the thoracic and lumbar areas of the spinal cord. The effect of this nervous system dominates in times of emergency and exertion; it regulates homeostatic mechanisms such as vasodilation, vasoconstriction and secretion by sweat gland etc.
2. The parasympathetic nervous system is made up of nerves which connect internal organs to several cranial nerves and spinal nerves of the sacral region. The effect of this nervous system dominates during normal relaxed periods.

Evaluation

1. Differentiate between the somatic and autonomic nervous system
2. Differentiate between the sympathetic and parasympathetic nervous system

The neurons (structure and functions)

Neurones (nerve cells) are the basic structural and functional unit of the nervous system and are responsible for the transmission of impulses within the body.

Each neurone is composed of some basic parts namely:

1. a cell body (soma)
2. One or more short extensions or processes called DENDRONS each of which branches to form many DENDRITES.
3. a long process called axons which branch at the free end and terminate at the synaptic knobs.

The cell body may be oval, polygonal or star-shaped with a large nucleus and dense granulated cytoplasm which give it its greyish colour. It relays impulses it receives from the dendrons to the axon.

The dendrons carry nerve impulses that their dendrites received to the cell body. The dendrites are the main receptive regions of the neurones and may be stimulated by sensory receptors and other neurones carrying impulses.

The axons carry electric impulses away from the cell bodies to the proper destination usually some distance away. The axon of a motor neurone going to a skeletal muscle may be several metres long.  It may also divide into several branches so that impulses from one neurone go to several places. Sometimes, a fatty sheath known as myelin sheath surrounds the axon.  The myelin is interrupted at intervals by constrictions known as nodes of Ranvier.  Myelin sheath acts as an electrical insulator. It also increases the speed at which impulses travel along the axon.

Evaluation

Describe the structure and function of the neurone

Classifications of neurons

In vertebrates, it is common to group neurones according to their functions, these are:

1. Sensory (afferent) neurones which transmit impulses from the receptors to the CNS.
2. Motor (efferent) neurones which transmit impulses away from the CNS to the effectors (muscles and glands).
3. Relay (association or intermediate) neurones which connect the pathways of sensory and motor impulses. They are found mainly in the central nervous system.

Evaluation

Classify neurones based on functions

Transmission of nerve impulses

Transmission of impulses along a neurone is by electrical (through a nerve fibre) and chemical (across a synapse) means. Three phases involved include

1. Resting-state: A state when a neurone is not transmitting an impulse. In this state, the nerve fibre is electrically polarized with excess sodium ions (Na⁺) outside the cell and excess potassium (K⁺) inside the cell. Therefore the cell becomes positively charged outside and negatively charged inside.
2. Action state: When the dendrites of a neurone receive impulses, the neurone becomes depolarized and the cell membrane suddenly permits the inflow of sodium ions and outflow of potassium ions. The cell body then transmits the impulse to its axon.
3. Transmission between neurons: Transmission here is by chemical means. When an impulse reaches the synapse, it stimulates the secretion of a chemical substance called acetylcholine which transmits the impulse to another neurone or muscle fibres.

Evaluation

Discuss briefly the transmission of impulse by a neurone

General evaluation

1. Outline the nervous system graphically.
2. State five functions each of sympathetic and parasympathetic nervous system.
3. Differentiate between somatic and autonomic nervous system.
4. What is a neurone
5. Differentiate between afferent and efferent neurones

Reading assignment

College Biology, chapter 10, page 211 – 217

Weekend assignment

1. The muscle and the gland together are referred to as ______ a) receptor b) effector c) neurone d) sensory cell
2. A neurone that transmits messages from the sense organ to the central nervous system is a) sensory neurone b) motor neurone c) relay neurone d) effector
3. Which of the following is not a structural part of a typical neurone a) axon b) dendron c) cell body d) dendrite
4. Which of the following neurons has no myelin sheath a) afferent b) efferent  c) relay  d) motor
5. A point where one neurone connects with another is called a _________        a) dendron b) myelin sheath c) synapse   d) ganglion

Theory

1. Give five examples of the antagonistic actions of the sympathetic and parasympathetic nervous system.
2. State the functions of five structural parts of a typical neurone

 

In our next class, we will be talking about Reflex and Voluntary Actions, Conditional Reflex.  We hope you enjoyed the class.

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