Nervous System: Peripheral Nervous System Flashcards
Divisions of the Nervous System
- The central nervous system (CNS) consists of the brain and spinal cord
- The peripheral nervous system (PNS) consists of nerve fibres that carry information to and from the central nervous system (CNS)
- These are nerves, and groups of nerve cell bodies, ganglia, which lie outside the brain and spinal cord
- The nerve fibres are arranged into nerves that arise from the brain and the spinal cord.
- Twelve pairs of nerves arise from the brain
- These are the cranial nerves
- Thirty-one pairs of spinal nerves arise from the spinal cord
- They are all mixed nerves, and each is joined to the spinal cord by two roots.
Naming pathways/divisions
- Sensory neurons carry impulses to the brain and spinal cord: this is called the afferent division
- Impulses originating from the skin or skeletal muscles travel along the somatic division
- Impulses from the internal organs travel up the visceral division
- Motor neurons carry impulses from the brain and spinal cord back out to the organs form the efferent division
- Impulses that travel to the skeletal muscles travel along the somatic division
- Impulses that travel to the involuntary organs, muscles and glands travel down the autonomic division
- The autonomic division has two pathways: sympathetic and the parasympathetic divisions.
Divisions of the Nervous System
- The ventral root contains the axons of motor neurons that have their cell bodies in the grey matter of the spinal cord
- The dorsal root contains the axons of sensory neurons that have their cell bodies in a small swelling on the dorsal root known as the dorsal root ganglion.
Autonomic Nervous System
- The autonomic nervous system (ANS) is a part of the peripheral nervous system
- It is responsible for the control of the body’s internal environment and is involved in many of the homeostatic mechanism that keep the internal environment constant
- It usually operates without conscious control and is regulated by groups of nerve cells in the medulla oblongata, hypothalamus and cerebral cortex.
- Some of the body functions regulated by the autonomic division include:
- Heart rate
- Blood pressure
- Body temperature
- Digestion
- Release of energy
- Pupil diameter
- Airflow to the lungs
- Defecation
- Urination
- The pathway travelled by an impulse from the CNS to an organ controlled by the ANS consists of two neurons
- One of these neurons has its cell body in the CNS but the cell body of the other is in a ganglion
- A ganglion (plural, ganglia) is a group of nerve cell bodies outside the central nervous system.
Somatic and Autonomic Pathways
- There are two other important differences between the autonomic and somatic divisions:
- The ANS is subdivided into sympathetic (fight or flight) and parasympathetic (rest and digest) divisions
- In the somatic nervous system, the neurotransmitter that carries the message from the neuron to the skeletal muscle is acetylcholine; in the autonomic nervous system either acetylcholine or noradrenaline carry the message to the effector
- This allows the body to either excite or inhibit muscles and glands, effectively controlling/balancing body functions as required.
Comparison of autonomic and somatic division: Autonomic Division
Characteristic: Autonomic Division
Effectors: Heart muscle, involuntary muscle, glands
General Function: Adjustment of the internal environment (homeostasis)
Efferent (outward) pathways: Two nerve fibres from the CNS to the effector with a synapse in a ganglion
Neurotransmitter at effector: Acetylcholine or noradrenaline
Control: Usually, involuntary
Nerves to target organ: Two sets – sympathetic and parasympathetic
Effect on target organ: Excitation or inhibition
Comparison of autonomic and somatic division: Somatic Division
Characteristic: Somatic Division
Effectors: Skeletal muscles
General Function: Response to the external environment
Efferent (outward) pathways: One nerve fibre from the CNS to the effector; no synapse or ganglion
Neurotransmitter at effector: Acetylcholine
Control: Usually, voluntary
Nerves to target organ: One set
Effect on target organ: Always excitation
Effects of the autonomic system: Sympathetic Stimulation
Heart: Increases rate and strength of contraction
Lungs: Dilates bronchioles
Stomach and Intestines: Decreases movement
Liver: Increases breakdown of glycogen and releases glucose
Iris: Dilates pupil
Sweat glands: Increases sweat secretion
Salivary Glands: Decreases secretion of saliva
Blood vessels of skin, skeletal muscle, internal organ:
- Skin: Constricts vessels
- Skeletal Muscle: Dilates vessels
- Internal Organ: Constricts vessels (except in heart and lung)
Bladder: Relaxes muscles of wall
Adrenal Medulla: Stimulates hormone secretion
Effects of the autonomic system: Parasympathetic Stimulation
Heart: Decrease rate and strength of contraction
Lungs: Constricts bronchioles
Stomach and Intestines: Increases movement
Liver: Increases the uptake of glucose and the synthesis of glycogen
Iris: Constricts pupil
Sweat Glands: No effects
