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Spinal nerves as a whole make up the peripheral nervous system and the peripheral nervous system along with the central nervous system makes the whole nervous system in the body. Spinal nerves are a part of the somatic nervous system and there are 31 pairs of spinal nerves coming out or reaching the spinal cord.
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Key Takeaways: Spinal nerves, Spinal cord, Neurons, Axon, Cervical, Lumber, Synaptic cleft, Acetylcholine, CNS, PNS, Cranial nerve
Meaning of Spinal Nerves
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A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals to and from the spinal cord and the body. Spinal nerves are part of the somatic nervous system. There are thirty-one sets of spinal nerves coming out or reaching the spinal cord. Each set has sensory and motor nerves. Spinal nerves have two functions. The sensory fibres of the spinal nerves collect sensory information from all over the body except the head region and send it to the spinal cord from where they are then carried out to the brain. Also, motor impulses coming down from the brain are sent to the muscles by the motor fibre of the spinal nerves.
Anatomy of Spinal Nerves
Each spinal nerve is a mixed nerve, i.e., it’s formed from the combination of nerve fibres from its dorsal and ventral roots. The dorsal root is the afferent sensory root that takes the sensory information to the brain. The ventral root is the different motor root that carries motor information from the brain. The spinal nerves originate from the spinal column through openings between adjacent vertebrae. After emerging from the vertebral column, the spinal nerves divide into branches.
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Types of Spinal Nerves
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In humans, a total of 31 pairs of spinal nerves are there, emerging from the spinal cord. They are classified according to which regions of the spine they originate from. Spinal nerves are divided into five types:
Cervical Nerves
Cervical means ‘of the neck’. 8 cervical nerves originate from the cervical spine (C1-C8). There are 8 pairs of cervical nerves.
Thoracic Nerves
Thoracic means ‘of the chest’. 12 thoracic nerves originate from the thoracic spine (T1-T12). There are 12 pairs of thoracic nerves.
Lumbar Nerves
Lumbar means from the lower back region. 5 lumbar nerves originate from the lumbar spine (L1-L5). There are 5 pairs of lumbar nerves.
Sacral Nerves
Sacral means ‘of the sacrum’, which is the bony plate at the basal end of the vertebral column. 5 sacral nerves come out from the sacral bone (S1-S5). There are 5 pairs of sacral nerves.
Coccygeal Nerves
Coccygeal means ‘of the tailbone’. There is 1 nerve that comes out from the coccygeal bone. There are 1 pair of coccygeal nerves.
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Location of Spinal Nerves
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Spinal nerves originate almost evenly along the spinal cord and spine. The spine, which is a column of vertebral bones, protects and surrounds the spinal cord. Each spinal nerve comes out from the spine by travelling through the foramen, which are openings at the lateral sides of the vertebral bones of the spine.
The spinal nerves are formed within a few centimetres of the spine on the lateral side. Some spinal nerves combine to form a large plexus. Whereas, some spinal nerves split into smaller branches, without forming a plexus. A plexus is a group of nerves that merge. There are five main plexuses formed by the spinal nerves:
- Cervical Plexus: Formed by merging of spinal nerves C1 to C 5. These carry sensory messages and provide motor control to the muscles of the neck and shoulders.
- Brachial Plexus: Composed of the merging of spinal nerves C5 to T1. This carries sensory messages and provides motor control to the muscles of the arm and upper back.
- Lumbar Plexus: Spinal nerves L1 to L4 combine to form the lumbar plexus. This plexus divides into nerves that carry sensory messages and control the abdominal and legs muscles.
- Sacral Plexus: Spinal nerves L4 to S4 merge and then branch out into nerves that carry sensory messages and provide motor control to the muscles of the legs.
- Coccygeal Plexus: Composed of the merging of nerves S4 toCo1, this plexus supplies motor and sensory control of the genitalia and the muscles that control defecation.
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Function of Spinal Nerves
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The spinal nerves have many sensory and motor branches. Each of the spinal nerves transmits out messages that correspond to a certain region of the body. A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals to and from the spinal cord and the body.
Motor
Motor function involves transmitting messages to the spinal nerves that originate in the brain. The brain initiates a response for muscle control. This command is sent to the spine via nerve impulses and then transmitted through the motor root to the spinal nerve.
Autonomic
The autonomic function of spinal nerves controls the body’s internal organs, such as the bladder and the intestines. There are fewer autonomic branches of the spinal nerves compared to those of the motor and sensory branches.
Sensory
The spinal nerves receive stimuli including touch, temperature, position, vibration, and pain from the small nerves in the skin, muscles, joints, and internal organs of the body. For example, sensation near the front trunk is sent to T10 and sensation from the hand is sent to C6, C7, and 8.
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Points to Remember
Following are some important points:
- The spinal cord controls both voluntary and involuntary movements, i.e., walking (voluntary) and digestion (involuntary).
- A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals to and from the spinal cord and the body.
- There are more than 1-billion neurons in the spinal cord.
