The Somatic and Autonomic Nervous Systems

Introduction

SoNS vs ANS

The Somatic Nervous System is the part of the peripheral nervous system that handles voluntary control of body movements. It contains all the neurons connected with skeletal muscles and skin.

The Autonomic Nervous System is the part of the peripheral nervous system that acts as an involuntary control system b(elow the level of consciousness), and controls visceral functions.

The ANS controls the connections between the brain, spinal cord and organs/glands, whereas the SNS connects external sensory organs through the brain to the muscles.

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Central vs Peripheral Nervous Systems

The CNS contains the brain and the spinal cord, the PNS includes all remaining nervous tissue within the body.

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Cranial Nerves

Cranial nerves emerge directly from the brainstem, as opposed to the spinal cords. Humans traditionally have 12 cranial nerves, labelled in Roman Numerals.
The nerves are a mix of sensory, motor and combinations. 4 of the nerves are ANS, as they serve organs/glands.

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Autonomic Nervous System

Purpose

The ANS regulates fundamental states of physiology, including heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, energy utilisation, temperature and sexual arousal.

This system sets the balance between relaxation and excitation required to meet environmental demands. As well as setting the balance for ongoing behaviour, such as foraging for food, the system is also necessary for short lived ‘emergency’ reactions such as predator evasion, and reproduction.

Sympathetic/Parasympathetic Divisions

The ANS can be split into three divisions; the sympathetic, parasympathetic and enteric nervous systems. We'll deal with the first two, which can be distinguished by anatomical arrangements.

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Sympathetic Nervous System

The Sympathetic Nervous System stems from the thoracic and lumber spinal cord (higher up).

The SNS acts upon organs and glands to mobilise resources for short-lived behaviour; predator avoidance and reproduction. It modifies salivation, digestion, heart rate, breather, blood pressure, and redirects blood towards the lungs for oxygen assimilation and the muscles to improve performance.

Parasympathetic Nervous System

The Parasympathetic Nervous System stems from the cranial nerves and the sacral spinal cord (lower down).

The PSNS is responsible for 'rest and digest' activities; maintaining stable bodily functioning in normal situations, handling energy acquisition and storage.

Endocrine Glands

The endocrine glands are a part of the ANS, and are a collection of organs that work to regulate the overall hormone levels of the brain.

Hormones act as neuromodulators, spreading diffusely throughout the body and regulating activity levels across a range of organs and neurons. They're involuntary, and regulated through the ANS.

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Smooth Muscles

Smooth muscles are an involuntary muscle group, regulated by the ANS. They are everything from pupil dilation to uterus contractions to respiratory tract breath rate modulation and heart beat.

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Spinal Injury and the ANS

Spinal injury produces abnormalities in the autonomic regulation that don't correspond as obviously as within the SoNS. Abnormalities can result in a new balance being reached, or persist for life, and they can include:

  • Temperature fluctuations
  • Poor fluid regulation; not sending the right thirst signals and hence receiving too much/little fluid intake
  • Bladder and bowl control
  • Irregular and insufficient breathing
  • Fluctuating heart rate/blood distribution

Somatic Nervous System

Composition

The somatic nervous system is composed of sensory receptors in the skin, which, when activated, send sensory information through dorsal root ganglion of the spine to synapses on inter-neurons which in turn synapse on motor neurons that project out of ventral root ganglion to activate muscle movements. Such simple circuits mediate the patellar reflex and withdrawal reflexes (e.g. from intense heat).

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More complex motor behaviour is achieved by a central circuit involving millions of neurons. Sensory information arriving at the spine ascends through the medulla, the pons, the midbrain, the thalamus and then arrives at the somatosensory sensory cortex.

From there information is relayed to the motor cortex (which has parallelisation of somatotopic organisation), and from there back through the thalamus, midbrain, pons, medulla, spine and down to the muscles.

The somatosensory cortex is thought to be the basis of intentional (voluntary) action.

Somatotopic Organisation

The somatosensory cortex has somatotopic organisation; neighbouring regions of the body provide sensory information to neighbouring areas of the brain.

Skeletal Muscles

Skeletal muscles are voluntary, and hence controlled by the SNS. They're one of the three major muscle groups, and are composed of muscles cells, all attached to bones by tendon fibres.

They're arranged in opposing groups; looking at the biceps and triceps we can see that the activation of each muscle is balanced so that the movement slows to a stop, rather than jerking, after activities such as throwing.

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SNS Diseases

Cerebellar Hypoplasia

Because of the cerebellum's rich interconnection with the pons and the fine tuning of voluntary movements, we see poor motor control and a lack of ability to perform complex coordination of muscle activity in those suffering from Cerebellar hypoplasia. This acts as evidence towards the role of the cerebellum in voluntary movement.

Parkinson's

Parkinson's is a degenerative disorder of the CNS, affecting voluntary motor control. The symptoms are slowness of movement, tremors, rigidity, and later on difficulty executing intentional actions.

Parkinson's arises from dopamine cell bodies originating in the Substantia Nigra Pars Compacta (SNc).

The SNc is connected to the thalamus to the striatum, and then to sensory-motor cortex, substantia nigra pars reticula (SNr), globus pallidus internal/external (GP[Ei]) and subthalamic nucleus (STN) as shown below:

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Parkinson's treatments look at increasing dopamine availability in the striatum.

Spinal Injury

Because of the links between areas of the body and their links to the spinal cord (so as to minimise the distance that signals need to travel) different vertebrae correlate to different body parts. Hence injury to different vertebrae causes damage to their corresponding body area.

The dorsal/ventral sections of the vertebrae control sensation (afferent nerves taking information to the CNS) and motor control (efferent nerves taking instructions to the muscles), respectively, and the left/right sections control the right/left sides of the body.

A complete injury means that there is no sensation or voluntary movement below the damaged site on both sides of the body. An incomplete injury means that there is some sensation or motor control below the damaged site on either or both sides of the body.

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