Structure of the mammalian nervous systemEdexcel A-Level Biology B: Revision notes
Section 1
Organisation of the nervous system
The mammalian nervous system has two parts:
- The central nervous system (CNS): the brain and spinal cord
- The peripheral nervous system (PNS): all the nerves outside the CNS, carrying impulses between the CNS and receptors and effectors
The PNS is divided into the voluntary (somatic) system, which controls skeletal muscle under conscious control, and the autonomic system, which works without conscious control.
Section 2
Structure of the spinal cord
In transverse section the spinal cord has a central H-shaped grey matter containing the cell bodies and synapses of relay and motor neurones. It is surrounded by white matter, which is pale because it is made of myelinated axons running up and down in tracts. A central canal filled with cerebrospinal fluid runs through the middle.
Spinal nerves join at two roots:
- Dorsal root: sensory neurones carry impulses into the cord; their cell bodies are in the dorsal root ganglion
- Ventral root: motor neurones carry impulses out to muscles and glands
Do not mix up the roots. Dorsal means sensory (in); ventral means motor (out). The cell bodies of motor neurones are in the grey matter, not in the ganglion.
Section 3
Regions of the brain
You must know the location and main function of four regions:
- Medulla oblongata: at the base of the brain; controls breathing and heart rate
- Cerebellum: at the back, below the cerebrum; controls balance and coordination of movement
- Cerebrum: the large folded upper part; initiates movement and handles conscious thought
- Hypothalamus: below the cerebrum; temperature regulation and osmoregulation
Link damage to symptoms: staggering means cerebellum; no breathing means medulla; failure to start a movement means cerebrum; poor temperature or water control means hypothalamus.
Section 4
Voluntary and autonomic systems
The voluntary (somatic) system carries impulses from the cerebrum along motor neurones to skeletal muscle.
The autonomic system controls cardiac muscle, smooth muscle and glands without conscious control. It is divided into two systems that act antagonistically:
- Sympathetic: prepares the body for activity ('fight or flight'): raises heart rate, dilates pupils, reduces digestion
- Parasympathetic: for rest and recovery ('rest and digest'): lowers heart rate, constricts pupils, promotes digestion
Having two opposing pathways lets the body make fast, fine adjustments in both directions, because the effect is the balance of the two.
'Antagonistic' does not mean one system is switched off. Both are active, and the effect depends on which has the greater activity.
Must know
- CNS = brain and spinal cord; PNS = voluntary and autonomic
- Spinal cord: grey matter (cell bodies), white matter (myelinated axons), dorsal and ventral roots
- Medulla: breathing and heart rate; cerebellum: balance; cerebrum: initiates movement; hypothalamus: temperature and osmoregulation
- Sympathetic and parasympathetic act antagonistically
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Structure of the mammalian nervous system
- A neurologist is teaching trainee nurses about the regions of the brain, using a patient who suffered brain damage in a road accident. The patient can still speak, and can decide to move his arm, but he staggers when walking and cannot touch his nose smoothly with a finger.Explain why severe damage to the medulla oblongata would be life-threatening.2 marks
- A student examines a transverse section through a mammalian spinal cord. A central H-shaped region of tissue contains many neurone cell bodies. It is surrounded by an outer region of paler tissue made of myelinated axons running up and down the cord. A narrow fluid-filled canal runs through the centre. Nerves join the cord at two roots, and the root nearer the back has a swelling (a ganglion) on it.Compare the roles of the dorsal root and the ventral root of a spinal nerve.2 marks
- A man runs a marathon on a hot day. His heart rate rises from 70 to 160 beats per minute during the race, and his core body temperature rises from 37.0 °C to 38.5 °C. He sweats heavily and loses a large volume of water.Explain how the hypothalamus helps the runner during the race.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).