- The nervous system uses electrical and chemical transmission to help all parts of the body to communicate with each other.
- There are thirty-one sets of spinal nerves coming out or reaching the spinal cord
- Spinal nerves all over the body are part of the peripheral nervous system.
Sample Questions
Ques: Differentiate between cranial and spinal nerves. (5 Marks)
Ans: Following are the differences:
| Basis of Comparison | Cranial nerves | Spinal nerves |
|---|---|---|
| Origin | Nerves emerge or terminate from/at the brain. | Nerves emerge or terminate from/at the spinal cord. |
| Number | 12 pairs of cranial nerves. | 31 pairs of spinal nerves. |
| Designation | By serial numbers and names. | Named according to the location on the spinal cord. |
| Type | Sensory, motor or mixed | Purely mixed |
| Function | Concerned primarily with the activities associated with head and neck. | Concerned with all the body parts, below the neck |
Ques: How spinal nerves function? (3 Marks)
Ans: The spinal nerves receive sensory inputs from tiny nerves located in areas such as the skin, internal organs, and bones. The spinal nerves send sensory messages to the sensory roots, then to sensory fibres in the dorsal (back) part of the spinal cord. The motor roots receive responses from the ventral (front) part of the spinal cord and send the nerve inputs to the spinal nerves, and eventually to many small nerve branches that activate muscles in the arms, legs, and other areas of the body for appropriate actions.
Ques: Write a short note on axon. (3 Marks)
Ans: Axon is typically a long, slender process of the nerve cell body that sends nerve impulses. It emerges from a cone-shaped hillock of the nerve cell body. The cytoplasm of the axon, known as axoplasm, is surrounded by axolemma. Many times axons branch along their lengths to form axon collaterals. At its end, each axon or axon collateral usually forms bulb shaped structures called synaptic knobs. The synaptic knobs contain neurotransmitters, chemicals that transmit nerve impulses, via synaptic cleft, to a muscle or another neuron.
Ques: What is the neural system in humans? (2 Marks)
Ans: The whole nervous system of humans is derived from the embryonic ectoderm layer. The human nervous system is divided into the Central Neural System (CNS) and the Peripheral Neural System (PNS). The CNS consists of the brain and the spinal cord and is the site of information collection, collation and control. The PNS consists of all the nerves of the body associated with the CNS i.e., brain and spinal cord.
Ques: Describe peripheral nervous system. (2 Marks)
Ans: The peripheral nervous system consists of:
- Somatic Neural System (SNS): The somatic neural system contains nerves that transmit impulses from CNS to skeletal muscles. These may be further categorised into cranial and spinal nerves based on their origin.
- Autonomic Neural System (ANS): The autonomic neural system comprises the sympathetic and parasympathetic nervous system.
Ques: How conduction of a nerve impulse along a nerve fibre? (4 Marks)
Ans: Nerves fibres can be of two types: myelinated and unmyelinated. In myelinated nerve fibre, the action potential is passed from node to node in a jumping manner. This is because the myelinated nerve fibre is covered with myelin sheath, except at points called ‘nodes of Ranvier’. The myelin sheath is impermeable to ions. As a result, the ionic exchange and depolarization of nerve fibre are not possible along the whole length of nerve fibre, the impulses jump through nodes of Ranvier toward the end of fibre. Whereas in non-myelinated nerve fibre, the fibre is not covered by myelin sheath, the ionic exchange and depolarization of nerve fibre take place along the whole length of the nerve fibre. Because of this ionic exchange, the depolarized area becomes depolarized and the next polarised area becomes depolarized and passes all along the fibre toward the end.
Ques: Describe Neural coordination. (3 Marks)
Ans: The nervous system establishes rapid coordination among the organs of the body. This coordination is by transmitting electric or chemical impulses via neurons. All the physiological processes in the body are linked and dependent upon each other. For example, during physical exercise, the body requires more oxygen and food. So, the breathing rate increases automatically and the heart beats faster to supply oxygenated blood to the muscles faster. This stimulus to muscle during exercise, to lungs to increase breathing, to heart for increasing heartbeat, etc are coordinated by the nervous system.
Ques: Write about the mechanism of synaptic transmission. (4 Marks)
Ans: The junction between two neurons is a synapse. It is present between the axon terminal of one neuron and the dendrite of the next neuron or between two axons ending, separated by a cleft. Synaptic transmission occurs in two ways:
- Chemical transmission: When a nerve impulse reaches the endplate of the axon, the nerve ending releases acetylcholine, a neurotransmitter, into the synaptic cleft. This acetylcholine diffuses across the synaptic cleft and binds to the receptors present on the membrane of adjacent neurons across the cleft. This causes depolarization of the membrane in the receiving neuron membrane and initiates an action potential.
- Electrical transmission: In this type of transmission, an electric current is generated in the neuron. This electric current forms an action potential and leads to transmission of nerve impulses across the nerve fibre. This transmission represents a faster method of nerve conduction than the chemical method of transmission.
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