Salivary Glands: Increases secretion of saliva
Blood Vessels of skin, skeletal muscle, internal organ:
- Skin: Little effect
- Skeletal Muscle: No effect
- Internal Organ: Little effect
Bladder: Constricts muscles of wall
Adrenal Medulla: No effect
Flight-or-Fight responses
- In threatening situations, the balance between sympathetic and parasympathetic stimulation is upset and the sympathetic becomes dominant
- Situations that invoke fear, anger, stress, danger or competition provoke what is called a flight-or-fight response or alarm reaction
- These responses prepare the body for increased activity (in other animals, to fight or to flee)
- Activation of the sympathetic division results in the following responses:
- Increase heart rate and force of contraction, which in turn increases in blood pressure
- Dilation of blood vessels in necessary organs: skeletal muscles, heart and live
- Constriction of blood vessels of non-essential organs: kidney, stomach, intestines and skin
- Dilation of airways in the lungs and increase breathing depth and rate
- Increase in blood glucose levels because the liver converts more glycogen to glucose
- Increase in secretion from sweat glands
- The adrenal medulla releases the hormones adrenaline and noradrenaline, which intensify and prolong the above responses
Receptors
- Receptors are structure in the cell membrane that are capable to detect a change in the body’s internal or external environments
- Osmoreceptors: detect osmotic pressure/water balance.
- Osmoreceptors are located in the hypothalamus
- Chemoreceptors: detect specific chemicals
- Chemoreceptors in the nose and mouth detect smell and taste
- Chemoreceptors in the blood vessels detect oxygen, carbon dioxide, pH so are involved in the regulation of heartrate and breathing rate.
- Thermoreceptors: detect heat and cold
- Thermoreceptors in the skin detect external temperature
- Thermoreceptors in the hypothalamus detect internal body temp
- Thermoreceptors detect either heat or cold
- Touch receptors: detect pressure
- Touch receptors are located in the skin and at the base of a hair follicle
- Concentrations vary across different locations of the body: example lots in the hands and face
- Touch receptors adapt/desensitize quickly
- Pressure receptors are located deeper under the skin
- Pain receptors: detect damage to the tissue
- Pain receptors are located in the skin and in most organs (but not the brain)
- Pain is stimulated by cuts, bumps, poor blood flow, heat or chemical damage to the cells
- Pain warns us damage is occurring, and receptors will continue to be stimulated for as long as the stimulus is present
Reflexes
- A reflex is a rapid, automatic response to a change in the external or internal environment.
- Reflexes are one of the ways in which the body achieves homeostasis.
- All reflexes have four important properties:
- A stimulus is required to trigger off a reflex.
- A reflex is involuntary
- A reflex response is rapid — only a small number of neurons are involved.
- A reflex response is stereotyped — it occurs in the same way each time it happens.
- Some reflexes involve the unconscious parts of the brain, but most are coordinated by the spinal cord.
- The impulse may be passed to motor neurons at the same level in the cord or may travel a few segments up or down the cord before travelling out through a motor neuron.
- In these cases, the reflex is carried out by the spinal cord alone and is known as a spinal reflex.
- The pathway a nerve impulse follows in travelling from a receptor to an effector is known as a reflex arc or, in the case of a spinal reflex, a spinal reflex arc.
- A reflex arc has the following basic components:
1. A receptor is either the ending of a sensory neuron or a specialised cell associated with the end of a sensory neuron. The receptor reacts to a change in the internal or external environment by initiating a nerve impulse in the sensory neuron.
2. A sensory neuron carries impulses from the receptor to the CNS.
3. There is at least one synapse. The nerve impulse may be passed directly to a motor neuron or there may be one or more interneurons, which direct the impulse to the correct motor neuron.
4. A motor neuron carries the nerve impulse to an effector.
5. An effector receives the nerve impulse and carries out the appropriate response. Effectors are muscle cells or secretory cells.
6. During a reflex the impulse will also be sent to the brain in order to inform the brain of the incident. Not part of the reflex arc but happens at the same time. - Learned reflexes:
- The protective reflexes mentioned above are present from birth.
- More complex motor patterns appear during a baby’s development — including reflexes such as suckling, chewing or following movements with the eyes.
- These innate reflexes are determined genetically.
- Some complex motor patterns are learned and are called acquired reflexes.
- Muscular adjustments required to maintain balance while riding a bike or catching a ball are all acquired reflexes.
- They are learned through constant repetition.
